Technetters 

For someone with a metallurgical backgroun, I turn on to whiskers which have
a specific metallurigcal definition and have nothing to with dendritic
growths or cathodic andodic filaments which are electrochemical in origin.

Metallic whiskers which are formed on tin and cadmium coatings and a few
other metals are a single crystal growth the may extend out from the coating
for about a half an inch.  The tin-whisker is usually only a few microns in
width and have been shown to be the cause of failure of electronic devices
including printed boards.  The tin whisker has no branches but may make a
sharp angular turn. The early papers on the suject came out of Bell Labs in
the 1950's where, I believe, some field failures were investigated.  Tin
platings or coatings when placed in compression in a humid atmosphere
produced tin whiskers in a few hours to a few months as the tin seem to
squirt out of the tin coating.  Unfortunately, some of the bright tin
deposits such as bright-acid-tin had a built in compressive stress which also
produced whiskers without external compression.  

Tin plating or fused coatings on brass were shown to be a large contributor
to tin whiskers, on copper the growth was less likely, and on nickel very
unlikely.  Tin coated brass terminals and tin plated screws is where I have
seen whiskers most often.  However, I did see a tin plated printed board that
seemed to have have had extensive Rogaine treatment, as it had sprouted
everywhere and caused extensive shorting.  The use of small amounts of other
metals such as lead or silver in the tin solder or plating/coating does
inhibit tin-whisker growth and there are some reports of heat treatment below
the melting point will also reduces whisker growth.

Some metals such as silver, lead, copper etc. do form dendritic growths in a
slightly humid atmospheres ( or in aqueos solutions) or in the presence of
some gasses and grow dendritic filaments that also cause shorting.  Often
these filaments have twigs or branches from which the name dendrite
originated.  These filaments may grow in air, through dielectric or between
soldermask and the printed board; however whisker growth is largely prevented
by  barriers other than gaseous atmospheres. 

It sounds to me that the failure reported by Rolf is a dendritic filament
rather than a whisker in the true metallugical sense. .

Thanks for listeniing 
Phil Hinton 
Hinton "PWB": Engineering 
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