Hi Pete, What you are seeing are conductive anodic filaments (CAF) aong separations between the glass fibers and the resin. Started as a less than perfect drilling of your PCBs and propagated in T-cycling [severe]. Werner -----Original Message----- From: [log in to unmask] To: [log in to unmask] Sent: Fri, Oct 30, 2009 10:34 am 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>>