Hi Steve:
Bill is right, a part can pass ICT then short out when heated or the board may have two traces almost touching. They shorted out when they expanded. Someone in testing can do some detective work and find out if there is a short in the board. Or better yet, send it to a board manufacturing company and have it checked for shorts when heated..
Regards,
Ramon
> -----Original Message-----
> From: Brooks,Bill [SMTP:[log in to unmask]]
> Sent: Tuesday, April 27, 2004 2:30 PM
> To: [log in to unmask]
> Subject: Re: [TN] Interesting Occurance...
>
> 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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