PROBABILISTIC ASSESSMENT OF COMPONENT LEAD-TO-LEAD TIN WHISKER BRIDGING
Authors: S. McCormack and S. Meschter Company: BAE Systems Date Published: 5/22/2009
ICSR (Soldering and Reliability)
Abstract: As a result of a global movement away from using lead (Pb) in electronic assemblies, component manufacturers are increasingly applying Tin-rich finishes to the leads of their devices. Unfortunately, this can create a risk of Tin whisker formation that can result in electrical failures if the gap between two adjacent component leads is bridged by a whisker. If bridging occurs, the effects can range from no effect to parametric system deviations to catastrophic circuit failure. Tin whisker risk management typically involves an evaluation of Tin plating attributes, base material of the component leads, underplating, lead-gap spacing, conformal coating, system use environment, and system reliability requirements. As an integral part of this management, a computational framework capable of quantifying the risk associated with Tin whisker-bridging failures is needed so that appropriate design, maintenance, and warranty decisions can be made. In the present work, a probabilistic model based on measured whisker observations is constructed. The model uses Monte Carlo simulation methods and quantifies the risk of Tin whiskers bridging the gap between two adjacent component leads. A dual-tail constraint strategy for constructing the whisker length probability density functions was established and applied to long and short-term bright Tin whisker length measurements. A bridging risk map also was developed to illustrate the relationship between lead-gap spacing, years of service life, and bridging risk. It was found that forming non-dimensional lead-gap spacing parameters provided useful insight into the risk posed by Tin whiskers.