Sounds like silicone. Lots of the high-temp kapton-type tapes have silicone
based adhesives, because they can take the heat without leaving visible
residue. You might try cleaning with Dow Corning OS fluid as it is purported
to effectively remove silicone residue where other solvents and cleaners
fail. Even this may not work if the board edges are porous, i.e., not already
coated with soldermask. In that case the silicone works up into the voids and
resists removal.

The edge stiffeners shouldn't be leaving residue according to this theory,
but it may be that they were clamped to the edge of a contaminated board and
transferred the silicone to the next boards.

Suggestions? Try silicone conformal coating. I know that's often not an
option, but it may fix the problem. There are also vast differences in the
non-silicone coatings made by different manufacturers. Some tolerate silicone
contamination far better than others. There are also application techniques
that can minimize the effect of the contamination.

For more concrete product recommendations or if you have any further
questions, please do not hesitate to call me.

Mike Moninger
Thermospray Company, Inc.
Sarasota, FL
1-800-952-5838
www.thermospray.com

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