Evidence of electric-field-accelerated growth of tin whiskers
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esearch Letters
Evidence of electric-field-accelerated growth of tin whiskers Anthony C. Vasko, Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA Corey R. Grice, Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA Andrew D. Kostic, Aerospace Corporation, 14745 Lee Road, Chantilly, VA 20151, USA Victor G. Karpov, Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA Address all correspondence to Victor G. Karpov at [email protected] (Received 13 July 2015; accepted 3 September 2015)
Abstract This paper presents experimental results supporting a theory that electric fields accelerate the growth of tin (Sn) whiskers. We have rapidly (within one week) grown long (∼100 µm) and dense (∼300 mm−2) whiskers on thin Sn films. Humidity and an applied electric field are found to have a strong effect when applied together, making whiskers orders of magnitude longer and denser. An evidence of explosive whisker development is presented.
Metal whiskers are hair-like protrusions growing on surfaces of some metals, including tin (Sn), Zn, and Cd. Tin whiskers are especially significant because they grow on plated films of electronic components and surfaces of soldering alloys. This can cause serious reliability problems in electronic systems.[1–7] In spite of almost 70 years of research, the physics behind metal whiskers remains a mystery. It is important that environmental regulations of restriction of hazardous substances (RoHS) adopted by the European Union in 2002, restricts the use of whisker suppressing lead in Sn solders. As the RoHS regulations are adopted across various industries, Sn whiskers are becoming a new, potentially hazardous reliability issue for electrical components. In the breadth of its impact, that issue can be similar to or even exceed the Y2K computer software flaw from the late 1990s.[8] Increasing concerns about vulnerability of electronic components calls for urgent responses in understanding and mitigating Sn whiskers. A recently proposed theory[9,10] points at the possibility of electric-field-induced effects attributing whisker growth to random electric fields in the near-surface regions of imperfect metals, which leads to some quantitative predictions. Here we describe our experimental results focused on the electric field effect and consistent with the theoretical predictions. So far, observations of the electric bias effect on whisker growth have remained scarce and inconclusive. While rapid whisker growth caused by electric bias in Sn films has been reported,[11,12] no separation between the current and field was attempted. In addition, zero effect of electric bias[13] and even a bias-induced effect of whisker suppression[14] have been reported. We have conducted the experiments of growing Sn whiskers in an electric field using true flat plate capacitor geometry
shown in Fig. 1, where the surface area of the plates, and the area from which whiskers may grow, is of vastly greater dimension than the inter-plate spacin
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