Very interesting thought, Robert.  I would agree with the statement "the
solution to one MAY be the solution to the other".  However, if tin pest
were tied to tin whisker formation, then this would only account for
some cases, as many (how many?) cases of tin whiskers are not formed
below the magic temperature of 13.2 C for tin pest.  This still leaves a
lot to explain.  Also, tin pest GENERALLY requires better than three 9's
tin purity and I have not heard of this requirement for tin whiskers.  I
look forward to being educated if I am wrong on this.  :)
Bev
RIM
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Robert J. Lang
Sent: May 2, 2006 11:02 PM
To: [log in to unmask]
Subject: [LF] Could Tin Disease Be An Underlying Cause Of Tin Whiskers?

Tin Disease, a phenomenon first noticed when it caused the
disintegration
of Organ Pipes in Cathedrals located in the cold climates of Europe,
starts
to occur when the temperature drops below 13.2 degrees C (~ 56 degrees
F).  The Beta or "white" Tin, a shiny, ductile metal, is transformed
into
a  Alpha or "gray" Tin powder.  Beta Tin has a tetragonal structure and
a
density of 7.31 g/cm3.  Alpha Tin has a cubic structure and a density of
5.77 g/cm3.  The decrease in density causes the Alpha Tin to expand and
exert  compressive forces on the remaining Beta Tin.

The resulting buildup of pressure retards the formation of the Alpha
phase.  Relieving the compressive force accelerates it.  Adding Copper
to
the Tin, even in large amounts, does nothing to impede the spread of the
Tin Disease.  Adding small amounts of Bismuth or Antimony to the Tin
solves
the problem.  Lead, in sufficient quantities, also solves the problem.

It has been shown that compressive forces can cause Tin
Whiskers.  Compressive forces can be caused by the diffusion of certain
impurities into the Tin crystal structure.  Impurities known to cause
the
problem include Copper.  A Nickel barrier layer between a Copper surface
and a Tin plating layer seems to reduce the growth of Tin Whiskers.  But
Tin Whiskers also occur in coatings of pure Tin such as those found on
many
of the component parts in use today.  The Japanese are apparently having
some success by adding Bismuth to Tin to control Tin Whisker growth.  Of
course Lead in sufficient quantities is known to solve the problem also.

It seems that there could be a relationship between the occurrence of
Tin
Disease and the growth of Tin Whiskers and it is even possible that an
underlying cause of Tin Whisker formation is Tin Disease.  The
compressive
stresses caused by the onset of the transformation from Beta to Alpha
Tin
could be sufficient to cause the onset of Tin Whisker growth.  The
temperature required to start the transformation could easily be reached
while parts are in transit and certainly would be reached during even
the
most benign of environmental thermal tests.  Also, the conditions that
are
favorable for both problems are similar, as are the conditions that are
detrimental to them.  It appears that there is a good possibility that
the
solution to the Tin Disease problem could also be the solution to the
Tin
Whisker problem.

I've not seen anything in the literature on this possible cause-effect
relationship.  Does anyone else have knowledge of work being done to
link
these two effects?


Robert J. Lang

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