Fundamentals of Sealing and Encapsulation

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1 Fundamentals of Sealing and Encapsulation

2 Sealing and Encapsulation Encapsulation and sealing are two of the major protecting functions of IC packaging. They are used to protect IC devices from adverse environmental and mechanical effects

3 What Is Encapsulation The protection can be an organic overcoat, in which case it is called encapsulation as in figure(a) or inorganic as illustrated in Figure (b), in which case it is called sealing.

4 Conti.. Protection Techniques Isolation from environmental pollutants Mechanical protection Performance(Stability, Thermal dissipation)

5 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

6 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

7 Protection from Atmospheric Contaminants Corrosive gasses in the atmosphere can be harmful to electronic devices Nitrogen oxides Sulfur dioxide Causes acid rain

8 Mechanical Protection Both wire bond and flip chip devices have very fine interconnects 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

9 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 cm 3 /s.

10 Moisture Absorption of Encapsulants Moisture Effects on Plastic Packages Moisture acts as a debonding agent though a combination of: Moisture-reacted metal surface can form a weak 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

11 Moisture Effects on Plastic Packages (continued) Organic materials are not hermetic and allow moisture to penetrate and be absorbed. 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.

12 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 of ICs and their interconnect.

13 Adhesion Is Very Critical Good interfacial adhesion between polymers and packages is important Corrosion protection

14 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 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

16 10/22/07 Chapter 15: Encapsulation

17 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

18 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(tg measures phasetransition of the polymers)

19 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)

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

21 Encapsulation Processes Molding Majority of processes use transfer molding Hard to apply to flip chip and PGA packages

22 Liquid Encapsulation Three most common liquid encapsulation processes: Cavity Filling Glob Topping Underfilling

23 Cavity Fill Used mostly in prefabricated ceramic (usually) chip carriers After die attach and wire bonding the cavity is flooded with liquid encapsulant

24 Glob Top Simple alternative to cavity-filling No need for premade mold or cavity Often used for extra protection on manufactured PCBs

25 Underfilling Typically used in flip-chip assembly Liquid injected under chip to seal and strengthen the chip to board/substrate bond

26 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

27 Fused Metal Seals Typical for hermetic packages with volumes >.1mm^3 Can be welded, soldered, or brazed(zinc and copper) 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

28 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 Brazing Stronger, more corrosion resistant seal than solder Does not require flux Glass Sealing Been in use since the 1950s Used to create hermetic glass-to-metal seals between metal lid and metallized alumina chip carrier

29 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.