Several ideas come to mind when you describe your problem.
1. Dendritic growth- if you don't see them on the surface, you might have
them growing from hole to hole (providing you have vias or pth on your PWB)
within the laminate matrix along the reinforcing fiber bundles. Classic work on the
within the laminate version of this done by Raytheon and reported by A.
derMaaderosian. (One of the other respondents also proposed this as a possible
cause.)
2. Non-ionic materials that attract water- some of the PWB fab chemicals will
do this, yet offer no response to ionic test instruments such as the
Ionograph or OmegaMeter. Cheap test for this are the TM 650 test methods 2.3.38 7
2.3.39
Another test would be to bake for several days in vacuum oven, then test the
PWA under very low humidy conditions. If works ok, try letting it equilibrate
at higher humidity conditions- 70% would be good. If it then croaks, dry it
out again and test at very low humidity conditions. If performance is restored,
then you have a moisture/current leakage problem. Honeywell Space folks
determined that 4 monolayers of water was enough to generate a current leakage path
when adsorbed onto PWA surfaces with residual organics to hold it.
Last, you don't need dendrites to get current leakage. In a previous life,
built an automated current leakage detection timing unit for testing purposes.
The timer auto- started upon detection of 4 microamps of current leakage, shut
off the digital timer when reaching 500 microamps of current leakage. Dramatic
contrast between solvent and aqueous processes, but the real point was of all
the measurements that reached the current leakage limit of 500 microamps,
only about 42% actually developed dendrites, the majority were just invisible
current leakage paths on the surface. The test boards had three comb patterns,
but no holes in them, so these were all surface current leakage measurements
without any complications from within the FR4 laminate.
This may give you some ideas for closing in on your problem. Anyone else wish
to comment?

Bill Kenyon
Global Centre Consulting
3336 Birmingham Drive
Fort Collins, CO   80526
Tel: 970.207.9586     Cell: 302.377.4272

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