Hair thin metal crystals in can grow from component pins causing shorts in cirquitry.  I have never heard of this before.
Really scary.
Mechaninsms yet not quite ynderstood!
Seen mostly on platings of tin, zinc, but also gold and silver.
Alloys seem less affected than pure material.
Can grow in different conditions and rate.

http://international2.farnell.com/NZ/online_magazines/rohs/rohs3/rohs-tin-whiskers.htm
http://nepp.nasa.gov/whisker/photos/pom/2004april.htm
http://nepp.nasa.gov/whisker/
http://nepp.nasa.gov/whisker/background/index.htm

/Morgan 
--
Morgan Olsson, Kivik, Sweden

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