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Chapter 15: Fundamentals of Sealing and 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

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Chapter 15: Fundamentals of Sealing and Encapsulation

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

  2. 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 Chapter 15: Encapsulation

  3. 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 Chapter 15: Encapsulation

  4. Protection from Moisture (continued) • Frick’s Law of Diffusion • Equilibrium water constant Chapter 15: Encapsulation

  5. 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 Chapter 15: Encapsulation

  6. Protection from Atmospheric Contaminants • Corrosive gasses in the atmosphere can be harmful to electronic devices • Nitrogen oxides • Sulfur dioxide • Causes acid rain Chapter 15: Encapsulation

  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 Chapter 15: Encapsulation

  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. Chapter 15: Encapsulation

  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 Chapter 15: Encapsulation

  10. 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. Chapter 15: Encapsulation

  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. Chapter 15: Encapsulation

  12. 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 Chapter 15: Encapsulation

  13. 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. Chapter 15: Encapsulation

  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 Chapter 15: Encapsulation

  15. Chapter 15: Encapsulation

  16. 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 Chapter 15: Encapsulation

  17. Residual Stress • Shrinkage of resin • Thermomechanical loading due to mismatch of CTEs of constituent materials between cure temperature and storage temperature. Chapter 15: Encapsulation

  18. 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 Chapter 15: Encapsulation

  19. 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 Chapter 15: Encapsulation

  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. Chapter 15: Encapsulation

  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 Chapter 15: Encapsulation

  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 • 90-240 second cycles • Modern gang-pot molds are balanced • Cycle time as low as 15 seconds Chapter 15: Encapsulation

  23. 15.6.2 Liquid Encapsulation • Viscosity controlled to meet fill requirements • Three most common liquid encapsulation processes: • Cavity Filling • Glob Topping • Underfilling Chapter 15: Encapsulation

  24. 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 Chapter 15: Encapsulation

  25. 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 Chapter 15: Encapsulation

  26. 15.6.2 Underfilling • Typically used in flip-chip assembly • Liquid injected under chip to seal and strengthen the chip to board/substrate bond Chapter 15: Encapsulation

  27. 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 Chapter 15: Encapsulation

  28. 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 Chapter 15: Encapsulation

  29. 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 Chapter 15: Encapsulation

  30. Sealing Examples Chapter 15: Encapsulation

  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. Chapter 15: Encapsulation

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