Interlayer shorts developed in modules subjected to elevated humidity,
temperature, voltage testing.  The shorts go through a 2 - 2.5 mils thick
epoxy glass dielectric (2 plies 1080, expectations were that the dielectric
would be closer to 4 mils thick).  Test conditions were 150V (DC), 150*F,
50%RH.  Time to failure varied from 130 to 1011 hours.  Current was limited
to 1mA.  Failure was defined as (leakage) current above 0.5mA.  The shorts
go nearly straight through the epoxy (not tree-like dendrites and do not
pass-through glass bundles), usually from a pad on one internal layer to the
solid plane on the adjacent internal layer.  Voids were observed in the
dielectric layer, but none were noted at the failure sites.  SEM analysis
did not identify any contamination.

Questions
1.  Are these "real" dielectric breakdown failures or have we induced some
odd (irrelevant) failure mode?

2.  Search of TechNet archives uncovered a 1998 (R Hersey) post stating that
dielectric strength should be derated to 50 V/mil for long-term reliability
of "thin" dielectrics.  Our results seem to support this post.  Is there
other data available to support (or refute) the 50 V/mil derating?

Thanks,
Wm. Dean May
NSWC Crane
Bldg. 38, Code 8086
300 Highway 361
Crane, IN 47522
812-854-3073

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