Fritz,
Evidently I need a faster server.

Pete, this is rather common with lesser resin materials.  Polyimide wetted nicely to the glass shouldn't do this, unless you tortured it.  Looks to me like you must have had some dryweave.  Drill induced fracture doesn't look to be an big issue.  For those purest out there- keep in mind that we don't have enough information yet to call it CAF.  It could be another flavor of internal migration.

Chris

-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Francis Byle
Sent: Friday, October 30, 2009 11:10 AM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] Latent defects on PWA

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 [log in to unmask]
Sent: Friday, October 30, 2009 9:34 AM
To: [log in to unmask]
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>>
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