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Likelihood of Metal Vapor Arc by Tin Whiskers
Keywords: Tin whisker, Metal vapor arc, Electrical shorting
Tin whiskers are hair-like conductive structures which grow spontaneously from tin and tin alloy surfaces and have been associated with mechanical and electrical reliability issues. The most common concern associated with tin whiskers is unintended electrical shorting due to a whisker bridging adjacent conductive surfaces. If sufficient electric current and voltage are available, a bridging tin whisker can initiate a destructive metal vapor arc that is capable of carrying hundreds of amps. Previous studies have demonstrated the vapor arc by tin whisker and addressed the some aspects of the requirement for arc formation; however, a practical method for assessing the likelihood of tin whisker induced vapor arc formation has not been provided. To characterize the likelihood of vapor arc by tin whiskers, a test circuit and test vehicles with tin whiskers having various geometries were subjected to the tests under various voltages and pressures. Under low pressure (74 torr) with 50V, arcs were initiated from the tin whiskers and sustained until the tin-plated copper electrodes of the test specimen were melted down. The initiated arcs under atmospheric pressure with 50V were extinguished quickly with relatively little damage to the tin-plated electrodes due to their being quenched by the surrounding gases. Experimental results show that the electrical resistance of the test specimen including the tin whisker appears to be the strongest factor in determining the likelihood of vapor arc formation. Based on the experimental evidence, a metric which combines a bias voltage and resistance was proposed to predict the likelihood of a vapor arc formation by tin whiskers. Test data indicates that arc formation is highly probable above a threshold value of the proposed metric. As a result, the arc formation metric may be used as a guide for circuit designs that minimize the vapor arc propensity via tin whiskers.
Sungwon Han, Graduate Student
University of Maryland - CALCE
College Park, MD

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