TED JONES wrote:
> 
> 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?

  Is there a chance that you are seeing  the CAF phenomenon ?
   good luck
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