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Pressureless Silver Nanopowder Sintered Bond for Liquid
Cooled IGBT Power Module for EVs and HEVs
by
Namjee Kim
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
The Edward S. Rogers Sr. Department of Electrical and Computer Engineering
University of Toronto
© Copyright by Namjee Kim 2018
ii
Pressureless Silver Nanopowder Sintered Bond for Liquid Cooled IGBT
Power Module for EVs and HEVs
Namjee Kim
Master of Applied Science
The Edward S. Rogers Sr. Department of Electrical and Computer Engineering
University of Toronto
2018
Abstract
Pressureless silver nanopowder sintering of the bonding layer between the insulated gate bipolar
transistor (IGBT) and the direct bonded copper (DBC) for the liquid cooled power modules in
electric vehicles and hybrid electric vehicles is studied. The pressureless silver nanopowder
sintering is analysed using the Differential Scanning Calorimeter / Thermogravimetry /
Simultaneous Thermal Analysis and the Scanning Electron Microscopy. Based on the analysis
results, the sintering is optimized at 200 ˚C for 60 minutes for reliable die attachments. The
bonding of the sintered silver to the IGBT is confirmed by intermetallic layers composition
analysis. The nanopowders are classified by the average sizes into the micro and nano-meter scaled
groups. The porosity of 30% is computed within the sintered silver layer. Overall, the experimental
results verify the feasibility of the pressureless silver nanopowder sintering of the bonding layer
between the IGBT and the DBC.
iii
Acknowledgments
I would like to express my deepest gratitude to my research supervisor, Professor Wai Tung Ng,
for his constructive guidance, continuing support and the patience throughout this project. It is a
great pleasure to have a chance working in the Smart Power Integration & Semiconductor Device
Group under his strong leadership. His knowledge and vision in power electronics and packaging
encourage me to build the insight of the study.
I would like to express my sincere gratitude to Professor Francis P. Dawson, Professor Thomas
W. Coyle and Professor Glenn D. Hibbard for their valuable advices and comments on my project.
Without their supports the project would have not been successfully driven.
I would like to express my appreciation to everyone involved in the project at Dana Ltd., Henkel
and Rogers Co. for their sponsorship.
Special thanks to the current and previous graduate students and researchers of the Smart Power
Integration & Semiconductor Device Group for their supports and companionship over the years.
I would like to thank graduate students of the Cellular Hybrid Materials Research Group and staff
of the TNFC for their technical supports as well.
Lastly but not least, I wish to thank my family and friends for their supports and encouragements.
I would like to acknowledge the financial supports from the Ontario Centre of Excellence (OCE)
and the Natural Sciences and Engineering Research Council of Canada (NSERC).
iv
Table of Contents
Acknowledgments.......................................................................................................................... iii
Table of Contents ........................................................................................................................... iv
List of Tables ................................................................................................................................. vi
List of Figures ............................................................................................................................... vii
List of Acronyms ........................................................................................................................... xi
List of Symbols ............................................................................................................................ xiv
Chapter 1 Introduction ..............................................................................................................1
1.1 Motivations ..........................................................................................................................1
1.2 Thermal Management ..........................................................................................................4
1.3 Research Objective and the Thesis Organization ..............................................................10
Chapter 2 Literature Review...................................................................................................12
2.1 Background ........................................................................................................................12
2.2 Sintering Theory ................................................................................................................13
2.2.1 Polymer Burnout ....................................................................................................13
2.2.2 Theoretical Analysis of Sintering ..........................................................................14
2.3 Sintering Process Management ..........................................................................................21
2.3.1 Sintering Temperature ...........................................................................................21
2.3.2 Sintering Pressure ..................................................................................................22
2.4 Nanopowder Sintering Challenges ....................................................................................23
v
2.5 Direct Bonded Copper (DBC) ...........................................................................................26
2.6 Chapter Summary ..............................................................................................................29
Chapter 3 Experimental Results and Discussion ....................................................................31
3.1 Experiment Design.............................................................................................................31
3.1.1 Structure Design.....................................................................................................31
3.1.2 Sample Preparation ................................................................................................34
3.1.2.1 Thermal Budget .......................................................................................37
3.1.2.2 Nickel/Gold Plating .................................................................................37
3.1.2.3 Silver Bonding Layer Process .................................................................41
3.1.2.4 Silver Nanopowder Pasting .....................................................................41
3.2 Silver Nanopowder Paste Analysis ....................................................................................43
3.2.1 DSC/TGA/STA ......................................................................................................44
3.2.2 SEM .......................................................................................................................47
3.3 Chapter Summary ..............................................................................................................56
Chapter 4 Conclusions and Future Work Plan .......................................................................58
4.1 Conclusions ........................................................................................................................58
4.2 Future Work Plan ...............................................................................................................61
References ......................................................................................................................................63
vi
List of Tables
Table 1.1 Thermophysical Properties of Liquid Coolants at Room Temperature [10] ................ 10
Table 2.1 Sintering Mechanisms in Solids [29]–[31] ................................................................... 18
Table 3.1 DBC Sample Data at 20 °C [51] ................................................................................... 36
Table 3.2 Summary of the DSC/TGA/STA Results ..................................................................... 47
Table 3.3 Weight Percentage at EDS Spots within the Intermetallic Layers ............................... 49
Table 3.4 Atomic Percentage at EDS Spots within the Intermetallic Layers ............................... 49
vii
List of Figures
Figure 1.1 The global electric car sales volume, 2010-2016 .......................................................... 1
Figure 1.2 DC bus voltage for the electric vehicle using boost converter ...................................... 2
Figure 1.3 The cross sectional view of an n-channel thin wafer IGBT cell structure .................... 3
Figure 1.4 Temperature dependence of failure rate of a power semiconductor device .................. 4
Figure 1.5 Illustration of a heat sink ............................................................................................... 7
Figure 1.6 Ranges of heat transfer coefficients for liquid coolants and cooling methods ............. 9
Figure 2.1 Evolution of the relative density during sintering process .......................................... 14
Figure 2.2 Schematics of the sintering process, (a) initial, (b) intermediate, and (c) final ........... 14
Figure 2.3 Geometrical evolution and instantaneous surface vacancy concentration level for the
two initially spherical crystalline particles of equal size sinter by combined surface, volume and
grain boundary diffusion (a) t=0, (b) t=0.1, (b) t=1, (d) t=5 (e) t=20 and (f) t=100 .................... 16
Figure 2.4 Definitions of radius of curvature and the center of curvature ................................... 17
Figure 2.5 Schematic diagram of two particles sintering model (a) diffusion paths and (b)
Dihedral angles at the initial state (120°), equilibrium state (150°) and final state (180°) ........... 19
Figure 2.6 Equisided tetrakaidecaheraon geometry ...................................................................... 20
Figure 2.7 Driving forces for nanopowder consolidation as a function of grain size .................. 23
viii
Figure 2.8 (a) Agglomeration and (b) Aggregation of the nanopowders .................................... 25
Figure 2.9 Sintering results from densification and non-densification diffusion ......................... 25
Figure 2.10 Changes in densification rate in temperature ............................................................ 26
Figure 2.11 Schematics for the eutectic bonding copper process and the Cu-O phase diagram .. 27
Figure 2.12 Cu/Al2O3 void formed at the interface ...................................................................... 28
Figure 3.1 Cross sectional view of the proposed IGBT attachment on a DBC substrate using
silver nanopowder sintered. The DBC is bonded to a liquid cooled aluminum heat sink ............ 33
Figure 3.2 Perspective view of the proposed IGBT attachment on a DBC substrate using silver
nanopowder sintered ..................................................................................................................... 33
Figure 3.3 The DBC design and the actual patterned DBC for a single IGBT die ....................... 34
Figure 3.4 The DBC design for a Half-bridge IGBT (with free wheeling diodes) module
assembly ........................................................................................................................................ 34
Figure 3.5 CTE differences in two layers produces interlayer stresses, leading to potential failure
....................................................................................................................................................... 36
Figure 3.6 Ni and Au electroplating wet bench set up. ................................................................. 38
Figure 3.7(a) Electroplating circuit for Ni and (b) Anode ............................................................ 39
Figure 3.8 DBC substrate samples in conditions of (a) bare (Cu on top), (b) Ni on top, and (c) Au
on top with the IGBT die mounted on pressureless sintered silver nanopowder with the Au wire
connections. .................................................................................................................................. 40
ix
Figure 3.9 Process flow for the IGBT die attachment process using silver nanopowder sintering
....................................................................................................................................................... 41
Figure 3.10 Illustration of the silver nanopowder paste stencil printing technique ...................... 42
Figure 3.11 Temperature profile for drying and sintering processes ............................................ 43
Figure 3.12 DSC/TGA/STA results for the silver nanopowder sintering experiment .................. 45
Figure 3.13 Isothermal DSC/TGA/STA results for silver nanopowder sintering at 200 ˚C ......... 46
Figure 3.14 Cross section image of the silver nanopowder sample sintered at 200 ˚C for 60
minutes, captured using Quanta FEG250 ESEM (magnification 500 X) ..................................... 48
Figure 3.15 (a) Cross-sectional view of sintered silver (magnification 10k X), (b) IGBT die and
sintered silver intermetallic layers EDS analysis on selected areas .............................................. 48
Figure 3.16 . (a) SEM cross-sectional image (magnification 5k X) for the porosity analysis and (b)
the porosity calculation result ....................................................................................................... 50
Figure 3.17 Sample preparations for the top view SEM image analysis along the temperature
profile ............................................................................................................................................ 51
Figure 3.18 Top view SEM images of the silver nanopowder paste samples with heating-up
process stopped at different temperatures before sintering .......................................................... 52
Figure 3.19 Top view SEM images of the silver nanopowder paste samples sintered at 200 ˚C (a)
for 1 min, (b) for 20 min, (c) for 40 min and (d) for 60 min (magnification 10k X) .................... 54
x
Figure 3.20 Images of grain sizes observed on the micro-sized powder after sintering at 200 °C
for 60 minutes, (a) magnification 80k X and (b) magnification 200k X. ..................................... 55
Figure 3.21 Top view SEM images of (a) the dried as-received silver nanopowder paste after the
heating stopped and maintained at 120 °C for 1 minute (magnification 5k X), (b) the silver
nanopowder paste sintered at 200°C for 60 minutes (magnification 5k X), (c) silver nanopowder
paste sintered at 200°C for 10 minutes (magnification 20k X), and (d) silver nanopowder paste
sintered at 200°C for 60 minutes (magnification 20k X) .............................................................. 56
xi
List of Acronyms
AF Acceleration Factor
Ag Silver
Al Aluminum
Al2O3 Alumina or Aluminum Oxide
AlN Aluminum Nitride
Au Gold
C Carbon
Cl- Chloride Ion
CTE Coefficient of Thermal Expansion
Cu Copper
Cu2O Copper Oxide
CuAlO2 Copper Aluminate or Copper Aluminum Oxide
DBC Directed Bonded Copper
DC Direct Current
DIW De-Ionized Water
DSC Differential Scanning Calorimeter
DTG Derivative Thermogravimetry
EDS Energy Dispersive Spectroscopy
EG Ethylene Glycol
EHD Electrohydrodynamics
ESEM Environment SEM
xii
EV Electric Vehicle
FAST Field Activated Sintering Technique
H+ Hydrogen Ion
HCl Hydrochloric Acid
HEV Hybrid Electric Vehicle
HRSEM High Resolution SEM
IGBT Insulated Gate Bipolar Transistor
KFO Potassium Formate
MOSFET Metal-Oxide-Semiconductor Field-Effect-Transistor
MTF Mean Time to Failure in Hours
Ni Nickel
O Oxygen
OH- Hydroxide Ion
PC Pulse Current
PCB Printed Circuit Board
PG Propylene Glycol
rpm Revolutions per Minute
SEM Scanning Electron Microscopy
Si Silicon
Si3N4 Silicon Nitride
SiC Silicon Carbide
SO42- Sulfate Ion
STA Simultaneous Thermal Analysis
TG Thermogravimetry
xiii
TGA Thermogravimetry Analysis
Ti Titanium
TIM Thermal Interface Material
TiO2 Titanium Oxide
ZrO2 Zirconium
xiv
List of Symbols
Rds,on Drain to source on-resistance
R1 & R2 Principal radius of curvature
B Coefficient of the mechanism characteristic
C Vacancy concentration under a curved surface
C0 Equilibrium vacancy concentration
D Particle diameter
ΔTt Test temperature difference between highest and lowest
ΔTo Operating temperature difference between on and off state
EA,v Activation energy of the vacancy formation
EA Activation energy (eV)
FT Failure factor
J Current density (A/cm2)
KB Boltzmann constant
L Length between two particle centers
P Vapor pressure
Pex External pressure
pH Potential of hydrogen
R Gas constant (8.314 J/K·mol)
r1 & r2 Particle radius
t Time
Tm,b Melting temperature of the bulk material (K)
xv
Tm,p Melting temperature of the powder (K)
Tmax,o Maximum operation temperature (K)
Tmax,t Maximum test temperature (K)
Ts Sintering temperature (K)
X Neck diameter
α Coefficient of the geometric and environmental factors
γ Surface energy
δ Material dependent parameter (1.8 - 2.65 nm)
ρ Density
σ Stress
Ω Atomic volume
1
Chapter 1
Introduction
1.1 Motivations
As the awareness on climate change and other environmental issues increases dramatically, the
reduction of greenhouse gas emission is one of the most pressing global interests. Electric vehicles
(EVs) and hybrid electric vehicles (HEVs) are regarded as the key products to lower carbon
emission within the automotive industry [1]. The production volume of EVs and HEVs is on an
exploding trend in the twenty-first century as shown in Figure 1.1 [2]. These worldwide activities
accelerate the development of high performance EVs and HEVs with better efficiency and longer
range. The automotive industry’s effort in reducing greenhouse gas emissions and limiting the
environmental pollution has also created exciting opportunities for the power semiconductor
market.
Figure 1.1 The global electric car sales volume, 2010-2016 [2].
2
The insulated gate bipolar transistor (IGBT) is a special type of power semiconductor device for
delivering electrical power to the drive trains, and it is the device of choice in most EV and HEV
power inverters and converters over the conventional power metal-oxide-semiconductor field-
effect-transistor (MOSFET) due to its better current conduction capability [3]. Common
applications of the IGBTs in EVs and HEVs include inverters for propulsion motors or generators
and DC/DC voltage boost converters for batteries or fuel cells.
IGBTs are used in the inverter to control the flow of the electrical power from the battery to the
propulsion motor in the EVs and HEVs. The commercially available IGBTs are designed to
optimize the power transfer efficiency from the DC bus to the electric motor with a low on state
voltage drop, fast switching speed and a high avalanche breakdown capability. Also, IGBT boost
converters are used to enhance the DC bus allowing more power available to the motor. As power
is calculate by multiplication of voltage and current, if the boost converter raises the DC link
voltage by 2.5 times, the torque and the power output of the motor is also raised by 2.5 times
without increasing the motor current, as the increased DC bus voltage is supplied to IGBT based
inverters to drive the generator and the motor [4]. The topology of the boost converter is illustrated
in Figure 1.2 below
Figure 1.2 DC bus voltage for the electric vehicle using boost converter [4].
3
The IGBT is a power semiconductor device with three terminals; gate, cathode and anode,
alternately known as the gate, emitter and collector, respectively. It is essentially a merged MOS-
bipolar device where the current carrying pnp bipolar transistor is controlled by a MOSFET. The
cell structure of IGBT is shown in Figure 1.3.
One of the major limitations of the power MOSFET is that the value of drain to source on resistance
(Rds,on) increases as the device voltage rating. This is because the doping in the drift region is
inversely proportional to the required breakdown voltage. Thus, the doping decreases as the
voltage rating increases, and the conductivity decreases. The IGBT overcomes this limitation by
using a forward biased pn-junction to inject minority carriers into the reduced doped region to
increase the conductivity. Hence, the structure of the IGBT is similar to the structure of the
MOSFET except that a heavily doped p-layer at the collector terminal is used to inject the minority
carriers [4], [5].
Figure 1.3 The cross sectional view of an n-channel thin wafer IGBT cell structure [4].
4
1.2 Thermal Management
One of the performance indicator for the power converter is its conversion efficiency. The output
power efficiency of the converter is defined as the percentage ratio of the electrical output power
against the total input power. The difference between the input and output power is dissipated as
wasted heat which causes increase in the operating temperature of the power device and the power
module. The typical maximum operation temperature of the silicon (Si) based IGBTs is typically
175 ˚C, and the maximum operation temperature can reach up to 250 ˚C for silicon carbide (SiC)
IGBTs [6]–[8]. To achieve a better power efficiency it is critical to increase the heat dissipation,
and the heat management system is designed to improve the reliability and the electrical
performance [9]. If the rate of the heat removal is not equivalent to or greater than the rate of the
heat generation, the temperature of the device and components increases constantly, therefore
reducing the device reliability and performance, and possibly causing a device failure. As
illustrated in Figure 1.4, the failure factor of a device is derived from the relative failure rate at any
temperature divided by the failure factor at 75 °C, and it increases exponentially with the
temperature increases within the device.
Figure 1.4 Temperature dependence of failure rate of a power semiconductor device [10].
5
The mean time to failure in hours (MTF) due to an increase in the operating temperature can be
estimated using Black’s correlation in forms of:
𝑀𝑇𝐹 =1
𝐴𝐽2 exp (𝐸𝐴
𝐾𝐵(
1
𝑇𝑟−
1
𝑇𝑡)) (1.1)
where A is a constant, J is the current density (A/cm2), EA is the active energy (eV), KB is the
Boltzmann constant, 𝑇𝑟 is the reference junction temperature (K) and 𝑇𝑡 is the junction
temperature during test (K). The Black’s correlation demonstrates that even a slight increase in the
operating temperature can exponentially decrease the MTF and increase the failure rate. The
exponential term in the Black’s correlation, known as the Arrhenius equation, relates how
increased temperature accelerates the aging of the product as compared to the aging under the
normal operating conditions. This term is also called acceleration factor (AF) [10], [11]. The
Black’s correlation is the theoretical failure model for the steady state test temperature. For the
non-steady state test temperature such as the thermal cycle test, the temperature difference between
the highest and the lowest test temperatures and the cycle frequency are addressed in the failure
model.
𝑀𝑇𝐹 =1
𝐴𝐽2 𝐴𝐹 (1.2)
𝐴𝐹 = (Δ𝑇𝑡
Δ𝑇𝑜)
1.9
(𝐹𝑜
𝐹𝑡)
13⁄
exp (𝐸𝐴
𝐾𝐵(
1
𝑇𝑚𝑎𝑥,𝑜−
1
𝑇𝑚𝑎𝑥,𝑡)) (1.3)
Δ𝑇𝑡 is the temperature difference between the highest and the lowest test temperatures, Δ𝑇𝑜 is the
operating temperature difference between the on- and off- states of the testing device, 𝐹𝑡 is the test
cycle frequency (number of cycles per 24 hours), 𝐹𝑜 is the cycle frequency in the device operating
6
condition (number of cycles per 24 hours), 𝑇𝑚𝑎𝑥,𝑜 is the maximum operating temperature, and
𝑇𝑚𝑎𝑥,𝑡 is the maximum test temperature [11], [12].
Most of the thermal management system designs are constrained by two requirements, the high
power dissipation and the size of the power module. The high power dissipation is necessary for
high performance while the high power efficiency and the smaller module size are desirable for a
higher packing density and miniaturization. The local thermal profile and the heat removal
technologies employed are key factors in determining the required heat sinking technique, and
therefore influencing the size and the cost of the module as well [13].
The heat transfer mechanisms for heat removal include conduction, natural convection, forced
convection and other effects such as thermoelectric cooling [13].
The solid state cooling uses solid state bulk materials to remove the heat from the source via heat
dissipation mechanisms. The most common method uses heat sinks, a conduction plate and thermal
interface materials (TIMs), based on the conduction heat transfer mechanism as illustrated in
Figure 1.5. Aluminum heat sinks and a conduction plate are often chosen due to their cost
efficiency, light weight and high thermal conductivity (200 W/m·K). Also, they are less reactive
to the conventional coolants and are non-toxic upon corrosion. A heat spreader maximizes the
contact surface area between the semiconductor and the cooling medium in order to improve the
heat transfer via natural air cooling. The TIMs are used to fill the gaps and to increase the thermal
transfer efficiency by removing the space between the heat sink and the packaging [13].
7
Figure 1.5 Illustration of a heat sink [13].
Magnetic cooling is another method of solid state cooling where changes in magnetic field on a
magnetocaloric material lead to temperature changes. However, the application of the magnetic
cooling is often constrained by the types of materials, volume and weight [13].
The air or gas cooling technologies have the advantages of increases in heat transfer and heat
exchange rates without implementation of the complex structure as often required in the liquid
cooling. The natural air convection is the one of the simplest methods. Assuming the same cooling
module structure as in Figure 1.5 with the heat source sitting on top of the heat sink, the
temperature difference causes the air/gas density to change due to the difference in energy of the
gaseous molecules. This then leads to the decrease in the surface temperature while the air
temperature increases. Only the thermal resistance between the solid surface of the heatsink and
the ambient acts as the controlling factor of the heat dissipation. Despite its advantage in the
simplicity, the heat flux for the natural air convection is as low as less than a few W/cm2. In order
to achieve lower thermal resistances between the heat sink and the ambient, forced convection can
be applied using a fan. The air/gas flow rate increases as the fan rotation speed in revolutions per
minute (rpm) increases. As a consequence, the heat flux is improved. The typical range of the heat
flux for forced air convection heat transfers is about tens of W/cm2 having significant improvement
8
over the heat flux rate of the natural convection. Other air or gas cooling methods employing a fan
using piezoelectric effect, an electrohydrodynamic (EHD) cooler using kinetic energy conversion
based on the corona effect and a thermoacoustic cooler are also available [13].
Liquid cooling is generally preferred over the solid or gas cooling for the cases of the thermal
management in higher power densities. Higher thermal conductivity and the higher heat density
are the main advantages of the liquid coolants. However, liquid cooling has a critical disadvantage
due to the possible leakages of coolants. A mechanical failure of the cooling system may cause
damages to the electronics. Furthermore, flammable or toxic liquid coolants may lead to the
corrosion of the device or module. There are three classifications in liquid coolants according to
their conductivity: where the direct immersion of the hot device is possible, where the direct
immersion is not possible, but a leakage does not damage the electronics, and where the direct
immersion is not possible, and leakage damages the electronics. Therefore, in the liquid cooling
system designs, it is imperative to select the coolant which is non-flammable, nontoxic and with
excellent thermophysical properties including high thermal conductivity, high specific heat, high
heat transfer coefficient and low viscosity.
The mechanisms for heat transfer within the liquid cooling system include free convection, forced
convection, boiling and condensation. The range of heat transfer coefficients of commonly used
coolants are compared in Figure 1.6. Water possesses the highest heat transfer coefficient; thus, it
is the most widely used. However due to its high freezing point at 0 °C it is not ideal for uses in
lower temperature conditions. Moreover, since its volume expands upon freezing water holds a
critical disadvantage to the mechanical design of the cooling system [10], [13].
9
Figure 1.6 Ranges of heat transfer coefficients for liquid coolants and cooling methods [10].
The liquid coolants are classified by their electrical conductivities into dielectric and non-dielectric
groups. Dielectric liquids are electrically insulating materials and non-dielectric liquids are
electrically conducting materials. The dielectric group includes liquids of aromatic base, aliphatic
base, silicone base and fluorocarbons base. The FC72 and the FC77 are two most commonly used
fluorocarbons based liquid coolants for the electronic cooling as they are non-flammable and inert.
Non-dielectric liquid coolants are normally aqueous solutions with high thermal conductivity and
high heat capacity. Water and ethylene glycol (EG) are two most widely used coolants. Propylene
glycol (PG), potassium formate (KFO) and liquid metals such as Ga-In-Sn are also commonly
used coolants. Thermophysical properties of these coolants at the room temperature are listed in
Table 1.1.
10
Table 1.1 Thermophysical Properties of Liquid Coolants at Room Temperature [10]
In addition to the conventional coolants mentioned above, nanofluids and ionic liquid based
nanofluids are newly classified liquid coolants. The nanofluids are made of nanoparticles dispersed
in the conventional liquid coolants to improve the thermal properties for the uses with reduced
sized electronics. Copper (Cu) in water, alumina (Al2O3) in water and titanium oxide (TiO2) in
water are examples of the nanofluids with enhanced heat transfer capabilities up to 18 % in
comparison to the heat transfer capability of water [10].
1.3 Research Objective and the Thesis Organization
In this study, an improved bonding technology using silver nanopowder sintering is proposed to
enhance the heat management and the power efficiency within the IGBT power module for the
HEVs and EVs. The direct IGBT mounting using pressureless silver nanopowder sintering is
studied as a bonding technology to reduce distance of the thermal path on the aluminum liquid
cooling module. The scope of this study is focused on the development and investigation of the
silver sintering bonds on the directed bonded copper (DBC) substrate, while the research on and
development of the liquid cooling module is coordinated by Dana Ltd.
11
This thesis consists of 4 chapters. Chapter 1 introduces the overview of the research background
on the power semiconductor module and the heat management using cooling systems. Chapter 2
contains extensive backgrounds on physics behind the sintering, material selection and failure
mechanisms. In Chapter 3 the sample fabrication processes, experimental and analysis results and
discussions are discussed. Chapter 4 presents the conclusions and future work suggestions.
12
Chapter 2
Literature Review
In this chapter, the background of this study and literature reviews are present and discussed
covering two main subjects of the silver sintering process and the direct bonded copper (DBC).
2.1 Background
The bonding materials between the insulated gate bipolar transistor (IGBT) dies and the direct
bonded copper (DBC) substrate must have a melting temperature above the maximum operating
temperatures of the IGBT dies to prevent delamination, and a high electrical conductivity to allow
electrical conduction between the collector of the IGBT and the DBC. Silver nanopowder is an
excellent candidate meeting these requirements, with the melting temperature at 960 ˚C, electrical
conductivity of 6.3×107 S/m, thermal conductivity of 406 W/m·K for the bulk silver and
240W/m·K for the nanopowder silver at 20 ˚C, and an excellent adhesive strength [6], [14]–[19].
However, the processing temperature of the bonding materials must be lower than the thermal
budget of the IGBT. The thermal budget is the maximum temperature that the IGBT is functional
without a failure during the bonding process. To reduce the thermal stress applied to the IGBT die
during the bonding process, a lower processing temperature is desirable. Sintering is an effective
method in lowering the temperature of the bonding process.
13
2.2 Sintering Theory
The sintering of powder is not a newly developed technique, but has been used for over thousands
of years in ceramic tool manufacturing. However, advanced sintering techniques are employed in
various manufacturing fields including automobile engines, rocket nozzles, dental implants and
semiconductor packaging substrates, etc. To understand the physics behind the sintering,
considerable researches have been carried out and the controllable variables established [20], [21].
The sintering is classified into solid state sintering and liquid state sintering. Solid state sintering
involves solid state diffusion of atoms. Solid state sintering materials include polycrystalline
materials and amorphous materials based on their lattice structures. These two materials exhibit
different diffusion mechanisms. Liquid state sintering technique involves the uses of liquid phase
materials. Liquid phase materials at an optimized condition can provide rapid mass and heat
transport paths therefore promoting fast sintering [22], [23]. In this study, the application of solid
state sintering is reviewed and further examined.
2.2.1 Polymer Burnout
Before the sintering takes place, polymers used as binders and lubricants are removed from the
sintering powder mixture. The polymer burnout is triggered when the powder mixture experiences
a temperature raise and at a certain temperature the polymer molecules become unstable. The
molecular bonds of the polymers are disconnected and the molecules are decomposed into smaller
molecules such as carbon, oxygen, water and other by-products of the combustion [24].
14
2.2.2 Theoretical Analysis of Sintering
The sintering is carried out to fabricate a bulk solid from powders at a low processing temperature.
Because a complete phase change is not required during the sintering, the processing temperature
is significantly lower and the link between the powders are determined by the diffusion
mechanisms. The density of the sintered product increases with the process time. The relative
density during the sintering process is illustrated in Figure 2.1. Starting from the porous structure
at the beginning, the pore sizes decreases as the density of the structure increases.
Figure 2.1 Evolution of the relative density during sintering process [25].
The sintering is classified into three stages; : (a) Initial-point contact, (b) intermediate-neck growth,
and (c) final reduction of pore sizes [24]. Figure 2.2 illustrates the schematics of the sintering
process among four particles in the same plane.
Figure 2.2 Schematics of the sintering process, (a) initial, (b) intermediate, and (c) final [14].
15
At the beginning, a point contact between two powder particles is required to initialize the atomic
diffusion. An empty volume surrounded by the solid particles and filled with gas is created. After
the initial point contact, the atomic diffusion is driven by the force to reduce the excess energy
associated with surfaces. The sintering can initiate by two mechanisms, (1) the reduction of the
total surface area by increasing the powder size, and (2) the elimination of the solid/vapor interface
and generation of grain boundaries.
The vapor pressure, the vacancy concentration and the stress of the sintering microstructure are
the factors considered at the initial stage. The vapor pressure is lower at the neck region where the
contact of two particles is made, and is above the equilibrium at the bulk region of the powder.
𝑃 = 𝑃𝑒𝑥 + 𝛾 (1
𝑅1+
1
𝑅2) (2.1)
where 𝑃 is the vapor pressure, 𝑃𝑒𝑥 is the external pressure applied, 𝛾 is the surface energy and 𝑅1
and 𝑅2 are the principal radius of curvature at the contact point [26]. Likewise, the vacancy
concentration is also far from the equilibrium at the curved surface.
C = 𝐶0 [1 −𝛾Ω
𝑘𝐵𝑇(
1
𝑅1+
1
𝑅2)] (2.2)
where C is the vacancy concentration under a curved surface, 𝐶0 is the equilibrium vacancy
concentration, Ω is the atomic volume, 𝑘𝐵 is the Boltzmann’s constant and T is the absolute
temperature. 𝐶0 has the Arrhenius temperature dependency as presented in Equiation 2.3 below;
𝐶0 ∝ exp (−𝐸𝐴,𝑣
𝑅𝑇) (2.3)
where 𝐸𝐴,𝑣 is the activation energy of the vacancy formation which is proportional to the melting
temperature of the solid, R is the gas constant of 8.314 J/K·mol. The geometrical evolution and
16
surface vacancy concentration levels are shown in Figure 2.3 at the different stages [27]. As the
simulation result shown, at the initial stage of (b), the vacancy concentration is high near the neck
region driving the atomic diffusion to the neck growth. The vacancy concentration finds its
equilibrium throughout the sintering process and two particles merge into one large particle.
Figure 2.3 Geometrical evolution and instantaneous surface vacancy concentration level for the
two initially spherical crystalline particles of equal size sinter by combined surface, volume and
grain boundary diffusion (a) t=0, (b) t=0.1, (b) t=1, (d) t=5 (e) t=20 and (f) t=100 [27].
The atoms are moved along the powder particle surface to fill the valleys in between the curved
surface of the particles. Equation 2.4 describes the driving force to initiate the sintering;
𝜎 = 𝛾
1
𝑅1+
1
𝑅2
(2.4)
17
where 𝜎 is the stress associated with a curved surface. The radius of the curvature is measured by
creating a fitted circle at the point of interest of the curve. The radius from the center of the fitted
circle is equal to the radius of the curvature. The definition of the radius of the curvature and the
center of the curvature is schematically described in Figure 2.4. As the radius of the curvature
decreases, the size of powder also decreases, thus the powder possesses the higher stress at the
surface. Consequently, the smaller powder tends to merge with another powder with a larger radius
of the curvature which in turn has lower surface energy.
Curve
Circle of Curvature
Radius of Curvature
Centre of Curvature
C
P
R
Figure 2.4 Definitions of radius of curvature and the center of curvature [28].
Throughout the sintering process, the surface area of powders decreases, and the pore volume
decreases at the same time. A neck is formed in between two contacted particles. For the solid
state crystalline powders, the neck formation rate depends on the mechanisms of sintering . The
major mechanisms inducing mass transformations are the grain boundary diffusion, the surface
diffusion, the volume (lattice) diffusion, and the viscous flow. Table 2.1 below summarizes the
properties of the solid state sintering mechanisms. Within the table, under the “Source of matter”
column the sources where the atoms are coming from are listed, and under the “Sink of matter”
18
column destinations where the atoms are relocated and diffuse into are listed. The diffusion
sourced from the grain boundary and the dislocation contributes to the densification at the later
intermediate and final stages. Other mechanisms sourced from the surface contribute more to the
surface reduction and the neck formation at the initiation and intermediate stages.
Table 2.1 Sintering Mechanisms in Solids [29]–[31]
Type of solid Mechanism Source of matter Sink of matter Densification
Polycrystalline Surface diffusion Surface Neck No
Lattice diffusion Surface Neck No
Grain boundary Neck Yes
Dislocation Neck Yes
Vapour transport Surface Neck No
Grain boundary diffusion Grain boundary Neck Yes
Amorphous Viscous flow Unspecified Unspecified Yes
Figure 2.5 (a) illustrates the paths of the diffusion for the different mechanisms. The dihedral angle
illustrated in (b) is the angle between the two spherical particles at the grain boundary. As the
dihedral angle increases, more greater degree of shrinkage and the neck growth is found indicating
the progress and the completion of the sintering [24], [27], [32].
19
X/2R
Lattice diffusion
Vacancy diffusion
Vacancy surface diffusion Dihedral angle
Triple junction
Grain boundary
Particle surface
Crystalline particle
(a)
(b)
Figure 2.5 Schematic diagram of two particles sintering model (a) diffusion paths and (b) Dihedral
angles at the initial state (120°), equilibrium state (150°) and final state (180°) [27], [33].
The relationship between the neck growth and the diffusion mechanism is defined by Equation
2.5.
(𝑋
𝐷)𝑛 =
𝐵𝑡
𝐷𝑚 (2.5)
where X is the neck diameter, D is the particle diameter, t is the isothermal sintering time, and n
and m are constants [24], [30], [34]. B is the coefficient of the mechanism characteristic, including
the temperature term;
𝐵 = 𝐵0 exp (−𝐸𝐴
𝑅𝑇) (2.6)
𝐵0 is the coefficient of mechanism, and 𝐸𝐴 is the activation energy. The values for n, m and B are
related to the mass transport. The values of n and m are different based on the mechanisms of the
diffusion. The B value is also different, depending on the diffusion mechanisms [24].
20
During the intermediate stage, pore rounding, grain growth and densification occur. The surface
transport still plays a role in pore rounding and pore migration at this stage. However, the volume
and grain boundary diffusion contribute more for densification. At the intermediate and final stages,
the grain size increases as the pore size increases and the porosity decreases. The geometric model
proposed by Coble is commonly used [34]. The lattice and grain boundary diffusion equation is
derived. The fourteen-sided tetrakaidecahedron in Figure 2.6 is assumed to be the final geometry
of the grains with the complete densification with the cylindrical pores along the edges [35]. Once
the pores are completely closed, the final stage of the sintering occurs. The pores at the final stage
has spherical shapes.. If the gas is trapped in the pore, the pore elimination process would be
extremely slowly or even prevented. For full densification, different sintering process conditions
and techniques may be accounted such as vacuum sintering [36].
Figure 2.6 Equisided tetrakaidecahedron geometry [36].
21
2.3 Sintering Process Management
The literatures and backgrounds on the process parameters for sintering are reviewed in this
section. The key factors reviewed include temperature (𝑇), pressure (𝑃) and time (𝑡).
2.3.1 Sintering Temperature
The sintering temperature is highly related to the sintering powder size, and the processing
temperature can give an advantage in terms of a significant drop in the sintering temperature by
using a nano-sized powder. The sintering temperature (𝑇𝑠) is expressed as in Equation 2.7.
𝑇𝑠 = 𝛼𝑇𝑚,𝑝 (2.7)
where 𝑇𝑚,𝑝 is the melting temperature of the powder, α is the coefficient of the geometric and
environmental factors, and is usually in the range from 0.5 to 0.8 for large sized powders. When
the particle size falls to the nano-meter scale, 𝑇𝑚 becomes lower than the melting temperature of
the bulk material (𝑇𝑚,𝑏). The following Equation 2.8 defines the relationship;
𝑇𝑚,𝑝 = 𝑇𝑚,𝑏(1 −𝛿
𝐷) (2.8)
The term 𝛿 is a material dependent parameter and its value depends on the atomic volume and
bonding energy of the crystalline powder. The reported values of 𝛿 range from 1.8 to 2.65 nm.
The value for α can be reduced to a range between 0.1 and 0.3. Therefore, the nano-meter scale
powder sintering is achieved at temperatures ranging from 0.1 to 0.3 𝑇𝑚,𝑏. For the sintering using
silver with the melting temperature of 960 °C, the sintering temperature can be as low as 110 °C
[26].
22
2.3.2 Sintering Pressure
Either the absence or the presence of the applied pressure during the sintering process classifies
the sintering as the pressureless or the pressure-assisted sintering. As mentioned earlier in Equation
2.1, the external pressure (Pex) leads to the higher flux of the atomic diffusion on the surface during
the sintering. With the pressure applied to the sample, the powder is physically deformed and the
contact surface area increases. Thus, the grain boundary is readily available even without
overcoming the activation energy of the diffusion. The advantage of the pressure-assisted sintering
is that the coarse powder with relatively larger sizes can be sintered at lower temperatures. One
drawback is the higher chance of mechanical failure of the IGBT die. Because the pressure applied
to the die can be as high as 40 MPa for the silver nanopowder, the corresponding force of 4000 N
is applied to a typical silicon die with an area of 100 mm2, which must be handled with care to
prevent the breakage [25].
Pressureless sintering is another option to overcome the drawback from the pressure-assisted
sintering. The Herring law in Equation 2.9 shows the relationship between the sintering times
(𝑡1, 𝑡2) and the particle radiuses(𝑟1, 𝑟2) of two particles. The integer constant 𝑚 ranges from 2 to
4 [24], [37].
(𝑟1
𝑟2)
𝑚
=𝑡2
𝑡1 (2.9)
From this relationship, it is clearly shown that the smaller particle has faster sintering time which
allows a reduction in sintering temperature and pressure. Overall, the pressure applied and the
curvature induced from the size of the particle affect the total driving force of the sintering process.
The pressure applied and the curvature contribution to the driving force is plotted in Figure 2.7.
As presented in this plot, with the smaller grain sizes less than 20-30 nm, the curvature brings the
23
most contribution to the total driving force. The pressure applied maintains the same factor of
contribution within the same range of the grain size. However, for the larger sizes of grains the
pressure applied contributes more to the driving force of the sintering.
Figure 2.7 Driving forces for nanopowder consolidation as a function of grain size [38], [39].
2.4 Nanopowder Sintering Challenges
The term “nanopowder” often refers to the powders with diameters ranging from 1 to 100 nm.
When compared to the conventional materials, a nanopowder has significantly larger surface area
to volume ratio, also meaning that the surface energy is larger. Consequently, the sintering
temperature can be lowered as explained in Section 2.3.1. However, this physical characteristic of
the nanopowders may cause problems. In this section two challenges are addressed – one from the
pre-sintering state and another after the sintering.
24
The agglomeration and the aggregation of the nanopowder brings one of the challenges in the pre-
sintering state of nanopowder. The agglomeration and aggregation are the phenomenon where the
powders are gathered to form a colony of powders without external forces applied and behave as
one large polycrystalline powder. Due to the fine powder sizes and the large surface to volume
ratio, the agglomeration and the aggregation commonly occur in the nanopowder paste. The major
difference between the agglomeration and the aggregation is the bonding strength. The
agglomeration is the state where the powders are weakly bonded by the Van der Waals force or
the electrostatic force. The aggregation is formed by the strong bond such as the metallic bonding
or covalent bonding. Therefore, to disperse the aggregated powder, an application of an external
energy is required. Both the agglomeration and the aggregation result in the inhomogeneous
powder distribution. Even before the sintering, the green density is lowered due to the
agglomerated and aggregated powders. The effective radius is used to characterize the
agglomerated and aggregated powders. The agglomerated and aggregated powder colonies behave
as the large sized powder with the effective radius. When the effective radius exceeds the nano-
scale range, the advantages of the nanopowder is no longer applicable. Figure 2.8 illustrates
descriptions of (a) the agglomerated powders and (b) the aggregated powders with the effective
radius [40]–[42].
25
Figure 2.8 (a) Agglomeration and (b) Aggregation of the nanopowders [22], [42].
The non-densifying diffusion at the low temperature is another challenge that the nanopowder
sintering possesses. The volume (lattice) diffusion from the grain boundary or the dislocations in
the neck region can lead to the densification. Figure 2.9 illustrates the results from the densification
diffusion and the non-densification diffusion. The length between two particle centers (L) differs
with the densification diffusion, but with the non-densification diffusion the length is not affected.
Figure 2.9 Sintering results from densification and non-densification diffusion.
26
The non-densification diffusion and densification diffusion are controlled mainly by the
temperature as shown in Figure 2.10. Within the relatively lower temperature range, the surface
diffusion is dominant thus not triggering the densification. Hence, using the nanopowder with the
higher surface area to volume ratio and processing at a lower temperature allow the sintering
process susceptible to the problem of non-densification. To reduce the effect of the non-
densification diffusion, the heating can be accelerated to bypass the low temperature region so that
the surface diffusion is not sufficiently performed during the short time interval. Techniques such
as the microwave sintering, the plasma activated sintering, the laser sintering and the field activated
sintering technique (FAST) are used for the rapid heating [43]–[47].
Figure 2.10 Changes in densification rate in temperature.
2.5 Direct Bonded Copper (DBC)
The direct bonded copper (DBC) substrate has the sandwiched structure of copper layers at the
bottom and on top and a ceramic layer in between for the electrical insulation. Alumina (Al2O3)
and aluminum nitride (AlN) are the most commonly used materials as the insulating ceramic layer.
27
To withstand a higher operation temperature of electronics and the heat cycling in a harsh
environment such as a large temperature difference between the maximum and minimum operating
temperatures, the module needs a higher thermal dissipation rate. Thus, characteristics such as a
higher thermal conductivity, a low coefficient of thermal expansion (CTE) on each layer and small
differences between CTEs of layers are desired for the module’s performances with better
reliabilities. The DBC is an excellent candidate satisfying these characteristics. ..
As the name DBC refers to, the copper layers are directly bonded to the surface of the ceramic
layer. The ceramic layer is generated first and the copper layers are bonded onto the ceramic layer.
During the bonding process, transition layers are created between the copper and the ceramic to
bond the layers via oxide bridge, the eutectic liquid from the Cu-O system [9], [48]. Figure 2.11
shows the partial binary phase diagram of the Cu-O system. The eutectic temperature is 1065 °C,
and the eutectic liquid forms at the temperature slightly above the eutectic temperature but below
1083 °C which is the melting temperature of copper, to allow the ceramic to bond. This technique
is not only used with Al2O3/Cu but also with AlN/Cu and Si3N4/Cu laminates [49].
Figure 2.11 Schematics for the eutectic bonding copper process and the Cu-O phase diagram [49].
28
The thermal resistance at the interface is considered important since the reaction phase Cu2O or
CuAlO2 is formed as isolated particles. The thermal conductivity of Cu2O and CuAlO2 is 6 W/mK
and 11 W/mK respectively. Compared to the thermal conductivity of Cu and Al2O3, 400 W/mK
and 24 W/mK, the reaction phases behave more likely as resistors and may interfere the current
path along the copper layers. Two critical issues are imposed in the DBC using the eutectic bonding.
First, the large void is formed at the interface. The typical size of the void is larger than 100 µm,
and sometimes void sizes of larger than 500 µm are also found. An image of a void is presented in
Figure 2.12. The void formation is due to the gas generated at the temperature range between 1065
°C and 1075 °C. Upon cooling below 1065 °C, the formed gases are pushed to the ceramic interface
and they are trapped at the interface as the solidification process ends. The voids are possibly the
weak points of failure, so they also impact the long term reliability of the DBC. The differences
between the thermal expansion coefficients of the ceramic and the copper is closely looked into as
the presence of stresses threatens the long term reliability [48], [49].
Figure 2.12 Cu/Al2O3 void formed at the interface [49].
29
2.6 Chapter Summary
In this chapter, the theoretical background and literatures of the sintering mechanisms and process
variables are reviewed discussed extensively. Also, the DBC manufacturing techniques and
possible failure factors are described.
The sintering uses powders to generate a bulk solid without melting. The solid state diffusion is
the driving force of the sintering. The key advantage of the sintering process is the lower process
temperature than the melting temperature of the solid. The sintering temperature, pressure, time,
the heating rate and the powder diameter are the controllable factors affecting the sintering process.
The sintering is further classified into types depending on the phase of powder and the solvent; the
polycrystalline solid state sintering, the amorphous solid state sintering and the liquid state
sintering.
The sintering process has three stages, initial, intermediate and final. At the initial stage, the
powders form a contact and a void/pore volume is created as the surface diffusion is dominant. At
the intermediate stage, the pore rounding and the volume diffusion is presented as the neck grows
among particles, which results in the pore size reduction. The final stage is where the densification
and the shrinkage are observed. The pores are closed and isolated with the gas trapped in. A
vacuum condition is needed to remove the gases completely and to reduce the porosity further.
The diffusion mechanisms are discussed and classified into the surface diffusion, the volume
(lattice) diffusion, the grain boundary diffusion, the dislocation diffusion and the viscous diffusion.
The surface diffusion is the main driving force at the beginning of the sintering and the volume
diffusion, the grain boundary diffusion and the dislocation diffusion play more roles during the
30
later stages. The densification is only governed by the grain boundary diffusion and the dislocation
diffusion.
Challenges of the sintering are also still encountered especially for the nano-scale powders. The
agglomeration and aggregation of the powder compact before the sintering affect the performance
of the sintering and the final porosity. Because the surface energy of the nanopowder is relatively
large, powders tend to form a weak bonds (agglomeration) or strong bonds (aggregation) such as
metallic bonds. By generating colonies of powder, the effective radius of the colonies tends to
behave as those of the large sized particles, and the nanopowder properties are lost. Another
challenge is the non-densification of the powder. Especially at the lower sintering temperature, the
surface diffusion is dominant rather than the grain boundary diffusion or the dislocation diffusion,
which results in the densification. As it has no chance to densify, the pores remain within the final
solidified structure. To resolve this problem and promote the densification at lower temperatures,
new sintering techniques such as the plasma activated sintering, the laser sintering and other rapid
heating sintering techniques are adopted.
The DBC manufacturing techniques and possible failure factors are described. The eutectic
bonding is used to fabricate the DBC using direct bonding between copper layer and the ceramic
layer. Typical ceramics such as Al2O3 and AlN which forms the oxygen bridge to the copper easily
are used. The thermal conductivity is the key factor considered during the selection of the ceramic
material. A drawback of the DBC is the void formation at the interface of Cu and ceramic layers,
as these voids affect the long term reliability of the DBC.
31
Chapter 3
Experimental Results and Discussion
In this chapter, the experimental results are presented and discussed. The contents are organized
as follow: Section 3.1 outlines the experiment design, Section 3.2 provides the analysis on the
silver nanopowder paste, Section 3.3 discusses the optimization for the sintering process, Section
3.4 examines the reliability of the die attachment, and finally Section 3.5 summarizes the chapter.
3.1 Experiment Design
The purpose of this experiment is to develop a solid bond between the silicon IGBT die and the
DBC substrate via pressureless silver nanopowder sintering.
3.1.1 Structure Design
The construction of the IGBT power module consists of two main parts, the IGBT die mounted
DBC substrate and the liquid cooled aluminum heat sink. Alumina (Al2O3) or aluminum nitride
(AlN) ceramic interlayered DBC is chosen due to its electrical isolation property and high thermal
resistivity. The overall structure is described in Figure 3.1. Liquid cooled aluminum plate with the
customized structure is brazed under the interface layers. The interface layer is indicated as the
added layer in Figure 3.1, which is the buffer layer to promote the better bonding between
aluminum cooling plate to copper. This layer is added because of the two reasons; first, to avoid
the brazing, and second, to try other bonding techniques. The ceramic layer in the DBC is too
brittle to experience brazing under high pressure and high temperature to build a direct bonding to
32
the aluminum cooling plate, so brazing is not preferred. Also, other bonding techniques than
brazing and soldering are tests by adopting the additional layers. The developments of the interface
layers and the liquid cooled aluminum plates are the proprietary technique of Dana Ltd. Further
researches on the development of the cooling module is conducted under the scope of Dana Ltd.
as well. A patterned DBC is bonded on the interface layers. The IGBT die is mounted on the DBC
via silver nanopowder sintering without applied pressure. The bondwires are used to connect the
IGBT die to the copper pads on the custom DBC design for the power module. Figure 3.1 illustrates
the perspective view of the IGBT power module. The IGBT is mounted on the patterned DBC with
a Ni/Au electroplated top layer via pressureless sintered silver nanopowder. The DBC in Figure
3.2 shows the single die test pattern that is the same as the one shown in Figure 3.3, also giving
the dimensions of the patterned DBC. The dimension of IGBT die used in the experiments is 5mm
5mm. The silver nanopowder paste is applied with a 2mm oversize to ensure proper coverage of
the IGBT die. The emitter, gate and collector pads are separated by a minimum distance of 2mm.
The IGBT and the pad is connected by using gold bondwires. In this study, only the IGBT die
bonded on DBC using pressureless silver nanopowder sintering is analyzed.
33
Figure 3.1 Cross sectional view of the proposed IGBT attachment on a DBC substrate using silver
nanopowder sintered. The DBC is bonded to a liquid cooled aluminum heat sink.
Figure 3.2 Perspective view of the proposed IGBT attachment on a DBC substrate using silver
nanopowder sintered.
34
Figure 3.3 The DBC design and the actual patterned DBC for a single IGBT die.
Half-bridge and full-bridge IGBT modules are also designed as shown in Figure 3.4. These DBCs
are used for the electrical and thermal analysis of the IGBT module and for the assembled
prototype. Experiment results in this study are limited to the single die DBC substrate.
Figure 3.4 The DBC design for a Half-bridge IGBT (with free wheeling diodes) module assembly.
3.1.2 Sample Preparation
The starting materials from Rogers Co. are 5.5 inches × 7.5 inches blank DBC. They are cut and
patterned according to the design illustrated in Figures 3.3 and 3.4. A variety of thicknesses and
35
ceramic materials are considered in this study. Thermal conductivity is the one of important
properties to be considered in order to prevent the heat build-up and to facilitate efficient heat
transfer. The coefficient of linear thermal expansion (CTE) of the ceramic must be as close as
possible to the CTE of the copper to minimize the mechanical stress applied to the
copper/ceramic/copper sandwiched DBC structure. The CTEs of the pure wrought copper and the
cast copper are in the range from 16 to 18 ppm/K [50]. As shown in Figure 3.5 (a), if the difference
between the CTE of Cu and the CTE of the ceramic is within the acceptable range where the
ceramic A can hold the stress within its elastic deformation range, thus no failure would occur.
However, near the interface the ceramic experiences tensile stress where the CTE of the metal is
larger than the CTE of the ceramic. At the opposite side away from the interface, the ceramic
experiences compression stress. Ceramics have excellent resistances to compression stress but are
less robust to the tensile stress in general. During the thermal expansion, the sandwiched structure
experiences both tensile and compression stresses, but the elastic deformation limit of the ceramic
is easily exceeded at where the tensile stress is applied. It causes the plastic deformation and the
breakage of the ceramic. Thus, it is important that the ceramic undergoes less tensile stress during
the thermal expansion, and that the two layers of metals on the top and at the bottom of the ceramic
are bonded to the ceramic. By bonding the top and bottom metal layers, the ceramic no longer
endures tensile stress on one side, but the stresses applied from both interfaces annihilate the
extreme tensile stress within the ceramic layer. This allows the DBC structure with better long-
term reliability. Figure 3.5 (b) illustrates the failure of DBC due to the breakage of the ceramic
layer. Failure of sandwiched structure increases the thermal resistivity rapidly, and decreases the
ability to dissipate heat.
36
(a) CTE Cu ≥ CTE Ceramic A
(b) CTE Cu ≫ CTE Ceramic B
Figure 3.5 CTE differences in two layers produces interlayer stresses, leading to potential failure.
Sample data including composition, thermal conductivity, CTE and thicknesses for the selected
DBC substrates are listed in Table 3.1.
Table 3.1 DBC Sample Data at 20 °C [51]
Ceramic
Substrate
Composition Thermal
conductivity
CTE Ceramic
thickness
Cu
thickness
Al2O3 Alumina 24 W/mK 6.8 ppm/K 0.32 mm 0.3 mm
0.64 mm 0.3 mm
HPS Alumina + 9%
ZrO2 doped
26 W/mK 7.1 ppm/K 0.32 mm 0.3 mm
AlN Aluminum Nitride 170 W/mK 4.7 ppm/K 0.64 mm 0.3 mm
The DBC substrates have two layers of copper; one at the top and the other at the bottom. The top
copper layer is customized in design and patterned, while the bottom copper layer is not processed
further during the preparation.
37
3.1.2.1 Thermal Budget
The silver nanopowder paste is capable of maximum sintering temperature up to 260 ˚C. The
recommended typical sintering temperatures are 250 ˚C on a silver coated DBC surface and 200
˚C on a gold coated DBC surface. Therefore, considering the maximum operation temperature of
175 ˚C for silicon (Si) based IGBTs, the gold coated DBC is selected in order to maintain low
thermal stress applied to the IGBT throughout the sintering process [6], [8], [52].
3.1.2.2 Nickel/Gold Plating
Once patterning is completed on the top copper layer, Ni and Au layers are then plated. Au is
chosen due to its good wettability, high electrical and thermal conductivity, and immunity to
oxidation. However, since Cu and Au interdiffuse easily, Ni is embedded to form a barrier layer
in between [53]. It is common to have electroless nickel and immersion gold (ENIG) for the
commercial printed circuit board (PCB) and DBC substrates. However due to the availability of
the laboratory equipment, electrolyzed Ni and Au coating is performed.
Watts Ni electroplating is applied and followed by the Au electroplating [54]. Ni and Au plating
set up is shown in Figure 3.6.
38
Figure 3.6 Ni and Au electroplating wet bench set up.
In preparation for the electroplating, the sample is cleaned to eliminate sources of contaminations,
including organic contaminants and native oxides. SP Cleaner manufactured by Caswell Canada
is used as a degreaser removing organic contaminants. 60 g/L of the SP Cleaner powder is
dissolved into 1 L of deionized water (DIW). The degreaser solution is then heated up to the
temperature ranging from 70 °C to 90 °C. The sample is fully dipped in the cleaner solution for a
time period of between 10 and 15 minutes depending on its condition. Once the organic
contaminants are all removed, the degreaser solution is rinsed off using DIW. In order to remove
native oxides, 10 vol% hydrochloric acid (HCl) solution at room temperature is used to etch the
copper surface for 2 minutes. DIW is used to rinse off the remaining HCl solution before the
sample surface is blow-dried using pressurized air. Upon completion of the cleaning processes, the
sample is ready for the electroplating.
The electroplating circuit is set up as illustrated in Figure 3.7 (a). The DBC is connected at the
cathode, and the Watts Ni in the titanium bath is connected at the anode. Watts Ni solution contains
39
300 g/L nickel sulfate, 45 g/L nickel chloride, 45 g/L boric acid, 0.5 g/L sodium saccharin, 0.2 g/L
sodium dodecyl sulfate in 1 L of deionized water (DIW) [54]. The temperature and pH of the bath
solution is maintained at 55 °C and 4.0 respectively. The pH is controlled by adjusting the amount
of 50 vol% sulfuric acid solution added to the bath prior to the Ni plating.
(a) Electroplating circuit (b) Anode
Figure 3.7(a) Electroplating circuit for Ni [55] and (b) Anode.
Pulse Current (PC) is supplied to the Ni bath to yield finer grain depositions. High PC density
results in a higher nucleation rate on the substrate with low porosity [54], [56]. In order to achieve
the minimum 3.5 µm of Ni thickness on the surface of the top copper layer, a peak current of 0.15
A is applied for 4 minutes, whereas the peak current is the amplitude of the current pulse. The
plating speed is maintained at around 1 µm/min·cm2 on both sides of the sample. Due to the nature
of PC, the current is supplied periodically. The time when the current flows is called ON time and
when the current does not flow is called OFF time. The average current is the level of energy
equivalent to the direct current (DC) level, and it is determined by the duty cycle and peak current
as in Equation 3.1. The duty cycle of the power supply is calculated as the ratio of the time during
40
which the power supply is ON against the total time as in Equation 3.2 [57]. The peak current of
0.75 A is applied at the duty cycle of 20 % with 20 ms of ON time and 100 ms of OFF time. The
maximum voltage is set to 40 V.
Average current = peak current × duty cycle (3.1)
Duty Cyle (%) = 𝑂𝑁 𝑡𝑖𝑚𝑒
𝑂𝑁 𝑡𝑖𝑚𝑒+𝑂𝐹𝐹 𝑡𝑖𝑚𝑒× 100 (3.2)
Upon completion of the Ni electroplating, the remaining bath solution is rinsed off using DIW.
The Au electroplating is prepared using a stainless-steel tip as an anode connected to the DC power
supply. The stainless-steel tip is wrapped-around by a cotton ball, and then soaked in the Au
solution in order to ensure continuous contact between the tip and the solution. The Ni plated DBC
is connected to the DC power supply as a cathode. 3 V DC is supplied.
The three images in Figure 3.8 show the surface finishes after each step of electroplating. Figure
3.8 (a) is the bare DBC after cleaning and native oxide etching. Figure 3.8 (b) shows the surface
of the DBC after the Ni electroplating. Shown in Figure 3.8 (c) is the Au plated DBC with the
sintered silver bonding layer and IGBT mounted on. Also, the Au wire is connected from the gate
and emitter terminals of IGBT to the gate and emitter pad, respectively.
(a) Bare DBC (Cu) (b) Ni (c) Au
Figure 3.8 DBC substrate samples in conditions of (a) bare (Cu on top), (b) Ni on top, and (c) Au
on top with the IGBT die mounted on pressureless sintered silver nanopowder with the Au wire
connections.
Ag
Au
IGBT Au wire
41
3.1.2.3 Silver Bonding Layer Process
The IGBT modules is bonded on the surface of the DBC via the silver bonding layer process using
silver nanopowder paste in order to allow electrical conductivity. The overall silver bonding layer
process flow is illustrated in Figure 3.9. The IGBT die is placed and mounted on the surface of the
DBC where the silver nanopowder paste is evenly printed. The sample with the IGBT die mounted
is then placed in the oven for drying and sintering.
Figure 3.9 Process flow for the IGBT die attachment process using silver nanopowder sintering.
3.1.2.4 Silver Nanopowder Pasting
The silver nanopowder paste, Loctite Ablestik SSP 2020-EN from Henkel is spread evenly on the
DBC substrate using the stencil printing technique as illustrated in Figure 3.10. First, the laser cut
stainless steel stencil of the thickness of 80 µm is placed on the surface of the Ni and Au
electroplated DBC, the silver nanopowder paste is applied on the stencil, and a stainless-steel
42
squeegee is used to press the silver paste for stencil printing as shown in Figure 3.10 (a) and (b).
Once the silver nanopowder paste is evenly spread, the stainless-steel stencil is removed, and the
IGBT die is mounted on the silver paste layer as illustrated in Figure 3.10 (c) and (d). Upon placing
the IGBT die on the sample surface, no pressure is applied, while the die and the silver nanopowder
paste layer are handled with care.
(a) Step 1: Stainless steel stencil placement on DBC (b) Step 2: Screen printing
(c) Step 3: paste spreading and removal of stencil (d) Step 4: die placement
Figure 3.10 Illustration of the silver nanopowder paste stencil printing technique.
3.1.2.4.1 Silver Nanopowder Drying and Sintering
After the IGBT die is placed and mounted on the sample surface, the DBC sample is placed in the
oven for drying and sintering processes. The sample is dried in the oven at 120 ˚C for 10 minutes
to ensure the complete removal of polymers such as solvent and additives.
43
After the drying process, the oven temperature is increased up to 200 ˚C for the sintering process.
The temperature is maintained at 200 ˚C for 60 minutes. Then, the sample is allowed to cool down
to the room temperature inside the oven. The temperature profile is depicted in Figure 3.11.
Ramping rates of 5 °C/min and 1 °C/min are used for heating up and cooling down, respectively.
The forced convection constant temperature drying oven DKN402 from Yamato is used. The
temperature distribution accuracy is ±2.5°C at 210°C with the oven in fully closed condition [58].
With no inert gas used, drying and sintering processes are executed with no pressure or vacuum
applied.
Figure 3.11 Temperature profile for drying and sintering processes.
3.2 Silver Nanopowder Paste Analysis
To analyze thermal responses during sintering processes, the silver nanopowder paste is examined
before and after sintering process, using analysis techniques of Differential Scanning Calorimeter
44
(DSC) / Thermogravimetry Analysis (TGA) / Simultaneous Thermal Analysis (STA) and
Scanning Electron Microscopy(SEM).
3.2.1 DSC/TGA/STA
The silver nanopowder paste is analyzed using Differential Scanning Calorimeter (DSC) /
Thermogravimetry Analysis (TGA) / Simultaneous Thermal Analysis (STA) to examine its
characteristic changes along the temperature profile during the sintering process. The analysis is
performed in Walter Curlook Materials Characterization & Processing Laboratory at the
University of Toronto.
The DSC measures the difference in heat required to raise the certain amount of the temperature
of the sample. By capturing the heat generation in cases of exothermic behavior or consumption
in cases of endothermic behaviour, the phase change of the sample is detected. An aluminum
reference sample is prepared in a N2 environment. The result data is extracted from the comparison
between the data measured using the silver nanopowder paste sample and the reference data.
The TGA records the mass changes of the sample along the temperature profile. The ramp rate
applied during the analysis is 5 ˚C/min from 30 ˚C to 300 ˚C. The silver nanopowder paste sample
of total mass of 55.56 mg is used for the analysis.
The results from the DSC/TGA/STA analysis are plotted in Figure 3.12 and Figure 3.13.
In Figure 3.12, it is observed that the endothermic reaction starts at 114 ˚C, indicating that the
liquids additives and solvent evaporate into gaseous forms. At 133 ˚C, the silver nanopowder paste
sample reaches its maximum in mass change, and the phase transformation is completed. These
results show that the evaporation of the polymer in liquids phases, such as additives and solvent is
45
accomplished at the temperature above 133 ˚C. The results also indicate that the additives and
solvent compose 10.9 % of the total sample mass. The plot shows an exothermic reaction starting
at 200 ˚C, therefore indicating that the silver nanopowder begins to diffuse and sinter at this
temperature. The exothermic reaction reaches its peak at 260 ˚C. The exothermic peak indicates
the sample experiencing the phase change from liquid to gas.
Figure 3.12 DSC/TGA/STA results for the silver nanopowder sintering experiment.
For further analysis of the sintering at 200˚C, an isothermal DSC/TGA/STA is performed for 60
minutes as shown in Figure 3.13. During the isothermal analysis, another exothermic reaction is
identified approximately 90 minutes from the beginning of the analysis and approximately 40
minutes after the beginning of the isothermal sintering process. A decrease in the sample mass by
0.2% is also detected.
46
Figure 3.13 Isothermal DSC/TGA/STA results for silver nanopowder sintering at 200 ˚C.
The overall DSC/TGA/STA results are summarized in Table 3.2. The onset temperature is
determined at the point where the mass change starts. While TGA measures the mass change of
the sample, Derivative Thermogravimetry (DTG) is computed as the percentage mass change. The
peak temperature is identified at the point where DTG is at minimum. The TGA result indicates
that the silver nanopowder paste sample is composed of 89 % silver nanopowder and 11 %
additives and solvent. Also based on the DSC and STA results the drying temperature above the
onset temperature of 114.4 ˚C and the sintering temperature above 200 ˚C are determined as
optimum temperatures for the processes. In addition, the results indicate that the sintering time
longer than 40 minutes is necessary for achieving stable sintered bonds among silver nanopowders.
47
Table 3.2 Summary of the DSC/TGA/STA Results
Onset T (˚C) Peak T (˚C) Δm (mg) Δm (%)
1. Solvent Evaporation 114.4 132.6 6.06 10.91
2. Heat up - 266.9 0.26 0.47
3. Isothermal - - 0.11 0.20
Total mass loss (300 ˚C) 6.43 11.58
Total mass loss (200 ˚C, 60 min) 6.17 11.11
3.2.2 SEM
Scanning Electron Microscope (SEM) images are analyzed to verify the results of the silver
nanopowder paste sintering. The instruments used are FEI Quanta FEG250 Environment SEM
(ESEM) and Hitachi S-5200 high resolution SEM (HRSEM) at the Centre for Nanostructure
Imaging in the Department of Chemistry, University of Toronto [59].
A cross sectional sample is prepared in order to analyze the bonding of layers with the sintered
silver nanopowder. Figure 3.14 is captured under the FEI Quanta FEG250 ESEM and shows the
cross section of the sample layers across the DBC, sintered silver nanopowder and IGBT. The
bonding of the sintered silver nanopowder layer with the Ni and Au coated Cu layer of the DBC
at the bottom, and with the IGBT at the top are visually inspected, and the cross sectional image
shows that the sintered silver nanopowder layer is firmly bonded to both the IGBT layer at the top
and the Ni and Au plated Cu layer of the DBC at the bottom.
48
Figure 3.14 Cross section image of the silver nanopowder sample sintered at 200 ˚C for 60
minutes, captured using Quanta FEG250 ESEM (magnification 500 X).
The chemical composition of the intermetallic layers between the sintered silver and the IGBT
bottom layer is examined using an Energy Dispersive Spectroscopy (EDS) analysis. The bottom
layers of IGBT die are identified as Ti and Ni. Figure 3. (a) is the image of the intermetallic layers
between the sintered silver and the IGBT bottom. Figure 3. (b) illustrates the four spots where the
EDS analysis is examined; EDS spot 1 at the deeper area of the sintered silver, EDS spot 2 near
the surface of the sintered silver layer, EDS spot 3 at the first intermetallic layer of the bottom of
the IGBT, and EDS spot 4 at the second intermetallic layer of the bottom of IGBT.
(a) (b)
Figure 3.15 (a) Cross-sectional view of sintered silver (magnification 10k X), (b) IGBT die and
sintered silver intermetallic layers EDS analysis on selected areas.
49
The weight percentage and atomic percentage of each EDS spot are analyzed and summarized in
Table 3.3 and Table 3.4 respectively. Carbon (C), oxygen (O) and aluminum (Al) are detected as
contaminants. Since the sample is processed and analyzed in the atmospheric surrounding, the C
and O contaminations are unavoidable. The Al contamination is introduced during the sample
preparation. During the sample preparation several grinding and polishing steps are required. Since
the alumina is used as an abrasive during the polishing step, excessive alumina abrasive remaining
on the sample surface is detected as a contaminant during the EDS analysis even after several
cleanings.
Table 3.3 Weight Percentage at EDS Spots within the Intermetallic Layers
EDS Spot 1 EDS Spot 2 EDS Spot 3 EDS Spot 4
C 6.02 6.28 8.04 7.58
O 2.71 9.22 5.54 8.15
Ni - - 71.33 7.53
Al 0.50 1.75 1.92 3.32
Si 0.26 2.39 2.39 18.19
Ag 90.52 80.35 10.78 1.14
Ti 0.50 - - 54.09
Ag Ag Ni+Ag Ti+Si
Table 3.4 Atomic Percentage at EDS Spots within the Intermetallic Layers
EDS Spot 1 EDS Spot 2 EDS Spot 3 EDS Spot 4
C 32.59 26.21 26.93 19.84
O 11.02 28.91 13.91 16.03
Ni - - 48.85 4.03
Al 1.20 3.25 2.86 3.87
Si 0.60 4.27 3.42 20.37
Ag 54.60 37.35 4.02 0.33
Ti - - - 35.51
Ag Ag Ni Ti+Si
50
The weight percentage and atomic percentage analyses indicates the EDS spots 1 and 2 within the
sintered silver layer, the EDS spot 3 consisting of Ni with some Ag diffused in, and EDS spot 4
within the Ti layer with some Si and Ni presence as well. The Si is detected at the spot 4 since the
Si layer is on top of the Ti layer. Consequently, the result verifies the formation of an intermetallic
layer by Ni and Ag therefore constructing a bond between the sintered silver layer and the IGBT.
The porosity of the sintered silver layer is examined on the image obtained from the cross-sectional
view of the sample. Figure 3. (a) is the SEM image for the porosity analysis, and the result of the
analysis as well. The porosity is measured by contrast comparison between the pores and the grains
on the image. The areas of dark regions and bright regions are considered as pores and sintered
silver grains respectively. Using the ImageJ software, the number of pixels within each dark and
bright regions is counted, and the result is plotted as shown in Figure 3. (b) [60]. The final porosity
of 29.6% is computed for the sample from the pressureless silver nanopowder sintering process.
Figure 3.16 (a) SEM cross-sectional image (magnification 5k X) for the porosity analysis and (b)
the porosity calculation result.
51
The SEM images of top views of the sample are examined under the Hitachi S-5200 HRSEM. The
shapes and sizes of the nanopowders are identified.
In order to determine the physical characteristics of the nanopowder during the heating-up process
before the sintering, samples are prepared by stopping the heating-up process at temperatures 120
°C, 130 °C, 150 °C and 170 °C and maintaining the temperature for 1 minute before proceeding to
the cooling-down step immediately. Figure 3.17 shows the points 1 to 5 indicating the different
temperatures at which the samples are taken during the heating-up process and after 1-minute
interval proceeded to the cooling-down. The temperature ranges starting from 120 °C is chosen as
the onset temperature of 114.4 °C is the minimum temperature at which the solvent and other
polymer mixtures are evaporated. Presence of remaining polymers from one sample analysis may
cause the cross contamination to another sample analysis, leading to the inaccurate analysis results
and an equipment failure due to debris causing a vacuum pump malfunctioning. Furthermore, since
the Hitachi S-5200 is a high-resolution SEM, the chamber is in high vacuum and thus no polymer
can be inserted into the chamber.
25
120
200
0
50
100
150
200
250
0 50 100 150 200 250
Tem
pera
ture
(ºC
)
Time (min)
120°C, 1min
RT
Te
mpe
ratu
re (
°C)
Time (min)
130°C, 1min
150°C, 1min
170°C, 1min
190°C, 1min
Figure 3.17 Sample preparations for the top view SEM image analysis along the temperature
profile.
52
(a) Process stopped at 120°C for 1 min
(b) Process stopped at 130°C for 1 min
(c) Process stopped at 150 °C for 1 min (d) Process stopped at 170 °C for 1 min
Figure 3.18 Top view SEM images of the silver nanopowder paste samples with heating-up
process stopped at different temperatures before sintering. All images are in
magnification of 8k X.
Top view SEM images (a)-(d) in Figure 3. are obtained at different temperatures. These SEM
images show that two different average powder sizes are present mixed in the paste. The samples
cooled down from 120 °C for 1 minute and from 130 °C for 1 minute are considered to be of the
same physical conditions of the original as-received silver nanopowder paste prior to the heating-
up, except that the solvent polymers are evaporated after heating-up. Thus, the analysis result
defines that the as-received silver nanopowder paste consists of powders of two different average
53
sizes, micro-meter scale powders and nano-meter scale powders. The mixture of micro-meter scale
and nano-meter scale powder has advantages of forming less agglomerated powders colony. As
the sample is not pressured during the sintering process, it is important to keep the nano-scale
powders uniformly mixed into the paste without making agglomerates or aggregates, since no
external energy is applied to break the bonds. However, agglomerated or aggregated powders are
found in all samples prepared below the sintering temperature which are indicated with the red
circles with in the images in Figure 3.. The physical characteristics of the silver nanopowder pastes
during the sintering is also analyzed using top view SEM images. The same method used above
for the sample analyses during the heating-up process is used. The sintering of the silver
nanopowder paste samples at 200 °C is stopped after different time intervals and started cooling-
down.
In Figure 3. the top view SEM images (a)-(d) are captured under the Hitachi S-5200 HRSEM. No
visible difference in porosities is observed among the samples of different sintering times. Also,
the size distribution of the micro-meter sized and nano-meter sized powders is maintained even
with the sintering process completed at 200 °C for 60 minutes.
54
(a) Process stopped at 200 °C for 1 min (b) Process stopped at 200 °C for 20 min
(c) Process stopped at 200 °C for 40 min (d) Process stopped at 200 °C for 60 min
Figure 3.19 Top view SEM images of the silver nanopowder paste samples sintered at 200 ˚C (a)
for 1 min, (b) for 20 min, (c) for 40 min and (d) for 60 min (magnification 10k X).
The overall average size of the silver nanopowder paste is measured at 3.2 µm, with a maximum
powder size at 22 µm and a minimum powder size at 58 nm. The grain size of the micro-meter
sized powder is measured using the images in Figure 3.. The average grain size is measured to be
142 nm (31 counts) with the minimum grain size of 78 nm and the maximum grain size of 602 nm.
The standard deviation is computed as 125.2 nm.
55
(a)
(b)
Figure 3.20 Images of grain sizes observed on the micro-sized powder after sintering at 200 °C
for 60 minutes, (a) magnification 80k X and (b) magnification 200k X.
In Figure 3. below, comparison is made between the top view SEM image (a) of the as-received
silver nanopowder paste and the image (b) of the sintered silver nanopowder paste under the same
magnification. Also, the images (c) of the nanopowder sintered for 10 minutes and (d) of the
nanopowder sintered for 60 minutes are compared to each other. From these comparisons, as a
result, no visible changes in porosity is observed during the sintering process, while the micro-
sized powders and nano-sized powders are observed maintaining their average sizes. The image in
Figure 3. (c), the circled powders show that the neck growth between the two spherical-shaped
powders occurs. In (d), the isolated pores are circled. Also, the non-densification is defined on the
sintered sample after completion of the process at 200 °C for 60 minutes. It is resulted due to the
low sintering temperature and the slow temperature raise rate. Thus, more surface diffusion is
promoted during sintering and not high enough temperature is applied to the powder sample to
lead the grain boundary and dislocation diffusion for the densification and reduction in porosity.
To achieve the more densification of the sample, faster temperature raising rate can be used by
trying other sintering techniques such as plasma sintering, laser sintering or field activated
sintering technique as mentioned in the previous chapter.
56
(a) Process stopped at 120 °C for 1 min,
(b) Process stopped at 200 °C for 60 min,
(c) Process stopped at 200°C for 10 min
(d) Process stopped at 200°C for 60 min
Figure 3.21 Top view SEM images of (a) the dried as-received silver nanopowder paste after the
heating stopped and maintained at 120 °C for 1 minute (magnification 5k X), (b) the
silver nanopowder paste sintered at 200°C for 60 minutes (magnification 5k X), (c)
silver nanopowder paste sintered at 200°C for 10 minutes (magnification 20k X), and
(d) silver nanopowder paste sintered at 200°C for 60 minutes (magnification 20k X).
3.3 Chapter Summary
In chapter 3, experimental results and following discussions are presented.
A DBC substrate is prepared to fabricate the sample. Before mounting the IGBT and examine the
silver paste sintering, Ni and Au electroplated layers is coated on bare Cu layer of DBC. The Au
57
is chosen to reduce the sintering temperature of silver nanopowder paste. Also, Au has good
electrical conductivity and immunity to the oxidation. Ni is used as the buffer layer between Cu
and Au to block the interdiffusions of Cu and Au atoms. When Cu diffuses into the Au layer and
is exposed to air, it oxides easily. Thus, the advantage of the Au layer being passive to the oxidation
is diminished. The sintering process is classified into four different steps: silver nanopowder paste
printing, IGBT die pick and mounting, silver nanopowder drying, and sintering. Stencil printing is
used to evenly spread the paste on DBC substrate. The drying and the sintering temperature and
time is optimized based on the results of DSC/TGA/STA, which is the analysis techniques to find
the phase changes and the mass changes of the sample along the temperature profile. The drying
of the sample is performed at the temperature of 120 °C for 10 minutes since the polymer solvent
evaporation occurs in the temperature range from 114 °C to 130 °C. From the result of isothermal
DSC/TGA/STA, sintering temperature of 200°C and sintering time of 60 minutes are defined as
optimized sintering condition. The sintering results are confirmed and inspected by capturing
images from the SEM analysis. SEM image analysis of cross sectioned sample is performed to
ensure the bonding via sintered silver nanopowder layers in between DBC and IGBT. EDS
analysis confirms the chemical compositions of the sintered silver nanopowder layer. The samples
to inspect the top surfaces is prepared for HRSEM analysis. The powder sizes are measured based
on the SEM images. Sizes of the powders are defined; the average size of 3.2 µm, the maximum
size of 22µm and the minimum size of 58nm. Also, the mixture of micro-scale and nano-scale
powder is defined. 29% porosity of the sintered silver nanopowder is counted using software. In
the as-received sample and pre-sintering samples, agglomerated or aggregated nano-meter scale
powders are found. After the sintering process, the sample has the non-densification problem.
58
Chapter 4
Conclusions and Future Work Plan
In this chapter, the conclusion from the literature review and the experimental result data are
discussed, and a future work plan is suggested for further improvements to the research and
development of the pressureless silver nanopowder sintering for the liquid cooled insulated gate
bipolar transistor (IGBT) power module.
4.1 Conclusions
Pressureless silver nanopowder sintering is studied and proposed for the liquid cooled IGBT power
module development for electric vehicles (EVs) and hybrid electric vehicles (HEVs). The
literatures on the sintering mechanisms, sintering variables, challenges present within the
nanopowder sintering, and the properties of the direct bonded copper (DBC) are reviewed. From
the literature reviews, the viability of the application of the pressureless nanopowder sintering at
the low temperature is reviewed. The processing parameters and the sample structure design for
the sintering analysis are determined accordingly. To determine the optimum sintering conditions,
the silver nanopowder paste is analyzed using the Differential Scanning Calorimeter (DSC) /
Thermogravimetry Analysis (TGA) / Simultaneous Thermal Analysis (STA), and the
characteristics of the sintered silver layer is examined under the Scanning Electron Microscope
(SEM). The result data obtained from the DSC / TGS / STA verify the thermal behaviours of the
silver nanopowder paste, and are used to optimize the sintering temperature profile. The SEM
image analysis verifies the formation of the bonding between the IGBT and the DBC resulting
59
from the pressureless silver nanopowder sintering at the low temperature. The SEM image analysis
results are also examined to identify possible improvements of the pressureless silver nanopowder
sintering due to the observed porosity and the nanopowder agglomeration within the sintered silver
layer. Based on the literature reviews and the experimental result data, the feasibility of the
pressureless silver nanopowder sintering application for the bonding between the IGBT and the
DBC within the liquid cooled power module, and possible improvements on the pressureless silver
nanopowder sintering are suggested.
The literatures on the sintering and the DBC are reviewed and used to determine the process
parameters and the feasibility of the pressureless silver nanopowder sintering. Since the minimum
particle size of the silver nanopowder paste and falls within the range where the effect of the
pressure on the sintering driving force is not significant, the feasibility of the pressureless sintering
is verified. The required sintering temperature is determined above the minimum silver sintering
temperature of 110 °C based on the particle size. The literatures on the sintering mechanisms are
reviewed. With the nano-meter scaled particles with the high surface area to volume ratio the
surface diffusion is determined as the dominant mechanism of the sintering at the low temperature,
therefore the high porosity remains throughout the sintering as no densification occurs. Due to the
high fabrication temperature above 1065 °C, the DBC is verified as a reliable substrate for the
power module. The alumina (Al2O3) and the aluminum nitride (AlN) are chosen as the ceramic
materials within the DBC based on the coefficient of thermal expansion (CTE) values from the
literature review.
The silver nanopowder paste is analyzed using the DSC/TGA/STA to examine the thermal
response and mass changes of the paste along the temperature profile. The evaporation of the
polymer solvent was identified at the temperature range from 114 °C to 130 °C. Therefore, the
60
drying temperature of 120 °C is selected. During the continuous temperature increase, a phase
change and a change in mass are detected at 260 °C. For the isothermal analysis, a phase change
and a change in mass are detected at 200 °C, 40 minutes after the isothermal process begins. Total
mass change of 11% is defined throughout the analysis, so 89% of the silver powder mass % is
verified. Consequently, the sintering temperature of 200 °C for the 60 minutes is selected and
applied to the pressureless silver nanopowder sintering.
From the SEM image analysis, the bonding between the IGBT and the DBC by the sintered silver
layer is inspected. The chemical composition of the layers between the sintered silver and the
bottom of the IGBT is analysed using the Energy Dispersive Spectroscopy (EDS). The
interdiffusions of the intermetallic layers indicates that the bonding is established between the
sintered silver layer and the IGBT. The porosity of 30 % within the final product is determined on
the cross-sectional image. On the top view images, the sizes of the nanopowder is measured with
the average size of 3.2 µm, the maximum size of 22 µm, and the minimum size of 58 nm. Two
different ranges of powder sizes are defined for the micro-meter sized and the nano-meter sized
powders. Also, the agglomerated or aggregated nanopowder colonies are detected within the
temperature range from 120 °C to 190 °C, and remain gathered as colonies throughout the sintering
process. These agglomerated or aggregated nanopowder colonies cause the non-densification of
the sintered silver nanopowder. Overall, the SEM image analysis validates the bonding capability
of the sintered silver nanopowder while also identifying aspects for possible improvements of the
pressureless silver nanopowder sintering.
In conclusion, the pressureless silver nanopowder sintering is capable to create the bonding layer
between the IGBT and DBC at the low sintering temperature. The literatures on the sintering and
the DBC are reviewed to verify the sintering parameters and controllable factors, and to identify
61
and avoid possible failures during the process. The DSC/TGA/STA identifies thermal behaviour
of the silver nanopowder paste and mass changes along the temperature profile. It allows the
optimization of the sintering temperature and time. The SEM analysis allows the visual inspection
of the nanopowder. The porosity of the sample and the sizes of the nanopowders are also
determined. The chemical composition analysis using the EDS confirms that the bonding is built
with the sintered silver nanopowder. Therefore, the experimental result data of the study verifies
that the pressureless silver nanopowder sintering at 200°C is a feasible technique for the bonding
within the liquid cooled IGBT power module. However, improvements can be made to minimize
possible impacts from the non-densification and the agglomerated / aggregated powder colonies.
The mechanical and electrical properties of the pressureless silver nanopowder sintering can also
be enhanced by rapid temperature raise sintering techniques.
4.2 Future Work Plan
Further study is planned for the development of the IGBT power module assembly with the
aluminum liquid cooling plate. The research on the application of the DBC bonding to the
aluminum liquid cooled plate will be executed under the coordination of Dana Ltd.
The thermal dissipation and power efficiency of the IGBT power module will be further analysed
to verify the performance of the module assembled with the aluminum liquid cooling plate. In
parallel, the heat dissipation from the IGBT to the liquid cooling plate through the sintered silver
layer and the DBC will be modeled using a software simulator in order to examine the thermal
paths within the power module and the liquid cooling plate.
The pressureless silver nanopowder sintering can be improved in aspects of the bonding quality
62
by minimizing the impact of the non-densification of the sintered product and the aggregation or
agglomeration of the nanopowder prior to the sintering.
63
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