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September 2007

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Subject:
From:
Wayne Thayer <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wayne Thayer <[log in to unmask]>
Date:
Tue, 4 Sep 2007 12:42:37 -0400
Content-Type:
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Hi Gil!

That's the way I like my failures!  Very dramatic!

We can presume that the observed phenomenon is the result of too much
current going through too little conductor.  How did that much current
get there?  To answer that question, you must first draw a "fishbone"
diagram which shows how many different ways you could get current in the
offending conductor high enough to cause it to melt.  For example, a
partial list:

-failed component on surface of the board
-overcurrent on board I/O
-reduction in cross sectional area for the conductor which melted
(unlikely, but possible)
-mechanical damage to board causing internal short
-CAF or other migration path
-overvoltage resulting in arcing and build-up of carbon deposits

You get the idea.  Then you form a consensus on the most likely/quickest
or cheapest to diagnose and weed out those fast.  After those are weeded
out, you re-investigate all of the data and come up with
experiments/research to indicate probabilities of the items on the list
you couldn't eliminate yet.

If your real question is whether CAF could do that, yes it can.  It
should definitely be on the list.  Unfortunately, it sounds like the
"smoking gun" probably got smoked a little too much.  But not to worry
since extended bias/humidity testing with the remaining pieces of
circuit board may be able to generate additional conductive paths for
you to analyze.

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Zilber Gil
Sent: Tuesday, September 04, 2007 1:26 PM
To: [log in to unmask]
Subject: [TN] Help needed on motherboard failure analysis

Hi,

Recently one of our motherboard failed. It reaches very high temperature
that melts the copper and explodes shooting (from the side of the board)
the melt copper to over 20 cm. the shooting hole cause delamination of
the board in the area. A lot of black smoke and was also seen. The max
voltage of the board is about 50V. The board worked fine for more then a
year. What could be the failure mechanism? Could it be due to conductive
filament formation (although the PCB should be free of that)?

Thanks

Gil Zilber
Elta Systems

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