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
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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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