Aerospace & Defense Technology - December 2021 - 32

Tech Briefs
prevent tin whisker electrical shorting.
Current widely used low-cost coating
spray processes used for humidity protection
generally provide incomplete
coverage and insufficient coating thickness
for whisker mitigation, or no coating
at all on many tin surfaces. The
most challenging areas are the backside
of the leads, vertical surfaces and edges.
There are vapor deposited coatings that
provide complete coverage, but have yet
to demonstrate complete whisker mitigation,
are difficult to repair, and have a
high manufacturing cost.
There continues to be increasing risk
1 cm
(A)
to DoD systems reliability and affordability
due to the growing gap between
consumer electronics designs and DoD
electronics needs. The DoD benefits
greatly by using consumer parts directly or using them with
modification. Environmental compliance, cost, and miniaturization
continually drive consumer designs, reducing unnecessary
design margins, as long as the relatively short consumer
warrantees are satisfied. As a result, significant
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32
Intro
Cov
ToC
+
-
(B)
Whisker growth on an electronic assembly after environmental exposure; (A) SEM image of a whisker (see
red arrow) between leads of a copper alloy lead 64 pin quad flat pack after 4,000 hours at 85 °C/85 %RH,
100x, and (B) an overall isometric optical image of the quad flat pack test board.
associated DoD risk continues to accumulate and evolve
with lead-free, conversion to greener chemistry, and other
design changes accompanying each generation of consumer
electronic parts and materials.
The main objectives of the project were to (1) develop and
evaluate nanoparticle-filled conformal coatings designed to
provide long-term whisker penetration resistance and coverage
on tin-rich metal surfaces prone to whisker growth in
modern DoD electronic assemblies containing commercial
lead-free consumer electronics, (2) utilize enhanced liquid
coating application processes to improve coverage, (3) perform
fundamental studies into the mechanisms by which
conformal coatings provide tin whisker penetration resistance
and inhibit nucleation/growth, and (4) evaluate coating
reworkability. An objective added later was to evaluate
the impact of long-term simulated power cycling thermal cycling
on coating whisker mitigation.
Desired coating qualities that will exhibit superior whisker
mitigation include complete coverage, high strength to
buckle whiskers and/or high elongation prior to breaking to
capture whiskers within the coatings. The key elements being
evaluated were (1) coating and particle chemistry, (2) coating
microstructure and mechanical properties enhancement, (3)
coverage quality, (4) layered coating application, (5) coating
rupture resistance on whisker test samples, and (6) reworkability.
Coating microstructural evaluation, nanoindentation,
tensile, and adhesion testing were used to characterize the
coatings and select candidates for lead-free electronic assembly
coating and testing. Finite element modeling was
used in conjunction with the experimental observations
and the measured coating properties to determine when
coatings can be expected to rupture from tin whisker or tin
nodule growth.
This work was done by Dr. Stephan Meschter of BAE Systemsfor the
Defense Strategic Environmental Research and Development Program.
For more information, download the Technical Support Package
(free white paper) at www.aerodefensetech.com/tsp under
the Manufacturing & Materials category. SERDP-0005
Aerospace & Defense Technology, December 2021
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Aerospace & Defense Technology - December 2021

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