Chapter 15: Fundamentals of Sealing and Encapsulation Jason Shin Derek Lindberg 10/22/07 Chapter 15: Encapsulation

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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation Protection from Moisture (continued) Frick’s Law of Diffusion Equilibrium water constant 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation Protection from Atmospheric Contaminants Corrosive gasses in the atmosphere can be harmful to electronic devices Nitrogen oxides Sulfur dioxide Causes acid rain 10/22/07 Chapter 15: Encapsulation

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 10/22/07 Chapter 15: Encapsulation

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. 10/22/07 Chapter 15: Encapsulation

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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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. 10/22/07 Chapter 15: Encapsulation

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. 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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. 10/22/07 Chapter 15: Encapsulation

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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation Residual Stress Shrinkage of resin Thermomechanical loading due to mismatch of CTEs of constituent materials between cure temperature and storage temperature. 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

15.5 Encapsulant Materials All encapsulants involve some form of polymerization and cross-linking reactions that enhance the mechanical properties of the packaging system. 10/22/07 Chapter 15: Encapsulation

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 10/22/07 Chapter 15: Encapsulation

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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 15.6.2 Liquid Encapsulation Viscosity controlled to meet fill requirements Three most common liquid encapsulation processes: Cavity Filling Glob Topping Underfilling 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 15.6.2 Underfilling Typically used in flip-chip assembly Liquid injected under chip to seal and strengthen the chip to board/substrate bond 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation 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 10/22/07 Chapter 15: Encapsulation

Chapter 15: Encapsulation Sealing Examples 10/22/07 Chapter 15: Encapsulation

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. 10/22/07 Chapter 15: Encapsulation