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Case Study: EasyFit Used by NASA to Better Assess the Tin Whisker Risk

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On April 17 2005, the Millstone nuclear generating plant in Connecticut shut down when a circuit board monitoring a steam pressure line short-circuited. "Tin whiskers" - microscopic growths of the metal from soldering points on a circuit board - were blamed for causing the problem. These whiskers are comprised of nearly pure tin, and are therefore electrically conductive. Tin whiskers have been found to form under a range of environments including: space, missile, airborne, shipboard, ground, medical implant, and office. The US military blamed them for malfunctioning F-15 radar systems and misguided Phoenix and Patriot missiles.

Tin Whiskers

Field failures attributable to tin whiskers have cost individual programs many millions of dollars each, and resulted in significant customer dissatisfaction. As a result, manufacturers of high-reliability systems are frequently forced to decide whether the use of tin poses an acceptable risk in a given application. Issues of cost, schedule and performance are then weighed against these risks.

Existing Monte Carlo risk simulations make the assumption that when a free tin whisker has bridged two adjacent exposed electrical conductors, the result is an electrical short circuit. This conservative assumption has been made because shorting is a random event that has a currently unknown probability associated with it. Due to contact resistance, electrical shorts may not occur at lower voltage levels.

EasyFit Used To Improve The Simulation Model

A group of scientists lead by Karim J. Courey, a Principal Engineer with the Orbiter Sustaining Engineering Office for the National Aeronautics and Space Administration (NASA), has studied the effect of varying voltage on the breakdown of the contact resistance which leads to a short circuit. This data was analyzed using EasyFit to estimate the probability of an electrical short as a function of voltage, given that a free tin whisker has bridged two adjacent exposed electrical conductors.

Specifically, EasyFit was used to fit more than 40 probability distributions to sample data and select the best fitting model based on the Kolmogorov-Smirnov goodness of fit test. As a result, the three-parameter Inverse Gaussian distribution was selected as the most adequate model to quantify the probability of occurrence of an electrical short circuit from tin whiskers as a function of voltage. This model can be used to improve existing Monte Carlo risk simulation models.