We have run a "no clean, low solids" operation for almost ten years now
without any contamination issues.  Recently a shorting failure has occurred
across a capacitor on a few field returned circuit boards.

The failure analysis report came back as electromigration of both tin and
lead based on 1) a SEM photo showing the feathery migration tracks similar
to silver migration and 2) a spectrum plot showing Sn and Pb peaks.

Questions for Technet:

Does lead actually electromigrate along with the tin as an alloy?  I
thought the metals separate during electromigration and the tin would be
the culprit.

This occurs on a potted board but only in one location which happens to be
a high frequency switching application.  Why wouldn't it happen all over
the board uniformly?

Is 2 volts enough to cause the electromigration to occur?

Our solderpaste has antimony (0.4%) and silver (0.6%) in it as well as 63%
tin ( and the rest is lead).  Shouldn't the spectrum be showing these
components as well or is anything under 1% not discernible?

Is it possible for the bare board to be the culprit as it is a HASL board?

Any insight would be appreciated.


Phil Bavaro

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