Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin is used as a final finish. They have been observed to grow to lengths of several millimeters, although typically they do not exceed 1mm, and in rare instances up to 10mm. They have been responsible for numerous electronic systems failures – causing short circuits between closely-spaced circuit elements.

The first published reports of tin whiskers date back to 1940-1950, and tin is only one of several materials known to be capable of growing whiskers. Other metals that may form whiskers include Zinc, Cadmium, Indium and Antimony.

The term “whiskers” is often confused with “dendrites” - a phenomenon that is far more familiar. A “whisker” normally takes the shape of a very thin, hair-like protrusion that emerges outwards from a surface. A “dendrite” forms along a surface rather than outwards from it; and the growth mechanism for dendrites is well known – requiring moisture capable of dissolving the metal into a solution redistributed in the presence of an electromagnetic field.

The growth mechanism for tin whiskers remains unknown, but their growth is believed to be influenced by the plating chemistry and process used, as well as the substrate and environmental factors – factors which increase stress or promote diffusion within the deposit are believed to cause greater whisker propensity. However, many experiments have shown contradictory results for these factors – research is still ongoing.

There are vast disparities in the observed growth rate, length, diameter, shape and incubation period of tin whiskers by different researchers. Little is known about how to prevent tin whiskers from occurring, and with so little progress made in understanding this phenomena tin whiskers pose a serious threat to the viability of long-term, high-reliability systems.

Tin whiskers have been responsible for the loss of billions of dollars worth of satellites, missiles and other equipment. Due to the desires of the commercial market for environmentally-friendly components, many users who need high-reliability systems (namely NASA and the DoD) are left with few alternatives given their small market share.

It is recommended that the use of pure tin plated components is avoided, and that restrictions against the use of pure tin plating are put into place. Studies have shown that by alloying tin with another metal, such as 3% lead by weight, will reduce the propensity of whisker growth.

Pictures and Sources

  • http://nepp.nasa.gov/whisker/
  • http://www.calce.umd.edu/lead-free/tin-whiskers/
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