10/22/07chapter 15: encapsulation chapter 15: fundamentals of sealing and encapsulation jason shin...

10/22/07 Chapter 15: Encapsulation

Chapter 15: Fundamentals of Sealing

and Encapsulation

Jason Shin

Derek Lindberg


15.1 What Is Encapsulation

• Protection Techniques– Typically low temperature polymers– Isolation from environmental pollutants– Mechanical protection– Performance

• Dimensional stability• Resistance to thermal excursions• Permeation (isolation of environmental pollutants)• Thermal dissipation


15.2.1 Chemical Protection

• Protection from Moisture– Major contributor to packaging failures– Rapid water desorption from polymeric

packaging during board assembly is a major cause of delamination

– Vapor pressure build-up within packages sometimes cracks the plastic cases

– Swelling of the encapsulants caused by moisture pickup is a major driving force of failures at the interconnection level


• Protection from Moisture (continued)– Frick’s Law of Diffusion

– Equilibrium water constant


• Protection from Salts– In the presents of salts, corrosion of the IC

metallization is accelerated– Operating voltages and materials used for

electrical performance may be sufficient to cause electrolytic corrosion

– Due to small line widths and micrometer or less pitch, small localized corrosion can produce major problems

• Protection from Biological Organisms– Insects can be attracted by the electric field

generated by an electronic device


• Protection from Atmospheric Contaminants

– Corrosive gasses in the atmosphere can be harmful to electronic devices• Nitrogen oxides

• Sulfur dioxide

• Causes acid rain


15.2.2 Mechanical Protection

• Both wirebond and flip chip devices have very fine interconnects

• Structural integrity provided by the interconnections is very minimal

• Protection achieved by:– Prevention of damage by encapsulation over

the IC– Minimization of strain in the solder joined by

underfill between IC and package substrate


15.3.1 Hermetic versus Non-Hermetic Sealing

• Compromise between cost and performance

• Inorganics are hermetic, organics are not

• Hermetic package is defined as one that prevents the diffusion of helium below a leak rate of 10-8 cm3/s.


13.2 Moisture Absorption of Encapsulants

• Moisture Effects on Plastic Packages– Moisture acts as a debonding agent though a

combination of:• Moisture-reacted metal surace can form a weak,

hydrated oxide surface• Moisture-assisted chemical bond breakdown• Moisture-related degradation or depolymerization

– Moisture diffusion rate depends on the material, as well as its thickness and the diffusion time


• Moisture Effects on Plastic Packages (continued)– Organic materials are not hermetic and allow

moisture to penetrate and be absorbed.– Improvements in plastic packaging materials

and processes have lead to reliability that approaches hermetic packages

– The word hermetic is defined as completely sealed by fusion, solder and so on, so as to keep air, moisture or gas from getting in or out.


15.3.3 Organics Came a Long Way

• Inadequate adhesion, contaminants within the material itself, incompatible thermal expansion, and stress-related problems all combined for early problems

• Now 90% of ICs are marketed in this form

• Better filler technology resulted in materials that do not impart stress-related failures.


• Adhesion Is Very Critical– Good interfacial adhesion between polymers

and packages is important– This adhesion is between metallic-organic

interfaces is facilitated by a combination of mechanical interlocking and chemical and physical bonding.

– Corrosion protection and adhesion properties are closely linked


• Accelerated Testing Helps to Select Right Material– The means by which non-hermetic packaging

is assessed during screening.– Temperature cycling is the most common

thermomechanical environmental test.


15.4.1 Encapsulation Requirements

• Mechanical Properties– Good stress-strain Behavior– An ideal encapsulant should exhibit

• >1% elongation at break• A tensile modulus of 5-8 GPa• Minimum shift in properties at temperatures close

to Tg


• Thermomechanical Considerations– Coefficients of thermal expansion– Ideally the CTE of a molding compond should

be as close to Si as possible– Also the CTE of an underfill should be as

close to the solder bump as possible


• Residual Stress– Shrinkage of resin– Thermomechanical loading due to mismatch

of CTEs of constituent materials between cure temperature and storage temperature.


15.4.1 Thermal Properties

• Coefficient of Thermal Expansion (CTE)– Requirements for CTE vary significantly with

the type of encapsulants in need

• Glass Transition Temperature (Tg)– The temperature at which the transition from

solid to liquid takes place

• Flow During Encapsulation– Flow characteristics of the molten compound

within the mold during the molding operation


15.4.3 Physical Properties

• Adhesion– Measure of the strength between two interfaces– Robust encapsulation system provides strong

adhesion to the device encapsulate interfaces such that the mechanical integrity of the package can be preserved under thermal stress

• Interfaces– Any physical or chemical layer (in atomic scale

between two materials)– first line of defense against adhesion failure


15.5 Encapsulant Materials

• All encapsulants involve some form of polymerization and cross-linking reactions that enhance the mechanical properties of the packaging system.


15.6.1 Encapsulation Processes

• Molding– Majority of processes use “transfer molding”

• Simple and mass producible • Molten material injected into mold cavity with IC at its center.• Held under pressure until compound cures• Hard to apply to flip chip and PGA packages


15.6.1 Molding Complications

• Early (70s & 80s) molds suffered from unbalanced EMC injection

• Different molds filled at different rates causing– “wire sweep”

– Variation in void sizes and quantity– Variation in size

• 90-240 second cycles• Modern gang-pot molds are balanced

– Cycle time as low as 15 seconds


15.6.2 Liquid Encapsulation

• Viscosity controlled to meet fill requirements• Three most common liquid encapsulation

processes:– Cavity Filling– Glob Topping– Underfilling


15.6.2 Cavity Fill

• Used mostly in prefabricated ceramic (usually) chip carriers

• After die attach and wire bonding the cavity is flooded with liquid encapsulant


15.6.2 Glob Top

• Simple alternative to cavity-filling• No need for premade mold or cavity• Dams may not be necessary based on application• Often used for extra protection on manufactured PCBs


15.6.2 Underfilling

• Typically used in flip-chip assembly• Liquid injected under chip to seal and strengthen

the chip to board/substrate bond


15.7 Hermetic Sealing

• The goal of sealing is to maintain the electronic package in an inert environment

• Several processes are used– Fused Metal Sealing– Soldering– Brazing– Welding– Glass Sealing


Fused Metal Seals

• Typical for hermetic packages with volumes >.1mm^3• Can be welded, soldered, or brazed• Welding is the most popular due to high yield, large

throughput, and reliability• Soldering and brazing are typically used if the metal lid

must be removed again later• Glass seals can also be used for reliable protection


Techniques

• Soldering– Solders are selected by temperature, strength, and cost– Melting temperature must be below that of the solder or brazing

process used to attach pins to the substrate– Must be above the temperature used to attach the part to a PCB

• Brazing– Stronger, more corrosion resistant seal than solder– Does not require flux– Usually tack-welded to a gold-plated Kovar (Co, Ni, Fe alloy) lid

• Glass Sealing– Been in use since the 1950s– Used to create hermetic glass-to-metal seals between metal lid

and metallized alumina chip carrier


Sealing Examples


Summary and Future Trends

• Early attempts at non-hermetic packages suffered from a number of problems, including: encapsulant contamination, poor moisture resistance, incompatible thermal expansion, stress-related problems.

• Low-cost polymeric plastic packaging has been dominant since the 1980s

• Use of polymeric packages is only expected to increase.

10/22/07chapter 15: encapsulation chapter 15: fundamentals of sealing and encapsulation jason shin...

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