Molded Electronic Package Warpage Predictive Modelling Methodologies

Ong, Kang Eu

Wei Keat Loh

Intel Technology SdnBhd

Malaysia

wei.keat.loh@intel.com

kang.eu.ong@intel.com

Chih Chung Hsu

Jenn An Wang

CoreTech System (Moldex3D)

Taiwan

jimhsu@moldex3d.com

jennanwang@moldex3d.com

Ron W. Kulterman

Flex Ltd, Austin Tx.

USA

ron.kulterman@flex.com

Haley Fu

iNEMI

Shanghai

haley.fu@inemi.org

iNEMI: Warpage Characteristics of Organic Packages, Phase 4

iNEMI Technology Roadmap 2013

•Identify package warpage as key challenge

Phase 1 (2011-13)

•Literature survey

•Establish metrology correlation between sites and increase awareness

•Reaching out to industry for component donation

•(Forming & Storming)

Phase 2 (2013-2014)

•Establish current technology package dynamic warpage (POP, PBGA, FCBGA)

•Measurement protocol (effect of “As Is”, “Bake” and Moisture Exposure Time (MET)) and sample size needed.

•(Norming)

Phase 3 (2015-2016)

•Continue establishing technology package dynamic warpage (all kind)

•Dynamic warpage measurement metrology assessment.

•(Performing)

Phase 4 (2016-2017)

•Continue establishing technology package dynamic warpage of new package technology (all kinds)

•Leverage previous effort to study the impact of Low Temperature Solder on dynamic warpage requirement

•Collaborate with simulation software to derive modeling approach in better predict the dynamic warpage

Project Scopes in Phase 4 SOW

• Characterize emerging electronic packaging technology dynamic warpage behavior to develop a better understanding of the current development of package construction and material development as listed in Table I.

• Silicon Interposer and Embedded Silicon Bridge with different package stiffeners and constructions for Heterogeneous Packaging Solution

• Next generation of POP packages that leverages wafer level process for package construction which include Panel and Wafer level molding. These also include the use of new fiber reinforced mold material for warpage control.

• System In Package/Multi Chip Package (BGA) with different configuration and layout.

• Embedded Package (embedded silicon, actives and passives)

Characterization of latest packaging technology

• To assess the impact of lower temperature solder on package warpage for those packages collected in Phase 2 and 3.

• To establish the risk level based on package technology with respect to warpage only.

Assess the impact of Low Temperature Solder (LTS) on package dynamic warpage requirement

• Establish modeling optimization approach and tools requirement to enable higher accuracy prediction technique

• Compare modeling with experiment data and identify potential gaps for further development in FEA technique

Assessment the use FEA in optimizing package warpage

3

Content

• Introduction

• Implementation details

• Results and discussions

• Conclusions

4

Disclaimer: The work here covers some feasibility study which collaborated

with iNEMI and we are not endorsing any software in particular.

Introduction: Multi-physics in Assembly Process

Incoming Materials

•Package Design –manufacturing variation

•Substrate warpage variation

•Die Warpage

•Raw materials (different material characteristics)

Electronic Package Assembly

•Process Steps

•Temperature exposure

•Time effect (Visco and creep)

•Mechanical loading

SMT Assembly

•Reflow Temperature/Transient Thermal exposure and distribution

•Mechanical interaction

•Board level SJR

•Yield

Chip Attach

•Mass Reflow

•TCB

•Non-Conductive Paste or Film

Encapsulation & Molding

•Underfill

•Injection or Compression Molding

•Molded Underfill

Lid Attach (if applicable)

•Mechanical loading

•Adhesive

•Thermal Interface Material

Ball Attach

•Mechanical loading

•Reflow temperature

Test and Final Inspection

•Mechanical loading

•Coplanarity/Warpage

Predicting electronic package warpage and managing dynamic warpage still remain as a heavy

lifting effort and difficult for industrial.

A practical warpage prediction model is critical to drive more advance package design and risk

management.

Introduction: Current Modeling Approach is it Adequate?

Create FEA Geometry,

Meshing and apply constraint

boundaries condition

and include incoming variations

Apply linear and visco-elasticity

material constitutive

properties and include different

processing steps of how materials are

added to the construction of the

package

Compute the model from the initial stress free

condition

How to account for the individual

residual stresses of the components during assembly

Post processing to extract predicted package warpage

for risk assessment with account for process, material

and incoming variation.

6

• There are a lot of work done in different scope and assumption and with different level of

success.

• Focus here include the study of the impact of

• visco-elastic constitutive property

• with time-temperature superposition (TTS) were included with a two steps shift

factor model of Arrhenius and WLF functions.

• include the chemical shrinkage (Pressure Volume Temperature Cure PVTC)

implementation. PVTC function is not widely included in general FEA tools. Some

researches found out that the pressure-volume-temperature-cure (PVTC) can

influence the amount of warpage of the package.

Bi-material model: Modeling implementation approach

• An arbitrary bi-material model evaluation was conducted to compare the material

model of linear elastic, visco-elastic, time-temperature superposition shift factor and

chemical shrinkage.

• Commercially available modeling software considered here were based on the

Moldex3D R17 and a general FEA tool called FEA-A.

• Stress analysis was computed for post mold cure induced shrinkage and followed by

time dependent thermal exposure loading to capture the evolution of warpage or

deformation.

• PVTC model is engaged to approximate the dynamic shrinkage based on transient

molding condition to predict the package behavior.

7

Materials Mold Silicon

Modulus (MPa) 18190 1172800

CTE (ppm)14.5 (T < 443 K)

53.6 (T≧443 K)2.3

Bi-material model geometry Bi-material mesh part Bi-material material properties

Mesh

1000um global size

2 elements across thickness

Total 1600 elements

Stress-strain constitutive properties

• Stress-strain constitutive properties coupled with non-linear

function of time, temperature and other field dependent variables

like cure conversion rate.

8

=

)--C( CureThermal =

)UU(2

1 T+=

TCTEhermal =T

( )( )

( )( ) klijjkiljlikijkl

tEtEC

211

)(

12

)(

−+++

+==C

Stress strain relationship

Stress equilibrium equation Stiffness tensor

Thermal induced strain

Visco-elastic constitutive behavior

• The visco-elastic properties of epoxy molding compound are in the

form of Prony series with time-temperature shift factor.

• A Prony series (also known as Generalized Maxwell model) can be

expressed as follows:

6.0E+08

2.1E+10

4.1E+10

6.1E+10

8.1E+10

1.0E+11

1.2E+11

25 50 75 100 125 150 175 200

Sh

ear M

od

uls

u

(dyn

e/c

m2)

Temperature (℃)

t = 0.1 s

t = 1 s

t = 10 s

6.0E+08

2.1E+10

4.1E+10

6.1E+10

8.1E+10

1.0E+11

1.2E+11

1.0E-10 1.0E-02 1.0E+06 1.0E+14 1.0E+22

Sh

ear M

od

uls

u (

dyn

e/c

m2)

Time (s)

T = 30 ℃

T = 60 ℃

T = 150 ℃

))(

exp(GG(t)0

1aT

tG

i

i

n

i

−+= =

))(

exp(KKK(t)0

1aT

t

i

i

n

i

−+= =

)(Taa T=Shift factorShear modulus

Bulk modulus

Visco-elastic TTS

• There are two models to describe the time-temperature

superposition (TTS):

– Arrhenius type equation

– WLF equation

• Most FEA tools can only considered one kind of TTS model.

Subroutine needed if TTS is impacted by Tref .

• Tref was 423 K (150C) in this case.

10

1.0E-10

1.0E-06

1.0E-02

1.0E+02

1.0E+06

1.0E+10

25 50 75 100 125 150 175 200

Sh

ift

Facto

r a

T

Temperature (℃)

Moldex3D

WLF

User Subroutine

( )referenceT

TTC

TTCa TT log

02

01 −+

−−=WLF equation

ref

0

T TT ))T

1-

T

1(

R

H(

=TaArrhenius type equation

C1=17.44, C2=51.6

∆HT: chemistry activation energy

R : gas constant

Curing Kinetics Mold Material Properties

11

• The combined model to investigate the curing kinetics of the given

mold because of its ability to accurately predict the experimental

data. The combined model can be expressed as follows:

( )( )nmkkdt

d

−+= 121

−=

RT

EAk 1

11 exp

−=

RT

EAk 2

22 exp

Conversion rate

A1, A2, E1, E2, m, n are model parameters

Modeling for chemical shrinkage without curing percentage effect

12

Pseudo-CTE shrinkage approach

Equivalent curing shrinkage CTE

Effective curing shrinkage CTE as function of Tg and

Filler %

Tan, Lin, et al. "Study of viscoelastic effect of EMC on FBGA block warpage by FEA simulation." 2013

14th International Conference on Electronic Packaging Technology. IEEE, 2013.

Modeling for chemical shrinkage with curing percentage effect

13

Mixing law based on CTEraw vs CTErubbery stage

PVTC - Pressure Volume Temperature Cure

approaches

Nawab, Yasir, et al. "Determination and modelling of the cure shrinkage of epoxy vinylester resin and

associated composites by considering thermal gradients." Composites Science and Technology 73

(2012): 81-87.

Hong, Li-Ching, and Sheng-Jye Hwang. "Study of warpage due to PVTC relation of EMC in IC

packaging." IEEE Transactions on Components and Packaging Technologies 27.2 (2004): 291-295.

Two domain modified Tait model

Chemical Shrinkage through PVTC with curing percentage effect

• The two domain modified Tait model is used to formulate the

specified volume of resin as below (Moldex3D implementation):

( )( )

0894.0,

if

if ,

,exp

exp

if

if ,

,

1ln1

1)1(

11

5

65

43

43

21

21

0

0/

=−=

+=

−

−=

+

+=

+−=

+−=

CbTT

PbbT

TT

TT

Tbb

TbbB

TT

TT

Tbb

TbbV

B

pCVV

VVV

t

t

t

LL

SS

t

t

LL

SS

uncuredcured

cureduncured

Conversion rate

Boundary conditions & steps

• Transient Coupled Temp-Displacement Analysis

– 2 steps; 100s each

– Convective heat transfer with 20 W/m2/K coefficient

– 5 faces (top horizontal + 4 vertical faces)

– Initial temp. 448 K / 175 ℃– Step 1 ambient temp. 298 K / 25℃– Step 2 ambient temp. 533 K / 260 ℃

Ambient temperature Temperature on chip top center node

Bi-material: Linear elastic and visco-elastic effects – no chemical shrinkage considered

16

Z displacement on top center node

• At lower temperature, the model deformed into concave shape because of the higher

contraction of the mold.

• For visco-elastic material model without the TTS, the z-displacement reduced significantly

compared to linear elastic.

• The difference is due to the modulus relaxation and stress relaxation which counters the

thermal strain.

-150

-100

-50

0

50

100

150

200

250

0 50 100 150 200

Z d

isp

lac

em

en

t (μ

m)

Time (s)

Moldex3D linear elastic

FEA-A linear elastic

Moldex3D VE-no TTS

FEA-A VE-no TTS

Visco-elastic effect

Visco elastic

(VE)

Linear elastic

Bi-material: Visco-elastic with/without TTS – no chemical shrinkage considered

17

• The concave warpage increases to -80um while convex warpage

reduces to 19.32um.

• As the temperature reduce, the modulus increases and hence the

warpage magnitude increases towards the linear elastic case.

Z displacement on top center nodeVisco-elastic with TTS

-100

-80

-60

-40

-20

0

20

40

60

80

0 50 100 150 200

Z d

isp

lac

em

en

t (μ

m)

Time (s)

Moldex3D VE-no TTS

FEA-A VE-no TTS

Moldex3D VE-TTS

FEA-A VE-TTS Visco elastic

(VE) with TTS

Visco elastic

(VE) without

TTS

Bi-material: Effects of PVTC based chemical shrinkage

18

Z displacement on top center node

• Including chemical shrinkage (1% or 2%) increase the bi-material z displacement during

the cool down.

• During the first 100 second, no significant difference in the z-displacement due to the

conversion rate only increased by 1%.

• The heating cycle speeds up the chemical reactions to reach the 100% conversion rate.

Temperature and conversion

-140

-120

-100

-80

-60

-40

-20

0

20

40

0 50 100 150 200

Z d

isp

lacem

en

t (μ

m)

Time (s)

VE-TTS

VE-TTS with 2% chemicalshrinkage

VE-TTS with 1% chemicalshrinkage

70

80

90

100

110

270

320

370

420

470

520

570

0 50 100 150 200

Co

nvers

ion

(%

)

Tem

peratu

re (K

)

Time (s)

Temperature (K)

Conversion %

Conclusions

1. The use of bi-material system in this paper demonstrates the impact of

transient thermal structural analysis coupled with the effect of the visco-

elastic, time-temperature superposition (TTS) and the pressure-volume-

temperature-cure (PVTC) material model implementation.

2. The structural and thermal solutions are consistent or closely match

between a general FEA and Moldex3D for both linear and non-linear

elastic cases.

3. The visco-elastic implementation on the bi-material model captures the

reduction of z-displacement compared to linear model.

4. The TTS principle which interpolated the material properties based on

time and temperature field must be taken into consideration for the

actual assembly process history.

5. PVTC should not be neglected as the predicted value can be highly

influenced by conversion percentage of the mold.

6. The logical step for this industrial project is to validate the effect seen on

the visco-elastic behavior and the chemical shrinkage impact and

evaluate all these different approaches

19

0.515

0.520

0.525

0.530

0.535

0.540

0.545

0.550

0.555

25 50 75 100 125 150 175 200 225 250 275 300

Sp

ecif

ic v

olu

me

(cm

3/g

)

Temperature (∘C)

Cured state, 0.1MPa

Uncured state (1.0%), 0.1MPa

Uncured state (2.0%), 0.1MPa

△V (2.0%), 260oC

△V (1.0%),

260oC

Two additional chemical shrinkage

values of 1% and 2%