Steve,
Was the board lying flat in the chamber during burn in testing?  Was it
possible that a drop of humidity could have formed on the chamber ceiling
or walls and gotten blown onto the energized part?  Smoking Hole 2 looks
like either a nasty dendrite or a crack in the component body.  I have seen
such failures in a previous life using low voltage high current testing.  I
began using current limiting resistors shortly thereafter.  Nice welding
demonstration though.

Doug Pauls

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