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November 2014

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From:
"Wenger, George M. [Contractor]" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Wenger, George M. [Contractor]
Date:
Tue, 18 Nov 2014 14:28:18 -0600
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Jim,

This looks like a "Foot-in-the-Mud" defect.  It could be caused by contamination but my best guess is it contamination isn't the cause.  You need to determine (i.e., measure) the coplanarity of QFP leads and look at your assembly process to see if it can be tolerated.  For example, if you are using a 5 mil thick stencil and the coplanarity of the QFN is above 3 or 4 mils you most likely have a "Foot-in-the-Mud" defect.  Since solder paste is approximately 50% by volume metal and 50% flux vehicle and you look at it very simplistically when during reflow the flux vehicle becomes a fluid before the metal melts and you wind up with 2.5 mils of flux vehicle on 2.5 mils of metal and the lead that is 3 or 4 mils above the others will be sitting is flux and when the solder melts it doesn't jump up through the flux to touch and wet the lead.  So what you are left with is an imprint of the foot of the lead in the solder that wet to the pad but not the lead.

Regards,
George
George M. Wenger
Failure Signature & Characterization Lab LLC
609 Cokesbury Road, High Bridge, NJ 08829
(908) 638-8771 Home  (732) 309-8964 Mobile
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jim West
Sent: Tuesday, November 18, 2014 3:13 PM
To: [log in to unmask]
Subject: [TN] Solder defect

Hi all,

Please see attached links. (Thanks Steve) http://stevezeva.homestead.com/IMG00002.jpg

http://stevezeva.homestead.com/IMG00004.jpg

http://stevezeva.homestead.com/IMG00008.jpg

We have recently come across two failures that were the result of one (1) pin not soldered on a 144 pin QFP.  Looking at the pictures, you'll see no wetting and the pin embedded in the solder.  We could say it's contamination, but we had no issues with any other pins on the QFP.  One more thing to add to the troubleshooting, the failure is the same pin (36) on the QFP.  Do I have a warping issue?  The pin doesn't appear to be bent, but I have not ruled that out until I have better means of verifying.  If it's contamination, why only one pin?  I would think that I would get at lease the heel or toe, or even the sides to solder, but in this case no wetting.  You can see the pin has made contact with the solder due to the indentation.  How does this not solder if it has made contact with the solder paste?  I'm at a loss and we will probably end up sending this to a lab, but I'm not sure if we will gain anything from a lab??

We are using leaded solder for this application.
Photo IMG00002 and 00004 are from the same failure Photo IMG00008 is the 2nd failure.

Your thoughts?

Thanks,
Jim


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