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>>