Steve Zeva has kindly posted a photomicrograph of a glass fiber in laminate
that came from the period when IPC was actively investigating laminate SIR
problems (see his web site for the photo).
The photo shows a thin black (in the photo) void along the glass fiber,
showing lack of adhesion to the resin. Work at the time by Raytheon personnel
showed that electromigration could take place from pin to pin in PTH assemblies....
thus the electromigration failures would be within the laminate structure, as
opposed to being on the surface.
Evidentally precise control of the pH in the adhesion promoter bath was found
to be critical in avoiding such problems (Dow Corning, supplier of the
adhesion promoters).
Perhaps Susan R has seen such internal metallic shorts in her microsection
work and can comment further. Also, laminate makers may also like to comment on
this. (I still have the original B/W photos of this and the solder ball/flux
residue, so the scans are as good as one can get...)

Bill Kenyon
Global Centre Consulting
3336 Birmingham Drive
Fort Collins, CO   80526
Tel: 970.207.9586   New Cell: 970.980.6373

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