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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