Rory,
Doug from CSL is correct.  Electromigration is like fire, without one of the
elements contamination, moisture, bias, you don't get migration.  What you may
be observing is dendritic growth of some lead based crystal (we have observed 
a crystal growth on silver conductors as the result of sulfer exposure.)
Aris Christou has recently published a compilation of electromigration papers;
"Electromigration and electronic device degradation". This may have some 
helpful information.
-Mitch Austin
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