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July 2011

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Fri, 1 Jul 2011 08:58:07 -0500
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I think the culprits are the little people who live underneath BGAs and other components. They are very similar in appearance to boll weevils but with tiny human heads, and they like to pull stunts like this just so they can sit around and laugh when we can't figure out what happened. Sometimes they cannot agree amongst themselves, and this causes government shutdowns. There seems to be a plague of them here in Minnesota this year.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Friday, July 01, 2011 7:33 AM
To: [log in to unmask]
Subject: Re: [TN] BGA ball dewet from substrate

Hi Wayne!  I think you hit the nail on the head for "what" happened in 
Julie's case - a metallurgical reaction that consumes the wettable surface 
and then becomes unstable/dewets. But I have never seen this happen with a 
BGA and ENIG which makes it very strange. If there was poor nickel I would 
have expected to see non-wetting on the first reflow but since there is a 
well established IMC layer, I don't think oxidation of the nickel is the 
culprit. Very strange case.

Dave Hillman
Rockwell Collins
[log in to unmask]




"Thayer, Wayne - IIW" <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
07/01/2011 06:09 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
"Thayer, Wayne - IIW" <[log in to unmask]>


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Subject
Re: [TN] BGA ball dewet from substrate






I have seen similar cases when thin films are overheated for too long: The 
metallurgical reactions can result in scavenging the wettable constituents 
from the surface of the pad.  Test by just leaving one of the components 
"balls up" on a hotplate at about 240C for 10 minutes or so.  This is a 
ridiculously long time, but what you'll be looking for is similar-looking 
failures to your observed problem.

This used to be common with ceramic substrates, when PdAg was used as a 
surface finish, and if you rework high Sn Pb-free alloys, it isn't 
uncommon to have similar problems, where metallurgical reactions "erode" 
the surface of the copper until the hole walls lose contact with the pads.

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Julie Silk
Sent: Thursday, June 30, 2011 6:05 PM
To: [log in to unmask]
Subject: [TN] BGA ball dewet from substrate

We have a failure analysis of "dewet" for a BGA ball that is separated 
from the component substrate.  Does anyone know more about this failure 
mode?  The part goes through reflow twice, so once while upside down.  It 
seems unusual, even unlikely, for separation to occur between the balls 
and the substrate due to the weight of the part rather than between the 
ball and the PCB.  Other parts on the same board do not have this issue. 
Do you have any experience with this problem or how to prevent it?  Or for 
the future, how to predict it and thus avoid it?  Would vapor phase 
processing influence the likelihood of this defect?  Any help appreciated!

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