POLYMERIC SYSTEMS
GENERAL REQUIREMENTS
polymeric systems

GENERAL REQUIREMENTS

Polymeric systems provide mechanical and cushioning support to components, improved thermal profile/thermal sinking and tamper-resistant, environmental packaging.

Encapsulation is a process in which electronic subassemblies (i.e. power supplies, amplifiers, hybrid circuits, etc.) are embedded in a polymer (i.e. silicon, epoxy gel) to produce a unitized, sealed assembly.

Underfill is a process in which a polymer (i.e. epoxy) is injected under an electronic component to improve thermal coefficient (tc) match and extend solder joint fatigue life.

encapsulant underfill

PREFERRED

ENCAPSULANT

Material is fully cured, with a smooth, continuous surface extending over all embedded components, and exhibits filly, wetted, continuous contact fillets with protruding devices (wires cable, connector, etc.) and the enclosure wall. No bubbles, cavities, striation marks or spillage.

PREFERRED

UNDERFILL

Material exhibits complete and uniform flow under the comonent body. Peripheral fillets are smooth and uniform, with a concave profile. No bubbles, cavities or spillage.

bubbles bubbles

ACCEPTABLE

BUBBLES/CAVITIES

Minor surface bubbles or cavities that do not extend to underlying components or conductive surfaces, are isolated, or bridge between conductors are acceptable.

Best Workmanship Practice

UNACCEPTABLE

BUBBLES/CAVITIES

Bubbles or cavities that bridge conductors are unacceptable.

Best Workmanship Practice

NASA WORKMANSHIP STANDARDS
NASA logo Released:
04.05.2002
Revision:
Revision Date:
Book:
9
Section:
9.01
Page:
1




POLYMERIC SYSTEMS
GENERAL REQUIREMENTS (cont.)
encapsulant fillets missing fillets

ACCEPTABLE

ENCAPSULANT FILLETS

Assembly exhibits proper fill, with comlete fillets extending a minimum of two (2) largest wire diameters above the encapsulant surface.

Best Workmanship Practice

UNACCEPTABLE

MISSING FILLETS

Assembly does not exhibit required fillets extending two (2) largest wire diameters above the encapsulant surface.

Best Workmanship Practice

minor roughness in finish excessive peaks in finish

ACCEPTABLE

PROFILE/FINISH

A rough profile/finish is caused by gel set (pre-cure) of the material, before the material has had sufficient time to smoothly settle and flow. Minor roughness that does nto interfere with the form, fit, or function or the device is acceptable.

Best Workmanship Practice

UNACCEPTABLE

PROFILE/FINISH

Excessive peaks, crests or folds indicate that the material was worked beyond its pot life. Assemblies typically exhibit improper adhesion, entrapped voids, etc. and shall be rejected.

Best Workmanship Practice

minor spillage excessive fill and spillage

ACCEPTABLE

SPILLAGE

Minor spillage that does not interfere with the form, fit or function of the device is acceptable.

Best Workmanship Practice

UNACCEPTABLE

EXCESSIVE FILL/SPILLAGE

Excessive fill or spillage that interferes with the form, fit or function shall be rejectable.

Best Workmanship Practice

NASA WORKMANSHIP STANDARDS
NASA logo Released:
04.05.2002
Revision:
Revision Date:
Book:
9
Section:
9.01
Page:
2




POLYMERIC SYSTEMS
GENERAL REQUIREMENTS (cont.)
flow marks stringing

ACCEPTABLE

STRIATION/FLOW MARKS

Striation/flow marks are an indicator of material flow during fill/pour, and are acceptable, provided no other defects are evident and the material meets wetting, cure and hardness requirements.

Best Workmanship Practice

ACCEPTABLE

STRINGING

Minor stringing is acceptable, provided the deposit is completely adhered, does not contact exposed conductive circuits, component leads, jumpers or glass-bodied components, and is not subject to flexure.

Best Workmanship Practice

underfill fillets partial underfill

ACCEPTABLE

UNDERFILL FILLETS

Assembly exhibits proper fill, with complete positive fillets extending around the periphery of the device body.

Best Workmanship Practice

UNACCEPTABLE

INCOMPLETE/PARTIAL UNDERFILL

Incomplete or partial underfill can result in solder joint failure and incorrect/uneven thermal profile.

Best Workmanship Practice

exposed component pull away of the encapsulant

UNACCEPTABLE

EXPOSED CIRCUITRY/COMPONENTS

Assemblies exhibiting exposed circuitry, components, conductive surfaces shall be rejected.

Best Workmanship Practice

UNACCEPTABLE

PULL-AWAY/PARTING

Assemblies exhibiting pull-away of the encapsulant shall be rejected. Pull-away is caused by improper adhesion, improper wetting or excessive shrinkage.

Best Workmanship Practice

NASA WORKMANSHIP STANDARDS
NASA logo Released:
04.05.2002
Revision:
Revision Date:
Book:
9
Section:
9.01
Page:
3




POLYMERIC SYSTEMS
GENERAL REQUIREMENTS (cont.)
recessed fillets tackiness

UNACCEPTABLE

RECESSED FILLETS

Recessed (negative) fillets indicate improper wetting/non-wetting and shall be grounds for rejection.

Best Workmanship Practice

UNACCEPTABLE

TACKINESS

Tackiness indicates an improper/incomplete cure. Tackiness usually manifests intself in the generation of residual fingerprint reliefs (permanent marks) and/or depressions, following light touching of the surface.

Best Workmanship Practice

















NASA WORKMANSHIP STANDARDS
NASA logo Released:
04.05.2002
Revision:
Revision Date:
Book:
9
Section:
9.01
Page:
4





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