Your image series is worth applause, reflects an experienced analyst...

We use exatly same power studs for power distribution, but we use denser
and harder boards. I have seen a lot of failure modes during my many
years in electronics business, and I understand your ambition to find
the root cause. I have seen something like and it was not corrosion, nor
CAF, but 'simply' Lead that had migrated from the stud finish and
deposited on the Copper. We realised, that cleaner liquid had penetrated
along the stud and been sucked into the board structure. Once on inside,
it ain't freely leaving, so some voltage difference and polar liquid
residues was supposed to start migration of Lead. The deposited Lead was
not pure Lead, but a salt. Don't remember what kind of. An EDAX would
tell you what the stuff is. In your case, light microscopy isn't enough
for finding the answer. 

My two farthings

Inge


 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of - Bogert
Sent: den 12 januari 2007 00:26
To: [log in to unmask]
Subject: [TN] Is this CAF?

January 11, 2007

This is an e-mail for all you CAF experts out there.

We have an OEM that manufactures a 10-layer VME backplane assembly that
has press-fit power studs installed in the backplane.  The manufacturer
has experienced system level test failures.  Some of these failures did
not occur during the initial testing but the assemblies failed at some
subsequent time during testing.  The failure mode was shorting of the
+12V layer 1 to the +5V layer 3 via the pressed-in power studs.  The OEM
had the backplane sent to an outside lab to determine the root cause of
the problem.  After looking at the pictures taken from the evaluation,
it appears to me that the problem could be due to CAF because of the
damage done to the glass fibers in the FR4 board material.  CAF is also
suspected since the test failures do not occur on initial testing of the
equipment.  We verified that the PWB conductor spacing meets IPC-2221
requirements.  Another factor is, we believe the OEM may have used some
Type H flux per J-STD-004.  Not sure this is an issue unless some of the
flux residue entered the PWB internally via the press-fit power studs.

Steve Gregory was kind enough to post 13 pictures on his web site.  They
are files GLB CAF # 1 through GLB CAF#13.

Can folks identify whether CAF could be the root cause of the problem?
Any help would be appreciated.  

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