Rory,

I would say that, no, it is not possible to get electromigration in an
unbiased test.  For electromigration to occur you need three elements: an
electrical potential, water, and a conductive contaminant.  Electrochemical
migration occurs when this conductive contaminant mixes with water (only a
few monolayers needed).  I would guess that you have a flux contamination
problem and the lead is re-deposited rather than migrated.

Doug Pauls
Contamination Studies Laboratories
317-457-8095