Hi Pete,
What's causing the low resistance is probably CAF, which stands for
Conductive Anodic Filament. Electrically it acts like dendritic growth
on a surface, but it's a somewhat different physical process. 
The separation along the glass bundles is what allowed the CAF to
happen. It looks like the laminate specified is not up to the processes
it is exposed to. A few questions:
 - What laminate is being used?
 - What's the overall PWB thickness?
 - What is the PWB final finish?
 - What thermal processes does the PWB see in assembly? Peak
Temperatures, number of exposures, etc.; specifically, how many SMT
reflows at what peak temperature, and any selective or wave soldering,
what pot temperature and contact time, any rework and if so by what
process and at what temperature?

Regards,

Fritz Byle

-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of
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Sent: Friday, October 30, 2009 9:34 AM
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Subject: [IPC-600-6012] Latent defects on PWA




All:

I have run into an anomaly that I am having trouble understanding. I am
including a couple cross section pictures to show the anomaly.

After as few as one thermal cycle (-55 to +60C) the boards are failing.
The failure is a reduced resistance between isolated circuits.  The
board shown in the pictures was  160 ohms.

I had the failed connection cross sectioned (both horizontal and
vertical views)  and it revealed a crack in the prepreg.  This is a 14
layer board and the crack is showing up between layers 7 and 8 - it is
B-stage that is cracking.  The laminate is polyimide.

As seen in the cross section pictures it appears to me that the crack is
happening after plating - most likely during my thermal cycle.  I see no
evidence of plating or any contamination in the crack. 

So, my question boils down to why is the crack manifesting itself as a
short or reduced resistance?  Has anyone ever come across something like
this or have an explanation?

Thanks for your help.

Pete Menuez
Supplier Quality Engineering Manager
L-3 Communications Cincinnati Electronics 7500 Innovation Way Mason,
Ohio 45040 [log in to unmask]
513-573-6401 Voice
513-573-6767 Fax 

 <<Cross section.jpg>>
 <<Z axis view.jpg>>