Hi, Phil,
1) I haven't had enough board volume to give you much of a feel for black
pad occurance, because we've only had about 40 ENIG boards made so far, but
with no problem. Some are well over a year old now and still solder well.
2) Pass - haven't had such a requirement yet.
3) I prescribed underfill for our 21mm sq CBGA and 35mm sq PBGA and several
other types as well - all 1.0 or 1.27mm pitch. The very first ENIG boards
we had with BGA's mounted were designed and assembled for us in the US
(we're in Singapore), and they failed miserably the first time they were
put into any level of vibration. The problem was very poor-looking assembly
processes - balls looked dry-jointed, very coarse with a lot of striations,
different shapes and sizes.
I had 3 more assembled locally, keeping a close eye on all the processes
used, and we got 3 good-looking boards. As a precaution (or extra
insurance), I opted to underfill them as well. The boards are military
avionics boards, subjected to all manner of TC and vibration, so I felt the
extra process was probably worthwhile. I chose a capillary-action type
underfill that is a) reworkable and b) doesn't limit me to the fluxes
contained in the 'no-flow' type of underfill that is applied to the BGA's
prior to assembly and tends not to be reworkable. So far, these new boards
have passed all the tests required of them.
Underfilling is reckoned to improve the reliability of BGA solder joints
under conditions of temperature cycling and vibration by a factor between 3
and 5 times that of joints with no underfill by helping to dissipate the
stresses built up in the joints through CTE mismatches during thermal
cycling, and through vibration. I should say that underfilling won't help
offset the effects of Black Pad and shouldn't be expected to. With Black
Pad, you won't have much of a solder joint, if any, to support with
underfill, and the more brittle nature of the oxidised Nickel plating won't
be made any less brittle with underfill either. Underfilling is intended to
further support good joints on a stable surface against the conditions
mentioned above. I'ld get your boards made right, rather than putting
sticking plasters over damage (if that's what you're [contemplating]
doing).
Hope this helps a bit with whatever you're looking into.
Peter
phil bavaro
<pbavaro@QUAL To: [log in to unmask]
COMM.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: Aero/ST Group)
TechNet Subject: [TN] ENIG black pad issues
<[log in to unmask]
ORG>
02/23/02
01:34 AM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
phil bavaro
We have used ENIG as our primary BGA board finish for many many, many
boards built for cellular communications without issues regarding this
problem.
We have a major PWB fabricator telling us that they cannot control what
appears to be a "black pad" problem and that we should be changing to
Entek's OSP combined with a selective gold process for our standard finish.
I cannot say that we have never had this problem before but usually it was
more a case of unrinsed soldermask on a pad rather than corroded nickel
plating beneath the gold finish.
I did the cross sectioning myself and took some great pictures of the
contaminated pad surfaces that were yielding when subjected to drop
testing.
I am convinced that ENIG is a solid finish but I know that it can be
susceptible to nickel wetting problems. I have three questions:
1) What kind of frequency are the rest of you finding "black pad" to occur
on ENIG boards?
2) What finish do you use for high density BGA/CSP double sided boards
that have surface contact (gold) plating requirements?
3) Are any of you using underfill to hopefully negate the solder joint
brittleness that the black pad surface inherently creates?
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