I remember hearing about this in the 60s, but haven't been concerned with it for decades. Here is a few links and comments on this situation::
http://nepp.nasa.gov/Whisker/background/
http://searchdatacenter.techtarget.com/definition/tin-whiskers
http://www.instructables.com/id/Don-t-throw-solder-away/
http://nepp.nasa.gov/Whisker/background/
http://searchdatacenter.techtarget.com/definition/tin-whiskers
http://www.instructables.com/id/Don-t-throw-solder-away/