-- [ From: Doug Jeffery * EMC.Ver #2.10P ] --

Ted,

I experienced a failure on a polyimide board several years ago. 
Althought it was not from a thru hole to a plane it may be of some use
in your failure analysis.

The failure ended up being due to an air pocket in a glass fiber that
got "excited" due to it's proximity 
between two planes.  The air pocket exploded and the current ran  down
the glass bundle to the edge of the board and shorted to the case.  

Now this was under a much higher voltage but the arc went about 1.4"
down the bundle from the failure site.  This happened on 14 out of 96
pieces run. 

We found that in thicker glass styles, i.e. 7628, there is a potential
for air pockets to be formed in the glass fibers themselves during the
process of forming the glass strands.

When we switched the construction to all 1080 style glass between these
two planes all failures went away.

Don't know that this relates to your situation but I am still today
wondering why the failure followed the glass bundle to the board edge
and not thru the .006" of dialectric to the plane.  It may be that
thicker glass styles also do not wet as well and the least resistance
is down the bundle. 
.

Doug Jeffery
Electrotek

-------- REPLY, Original message follows --------

> Date: Friday, 29-Mar-96 11:29 AM
> 
> From: Ted Jones                \ Internet:   
> ([log in to unmask])
> To:   Doug Jeffery             \ PRODIGY:     (TCGE34A)
> 
> Subject: Military Polyimide Board Failures
> 
> 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?
> 
> 
> 

-------- REPLY, End of original message --------