Matt -

From your description of the dark vs. light areas of copper, I suspect you are
correct about a delaminated or debonded condition.

The acrylic adhesive used in your flex has a glass transition (Tg) temperature
of about 45C, as opposed to a Tg of 240-250C for the polyimide materials in
the structure.  This means your process temperatures can temporarily turn the
acrylic adhesive to something rather like grease.  In this state, it is
subject to debonding/delaminating rather easily from any form of stress.

I would suggest the following as a quick check to confirm: Place the specimen
under low magnification (10-20X) and gently squeeze the suspect area with
tweezers or similar.  If debonded/delaminated you should be able to observe
movement in the structure between the cover film and the circuit.

Several years ago, DuPont, Sheldahl and others standardized and commenced
marketing "all polyimide", "adhesiveless flex" or "bondable polyimide", or
whatever you want to call it.  This material has a Tg in the 230C or above
range, which is much more durable.  We have switched over most of our designs
and have seen improved product yields at the fab houses and in our own
assembly operations, as well.  In our case, this has pretty well turned out to
be a true "drop in replacement".  All of our government sector customers, to
date, have agreed that it is a class 2 change..

Good luck - I'm at 256.882.4536 if you need to talk.

Regards - Kelly

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