The Navy has recently experienced field failures (shorts) on three
7-layer polyimide PWBs.  The boards had operated for 300 hours in a
sealed chamber.  The chamber is maintained at 50C, with constant
humidity.

The shorts (about 8 ohms of resistance) developed between a PTH and
ground plane on layer 5.  There is a voltage potential of 30 volts
(+15/-15) between the PTH and plane.  The clearance between the PTH's
pad and the plane is 10 mils.

Horizontal microsections of the failed areas do not show any obvious
migration path.  The only anomaly noted on the failed pad is that it
demonstrates much more pink ring than pads on adjacent PTHs.

We're looking for ideas on the most likely cause of the shorts.

Have there been any studies done that indicate pink ring will or will
not contribute to this type of failure?

Has anyone seen similar failures?