07 December 2003
Tin whiskers are single crystals of b-tin that grow from the surface of some tin coatings. There are several types of growth. Usually these are described as filament, nodule and mound. Filaments are the typically high aspect ratio growths that look like whiskers, hence the name. They may contain one or more kinks and often have striations on their surface along their length. The filament-type can achieve lengths of several millimetres. Nodules are irregular filamentary growths with no predominant growth direction resulting in structures resembling worm casts. These can project tens of microns from the surface. Mounds are much smaller features that may be the beginning of filament or nodular growths. These almost always project less than 10 microns from a surface.
Whiskers are only normally found on chemically-plated tin or tin alloy coatings. It is possible that whiskers grow elsewhere but are not noticed because they do not pose a problem. The thickness of the coatings is important. Whisker growth does not tend to occur in coatings below 5 microns in thickness or above 10 microns. Unfortunately this tends to coincide with the thicknesses typically used in solderable coatings. Hot-dipped and reflowed coatings are usually immune from whisker growth.
Tin whisker can carry an electrical current and therefore have a potential for causing short-circuits. Filamentary whiskers are considered the highest risk as they can grow to the longest lengths. It should be pointed out that only a handful of in-service failures are directly accredited to tin whiskers.
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