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April 2004

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Subject:
From:
"Brooks,Bill" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 27 Apr 2004 11:30:20 -0700
Content-Type:
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text/plain (85 lines)
There does not need to be more voltage... just lots of current and a low
resistance path.


Bill Brooks
PCB Design Engineer , C.I.D., C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510


-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]]
Sent: Tuesday, April 27, 2004 11:08 AM
To: [log in to unmask]
Subject: [TN] Interesting Occurance...

Hi All!

Just had an interesting occurance that I thought I'd share with you. We
build
this 14-layer assembly here that has 10 Altera FLEX 240-pin QFP's on it...4
on one side, 6 on the other. We've been building this assembly for some time
now with no problems.

After SMT, the board goes through AOI, gets some connectors hand soldered
on,
then goes to ICT. After ICT it goes into a Thermotron for some power-on
thermal cycling for eight hours.

Last week, after a cycle was done, the test technician went to retrieve the
boards from the oven. Upon opening the oven door, he noticed this funky
smell....kinda like burnt fiber glass. At one of the twelve positions in the
test
rack he found this:

Go to http://www.stevezeva.homestead.com and look at "Smoking Hole" and
"Smoking Hole 2" (NOTE: DUE TO THE GRAPHIC NATURE OF THESE IMAGES, PARENTAL
DISCRETION IS ADVISED)

We don't have a clue how this happened. ICT on this assembly was fine, there
were no problems found in the test fixture position in the oven (other
assemblies have gone through the temp cycle in the same position without a
problem).
And supposedly, there is only 5V and 3.3V being applied during the thermal
cycle.

Obviously there had to be more voltage than that to cause the kind of damage
you see in the pictures, but nobody knows where it came from. Smoking Hole
is
a QFP on one side of the board, and Smoking Hole 2 is on the other. The
leads
are vaporized in the pictures. This only happened to one board, and the
fixture was loaded up with another 11-assemblies.

I don't expect we'll ever figure out what happened unless it somehow happens
again. I know one thing, if it does, I don't want to be anywhere near the
oven!

Thought you might find the pictures interesting...these are $4000 images.

-Steve Gregory-

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