Detection of tin plating and tin whisker mitigation

The problem of tin whisker growth from pure tin-plated surfaces is a well-documented phenomenon. For the space industry in particular, vacuum plasma arcing as a result of tin whisker fusing has been known to cause the failure of on-orbit satellites. The complete prevention of tin plated parts in space hardware has been a very difficult goal to achieve. Visual inspection is not reliable because tin-plated parts can be mistaken for lead-tin solder plated parts, Supplier injunctions have not worked either. First tier suppliers may provide parts in their hardware that have been plated by second, third or even fourth tier suppliers, and it is difficult to have constant awareness across all those levels. Even with injunctions, all companies providing flight hardware have had the experience of "'discovering" tin whiskers in their assembled hardware. Also, with the commercial markets moving to lead-free solders, the risk of pure tin making its way into hardware increases. With the shrinkage of dimensions and reductions in voltages for electronic components and packaging, tin whisker growth is becoming more of a threat. Elemental analysis using x-ray techniques have been used to successfully screen and identify tin plating. They have also been used to identify other prohibited materials such as cadmium and zinc. The difficulty in the past has been that the x-ray detector systems which were used EDX attachments to scanning electron microscopes (SEMs). These lack portability and require significant span times because of sample preparation and vacuum pump-down. Recently however, handheld portable x-ray fluorescence (XRF) analyzers have become available which offer accurate analyses. The purpose of this poster session is not to describe or redescribe the tin whisker phenomenon. This is well described the literature. Rather, we describe the processes that we used in removing tin-plated parts from our inventories. There were three different parts to the process. First, we prioritized the part commodity types according to their likelihood of containing pure tin plating. Then, the prioritized inventory was evaluated using portable XRF detectors and where this was not possible using EDX analysis in a SEM system. Then, with tin plated lots that we knew had found their way into flight hardware, we followed a disposition process similar to that described by McDowell [2].