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.

Novel Whisker Mitigating Composite Conformal Coat Assessment

European Union RoHS compliant commercial lead-free electronics used in DoD electronics have an increased failure risk due to tin whisker short circuits unless properly mitigated by design.

An electronic assembly is created by integrating thousands of parts from multiple suppliers utilizing a host of circuit card manufacturing processes. With the lead elimination from electronics resulting from the European Union Reduction of Hazardous Substances (RoHS) legislation, many of the heritage aerospace and defense commercial off the shelf (COTS) solder materials with tin–lead have become obsolete. Most notably there has been an increasing cost and schedule pressure to use commercially available pure tin part finishes and lead–free solders in aerospace and defense electronic systems.

Since these materials have an increased tendency to grow tin whiskers that can cause short circuit failures and decrease reliability, original equipment manufacturers are relying on multiple mitigation methods to manage the tin whisker risk. One of the main mitigations is the use of conformal coating to 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 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 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) below. SERDP-0005

This Brief includes a Technical Support Package (TSP).
Novel Whisker Mitigating Composite Conformal Coat Assessment

(reference SERDP-0005) is currently available for download from the TSP library.

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