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Tin Whiskers
The term 'Tin Whiskers' refers to 'needle-like' crystalline structures of tin (Sn) that form and grow on surfaces that use pure or nearly-pure tin as final finish. Tin whiskers commonly (but not exclusively) appear as thin strands of tin, and can indeed look like whiskers, hence its name. Other metals such as zinc, cadmium, indium, and antimony also exhibit this whisker-growing phenomenon.
Tin whiskers have been observed to grow to several millimeters in length, with records showing them attaining lengths of up to 10 mm in rare instances. Whisker diameters, on the other hand, can go up to as high as 10 microns.
Tin whiskers are highly undesirable in the external pins or leads of semiconductor devices, since they can bridge two adjacent leads together and form an electrical short. The short will be transient if the resulting current flow is enough to 'fuse' open the whisker. Otherwise, the short will be stable and can result in real device failures.
The problem of tin whiskers is not a new phenomenon, having been documented as early as the 1940's. Its resurgence as a critical issue in the semiconductor industry, however, was heightened by recent efforts of the industry to move away from the use of lead (Pb) in its manufacturing processes. Early explorations revealed pure or nearly-pure tin systems to be viable alternative Pb-free lead finish materials. Their disadvantage, of course, is their tendency to exhibit tin whiskers.
Not all tin whiskers look like whiskers, and even those that do also vary in form - they can be straight, kinked, hooked, or forked. Those that do not look like whiskers at all can appear as nodules or in pyramidal structure. A word of caution though - many people confuse tin whiskers with a more commonly-encountered attribute, i.e., dendrites, so novice engineers must be trained to distinguish between the two.
Figure 1. Photo of a straight filament whisker; source: http://nepp.nasa.gov
Dendrites exhibit fern-like or snowflake-like patterns that propagate along the surface, whereas whiskers protrude out of the surface. Dendritic formation involves the dissolution of the metal atoms in moisture and their redistribution on the surface under the influence of an electric field, such as when the device is biased.
The amount of time needed for whiskers to grow varies as well from just a few days to a few years, with reported growth rates ranging from 0.03 mm to 0.9 mm per year. This is one reason why whiskers are a major reliability concern - they can not be screened out at t=0 and can appear when least expected.
There is still a lack of thorough understanding as to why whiskers form and grow. In fact, many independent studies on the whisker phenomenon have yielded contradictory results, underscoring the fact that whisker formation mechanisms are complex phenomena.
See also: Lead Finish; Pb-free Manufacturing
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