SERDP Tin Whisker Testing and Modeling: Simplified Whisker Risk Model Development
Authors: Stephen A. McKeown, Dr. Stephan J. Meschter, Polina Snugovsky, and Jeffery Kennedy Company: BAE Systems and Celestica Inc. Date Published: 1/31/2015
Abstract: Most commercial electronics manufacturers began a large-scale movement toward tin rich finishes and solders in 2006 due to European Union Reduction of Hazardous Substances (RoHS) legislation banning lead. Unfortunately, this can create an increased risk of tin whisker induced electrical failures, particularly for defense and aerospace equipment using commercial off the shelf (COTS) items. This paper presents a statistical tin whisker short circuit risk modeling framework for surface mount assemblies having various combinations of tin-lead and lead-free materials. While industry and academia have not developed a robust model correlating whisker length to environmental exposure, the framework does include the results of the multi-year SERDP testing program that is assessing tin whisker growth on lead-free manufactured assemblies in various environments. Since tin whisker length data is expected to mature over the next decade as more measurements are made in the field, a novel technique is employed to facilitate rapid recalculation of short circuit risk as new whisker growth characteristics become available. This is achieved by first determining the geometric leadto- lead spacing characteristics for various parts. The geometric modeling includes manufacturing variation not readily apparent from the drawings such as printed wiring board conductor spacing reductions due to etching and bulbous solder that decreased conductor-to-conductor spacing. The spacing distributions are then compared to the whisker growth length distribution to determine the probability of a bridging occurrence. Then, the short circuit probability is determined for a given circuit voltage by using NASA data. The computational framework is also used to evaluate the effectiveness of tin-lead hot solder dip and partial conformal coating whisker mitigations.
Tin Whiskers, Lead Free, Assembly, Testing, Short Circuit Risk Modeling, Statistics