...JUST SCRATCHING THE SURFACE...

There has been years and years of exposed copper if you factor the edge
traces.  They are never solder coated.  Same is true for any selective strip
process.  The issue of exposed copper being a bad reliability issue comes
from inside the plated through hole.  The fear or more appropriately; the
worst case analogy is the weakest point of copper plating is the location of
least plating.  This would be the vertical mid point of the hole.  Given a
"quasi-uncontrolled" methodology: copper plating (back in the day) and
potential large nodule size; copper oxidization would perpetuate lessening of
elastic limit and induce work hardening then -walaa- add Z axis rotation and
PWB failure.  Condense this all down to a simple rule: NO BARE COPPER, and
conjecture upon conjecture has bare copper an ominous foe lurking in the
shadows.

Well, at least that's how I recall the "bare copper" fail safe stories back
when suggesting polyimide was seen as witchcraft.

Boston Brad

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