Hi Jim! There is a fairly high probability the copper-tin crystallites you
have seen are copper/tin intermetallics (Cu6Sn5 phase). There are some good
studies in the industry literature by Morris, Yost, Romig, etc. showing
that the Cu6Sn5 phase can be evenly distributed within a solder joint as a
normal occurrence of the soldering process (there are some neat pictures of
the phase as a hexagonal, hollow shaped geometry).  Take a look in Klein
Wassink's book for more info (ISBN 0-901150-14-2) pages 93-98. Good Luck.

Dave Hillman
Rockwell Collins
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Jim Marsico 516-595-5879 <[log in to unmask]> on 05/13/99 07:26:00 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Jim Marsico 516-595-5879 <[log in to unmask]>

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Subject:  [TN] BGA FAILURE ANALYSIS




Good Day Technet:

First, a little background...  We're in production using a 228 ceramic BGA
for
a military contract.  The base material of the component is gold plated
nickel,
prior to solder ball attachment.  The gold plating is very thin, around .25
microns.  We've been experiencing approximately 5% (components, that is,
not
solder joints) open circuits found at in-circuit testing (ICT).  Upon
removal
of the BGA (Air-Vac system) from the board, the specific ball location
where
the open was identified at ICT shows no solder remaining on the component,
where all other locations has solder remaining.  What does remain is a
dull,
gray, crystalline metallization.

SEM/EDX analysis shows Ni/Sn and Cu/Sn.  It is presumed that the Ni/Sn is
the
intermetallic layer created from the nickel base material and the tin in
the
solder.  What puzzles me is the presence of the copper-tin crystallites.
Since
there is no copper in the solder or on the component, is it possible that
this
copper has migrated all the way up from the board pad to the
component/solder
interface?

Thanks in advance...

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