Hello Prasad,

Thanks for the off line photo. It was the "thousand words".

Based on the etch profile of layer two, the construction appears to be a "cap
foil" construction, thus prepreg was used in the construction of the outer
layers which fits with my first thoughts. I cannot see any evidence of the
effect in the small area or core material that is in view. If the effect is an
intrinsic artifact of the material, I would expect to find it everywhere and not
just in the prepreg zone.

I confess that I have not seen anything exactly like it before but I am still
leaning towards my earlier thoughts regarding contamination of some sort in
and between prepreg layers (perhaps it is simply moisture in the prepreg as
polyimide is quite hygroscopic).

I am not an expert is resin formulations and thus would defer to Brian's
comments which make sense to me but I believe that EDX elemental mapping analysis
would find bromine if it is in fact present and dominant in the voided areas.
You might send your photo to Steve Zeva to have him post on his website and
let others take a look and share their thoughts.

Best of luck in finding cause and resolution.
Joe

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