I agree with Werner. The only way you will be able to determine if you
have a solder reliability issue is to either microsection the failed
node(s) or perform dye penetration testing, both of which rely on a
destructive method. I doubt very much that C-SAM will show anything.
However, I have found that when doing the dye penetration, I can almost
always pull off the BGA mechanically without lifting any of the pads on
circuit boards with ENIG finish. This in itself is also supporting
evidence of some type of brittle intermetallic failure.
There are different levels of "black pad" (not a good term) and there
are different levels of brittle intermetallic failure associated with
soldering to ENIG. Anytime you use ENIG and you also have design issues
such as severe CTE mismatch or stress imparted to the PWB after
soldering (attachment of the assembly to a metal frame, flex during
testing, etc.) you can end up with a very short mean time before
failure. I have seen the ENIG BGA solder joints fail at stress screen
testing after having passed ICT and full functional test, and I have
seen them pass that but fail shortly after a few cycles of service in
the field. One thing I always try to do is have a "golden" board built
up with an emulation socket utilizing crowned pins. This is always an
extremely valuable failure analysis tool. I use it to take the "failed"
BGA and test it on the golden board. If it works then I am pretty
certain that the problem was in the solder joint to begin with, and it
was not a component performance or design performance (timing,
crosstalk) issue. The crowned pins in the emulation socket allows me to
test BGAs that have been removed in that I do not need the solder balls,
which are lost upon removal of the BGA. I just have the rework operator
carefully remove the remaining solder with an iron and plenty of tacky
flux. The crowned pins also allow me to test BGAs with the solder balls
attached, so if I suspect a bad lot of BGAs I can pre-test them with the
golden emulation board and know that they are good prior to assembly and
reflow.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
Sent: Tuesday, December 18, 2007 9:24 PM
To: [log in to unmask]
Subject: Re: [TN] BGA failures after OMPAC underfill
Hi Tom,
Maybe the pin location are not that random. With the exception of P16,
they may all be at the outline of the die-I do not know your die size,
but I am guessing. You did not give a thermal history; is it possible
that you have creep-fatigue failures?
214.5 certainly should be sufficient for proper wetting of eutectic
SnPb, if you have solderability.
In order to find out, you need to sacrifice some of your product to
cross-sectioning.
Werner
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