1. Chapter 15: Fundamentals of Sealing and Encapsulation
Jason Shin
Derek Lindberg
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Chapter 15: Encapsulation

2. Chapter 15: Encapsulation
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
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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
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Protection from Moisture (continued)
Frick’s Law of Diffusion
Equilibrium water constant
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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
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Protection from Atmospheric Contaminants
Corrosive gasses in the atmosphere can be harmful to electronic devices
Nitrogen oxides
Sulfur dioxide
Causes acid rain
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7. 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
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8. 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.
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9. 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
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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.
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11. 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.
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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
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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.
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14. 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
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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
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Residual Stress
Shrinkage of resin
Thermomechanical loading due to mismatch of CTEs of constituent materials between cure temperature and storage temperature.
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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
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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
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20. 15.5 Encapsulant Materials
All encapsulants involve some form of polymerization and cross-linking reactions that enhance the mechanical properties of the packaging system.
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21. 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
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22. 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
second cycles
Modern gang-pot molds are balanced
Cycle time as low as 15 seconds
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Liquid Encapsulation
Viscosity controlled to meet fill requirements
Three most common liquid encapsulation processes:
Cavity Filling
Glob Topping
Underfilling
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Cavity Fill
Used mostly in prefabricated ceramic (usually) chip carriers
After die attach and wire bonding the cavity is flooded with liquid encapsulant
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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
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Underfilling
Typically used in flip-chip assembly
Liquid injected under chip to seal and strengthen the chip to board/substrate bond
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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
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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
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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
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Sealing Examples
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31. 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.
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