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Reply To: | TechNet E-Mail Forum. |
Date: | Wed, 5 Apr 2000 21:04:23 EDT |
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Steve/Alcon:
- Re exposed copper. Not a problem per 600, A-610. 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 (it 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, Technical Director, Electronics Training Advantage,
Indianapolis IN
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