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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