Hi, Arturo,
As recently as last Friday, I removed a Xilinx BGA (eventually) that was
suffering from a lack of contact at one ball. I discovered that about one
third of the pads on the board were blackened, and my heart skipped a beat
or two as I suspected Black Pad. However, since no other areas of the board
exhibited this symptom, I have put it down to improper reflow, improper
cleaning and general charring of residual flux resulting from 3 attempts to
remove the BGA from the board.
Reason for 3 removal attempts? The board had been conformally coated, and
although the coating had been removed, I suspected that enough had
penetrated underneath a pad of encapsulant beneath (and part of) the Xilinx
device to have stuck it down. The hot air rework station, initially used to
try and remove the component did not have a strong enough vacuum pick-up
tool to break the BGA free of the board. The operator, thinking that maybe
the whole thing wasn't hot enough, cranked up the temperature and duration
to 265 deg C for 50 seconds and still failed to remove the device.
I then decided to use our own new IR Rework station (as opposed to our
CEM's subcontractor for removing BGA's), and removed the BGA no problem
with some gentle assistance from a pair of tweezers to help break the bond
caused by the coating.
I will take some pictures of the BGA and the board as-found post-removal,
and others as I attempt to clean things up in preparation for a replacement
BGA going down. If they look interesting, I'll post some on Steve's website
with his kind permission, but roughly, many of the balls on the device do
not look well reflowed, and the blackened pads appear to be cup-shaped -
i.e. flux residue had collected around the base of poorly reflowed balls
and solidified there. When the BGA was removed, it was like pulling acorns
out of their cups, leaving the cups behind on the board. I'll know more
when I start to clean up.
In your case, you can maybe do what I plan to, and that's clean off any
residual solder from the board, then use liquid flux as a cleaner and a
good cloth to remove the baked-on flux residue. That done, you can try to
prep the pads with fresh solder to see how well they wet, and wick it off
again before putting a new BGA back on.
Some serious process investigation may be required - into the thermal
profiling of the board, since reflow was obviously not adequate in our
case, into the cleaning process, since so much flux residue appears to have
remained, into the BGA inspection process, since such a widespread defect
was not detected, and into our test methods, since I would have expected
much more intermittency or noise on the board. This particular board worked
for over a year, by the way.
Hope ths helps you a bit.
Peter
Arturo Medellin <[log in to unmask]> 30/03/2003 12:59 AM
Sent by: TechNet <[log in to unmask]>
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Medellin
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cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: [TN] Black Contamination
Hi All .
Actually we are working with the PCB finished EMIG and
during the process specially after the ICTand FT test we found
problems with the BGA Not contact so removed the BGA and found a black
contamination on the BGA pads .and we can not removed
Any help is really apreciated
Arturo Medellin
Solectron
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