Chapter 15: Fundamentals of Sealing and Encapsulation

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Chapter 15 Fundamentals of Sealing and
Encapsulation

• Jason Shin
• Derek Lindberg

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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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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

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• Protection from Moisture (continued)
• Fricks 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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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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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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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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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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15.4.1 Encapsulation Requirements

• Mechanical Properties
• Good stress-strain Behavior
• An ideal encapsulant should exhibit
• gt1 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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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

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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

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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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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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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

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15.6.2 Liquid Encapsulation

• Viscosity controlled to meet fill requirements
• Three most common liquid encapsulation processes
• Cavity Filling
• Glob Topping
• Underfilling

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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

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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

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15.6.2 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
  gt.1mm3
• 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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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.