Tuesday, September 12, 2006

Tin Whiskers do billions in damage

What's a Tin Whisker you're wondering... Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish. Tin whiskers have been observed to grow to lengths of several millimeters (mm) and in rare instances to lengths up to 10 mm. Numerous electronic system failures have been attributed to short circuits caused by tin whiskers that bridge closely-spaced circuit elements maintained at different electrical potentials.

Tin whiskers are not a new phenomenon. Indeed, the first published reports of tin whiskers date back to the 1940s and 1950s. Tin is only one of several metals that is known to be capable of growing whiskers. Other examples of metals that may form whiskers include Zinc, Cadmium, Indium and Antimony.

People sometimes confuse the term "whiskers" with a more familiar phenomenon known as "dendrites". Therefore, it is important to note here that whiskers and dendrites are two very different phenomena. A "Whisker" generally has the shape of a very thin, single filament or hair-like protrusion that emerges outward (z-axis) from a surface. "Dendrites", on the other hand, form in fern-like or snowflake-like patterns growing along a surface (x-y plane) rather than outward from it. The growth mechanism for dendrites is well-understood and requires some type of moisture capable of dissolving the metal (e.g., tin) into a solution of metal ions which are then redistributed by electromigration in the presence of an electromagnetic field. While the precise mechanism for whisker formation remains unknown, it is known that whisker formation does NOT require either dissolution of the metal NOR the presence of electromagnetic field.

Tin "Whisker" shown above growing between pure tin-plated hook terminals of an electromagnetic relay similar to MIL-R-6106 (LDC 8913)
Photo Courtesy of Andre Pelham (Intern)
NASA Goddard Space Flight Center

Why the damage?

Tin Whisker Alert

A failure mode is re-emerging that has been responsible for the loss of billions of dollars worth of satellites, missiles and other equipment - electrically conductive 'tin whiskers'. Tin whiskers can develop under typical operating conditions on any product type that uses lead-free pure tin coatings. Driven by the accelerating movement to lead-free products, tin whiskers pose major safety, reliability and potential liability threats to all makers and users of high reliability electronics and associated hardware. Existing approaches are not sufficient to control tin whiskering in high-reliability systems. The risk is here now, and unless decisive action is taken soon to fund development and implementation of a strategic action plan to devise short-term stopgap procedures and medium-term investigation of mitigation alternatives, serious consequences are inevitable.


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