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Experimental Investigations of Whisker Formation on Tin Platings
Pecht, Michael G
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With the global transition to lead-free electronics, the electronic component market has seen an increase in the selection of pure tin and tin-rich alloys as lead-free component finishes. The adoption of tin-rich finishes has enhanced a reliability issue associated with the formation of electrically conductive whiskers, emanating from tin finished surface. A spontaneous growth of whisker may bridge adjacent conductors, leading to current leakage or electrical shorts. Whiskers tend to grow over many months. However, due to a lack of the factors accelerating whisker growth, prediction of whisker formation is extremely difficult. Therefore, the effective mitigation strategies are necessary, particularly for high-reliability applications, which require a long product operational life. The objective of this study is to investigate a method for characterizing whisker growth, which can further enable measuring the effectiveness of mitigation strategies. To achieve this objective, a set of experiments was conducted using matte and bright tin platings on copper, Alloy-42, and brass metal coupons. The plated coupons were subjected to high temperature exposures, including annealing (at 150ºC/one hour). Whisker growth on tin-plated samples was characterized using environmental scanning electron microscopy, in terms of the maximum whisker length, length distribution, and whisker density, at different time periods up to 24 months. The experimental results have shown different behaviors of whisker growth (length and density) between bright and matte tin, depending on the materials and exposure conditions. It was experimentally demonstrated that bright tin over brass could be considered a worst-case scenario for assessing the risks associated with tin whiskers. This work has further revealed that the current industry practice of testing for 3000 hours for monitoring the propensity of tin whiskers is insufficient to cover a saturation of whisker density and capture the temporal nature of whiskers. In order to overcome such insufficiencies, the use of time-based distribution data for whisker length and whisker density was proposed as an alternative method for characterizing whisker growth. With the application of this proposed method, the effect of annealing (150ºC/one hour) and its effect under the presence of electrical current were investigated for retarding whisker formation and growth on tin-plating.