Peter/All Concerned:
- Re exposed copper.  Not a problem per 600, A-610 or J-STD-001.  Exposed
copper just turns green and as soon as the surface is protected from the
environment by the coating, the oxidation/tarnishing stops.  There has been
some conjecture that exposed copper would lead to metal migration, but as far
as I know neither DOD or anyone else has found a smoking gun to indicate that
this has ever caused or contributed to a field failure.
- History Lesson:  Once upon a time DOD had a bad problem with exposed
Kovar/Alloy-42 (being iron based alloys they kept oxidizing and eventually
would result in an open circuit when the lead rusted away from the solder)
[bad things happen to a Naval or Marine Corp. airplane and onboard systems in
a perpetual salt-fog environment].  Consequently, DOD prohibited exposed
Kovar/Alloy 42.  During one revision of the Mil-Specs the DOD contractors
cried hard times and got the requirement for optimum inspection lighting
(centered at 3000K, e.g. quartz halogen) deleted (cost too much to have a
good light).  Without the broad color spectrum available with optimum
lighting it is difficult or impossible to differentiate between exposed
Kova/Alloy 42 and exposed copper, so DOD simply said "no exposed basis
metal", and didn't really worry about exposed copper.  For a while industry
standards/specifications mimicked the DOD requirement for no exposed basis
metal.  Eventually someone figured out why the Mil-Stds said no exposed basis
metal and then the requirement was dropped.   Recent industry specifications
and standards require neither optimum lighting or (in some cases)
magnification aids.
- Exposed copper (on lead ends and the vertical sections edges of circuit
traces/pads/etc.) should not be considered a problem (as far as I know no
failure mode is/has-been associated with exposed copper).   Exposed copper
due to mechanical, thermal, or chemical attack is another kettle of fish and
must be examined on a case-by-case basis to determine if it constitutes a
defect.
Cheers, Jim Moffitt, Moffitt Consulting Services, Noblesville IN

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