Nancy,

Electrochemical failures are the result of contaminants residing on the
surface of the laminate and/or components of your assembly.  To create the
reverse plating cell that produces electromigration, three components are
required: bias voltage, moisture, and active ionic material.  By increasing
any one of the three required components, an increase in electrochemical
migration will be seen.  The circuit design dictates the bias voltage
present, which leaves you with control of the active ionic material present
and to some extent the moisture penetration through the conformal coating
(type and thickness).  Conformal coating does not completely stop moisture
penetration, so any surface ionic material will react under bias to create
the electromigration condition.

Proper cleaning of the laminate surface during the fabrication processes,
adequate flux activation during all thermal excursions, and subsequent
cleaning (as required by flux vehicle used) in the assembly process will
produce a finished assembly with low risk for electrochemical failure.

Halide residues are usually the root cause of dendritic growth.  Halide
residue may be from fluxes in the assembly operation or from HASL fluxes used
in PCB fabrication.  You should investigate both areas, bare board and
assembly, as either or both could be contributing to the problem you describe.

If you would like to discuss this further offline, please contact me at

Pat Kane
Contamination Studies Laboratories
765-457-8095

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