Black pad defect on gold plated boards

Well, for some reason the board house will pay for the boards but has some issue with paying for the parts on them. Why am I not supprised?

Perhaps I have mis-diagnosed the problem. We have found two different board p/n's with the same type of defect. Both from the same supplier. One production lot (processed at the same time) had two date codes, only on date code displayed the probelm and it was on every board with that date. If black pad is caused by oxidation of the nickel before the gold plating, it would seem the severity would be dependent on the severity of the oxidation. Our supplier outsources the gold plating so It would seem pretty possible the boards sat around awhile before gold plating. The defect looks like black goo mixed into the joints and once removed the pads do not take solder well if at all. I can not flick parts off the board though. The first time it occured we thought it must have been contaminated some how in our facility. We cleaned up the joints as best we could and shipped it since it was just a prototype. Electrically, the boards worked fine but the engineer removed an so-8 to try another part and the pads would not accept solder. I can not imagine that this is not some kind of plating probelm. We sent some of the first batch back to the supplier and they claimed it was surface contamination and that the plating was fine. We use boards from a number of houses with this type of plating and I have never had a problem in the 4 to 5 years we have used it. Also the problem batches of boards were run 2 to 3 weeks from each other, not at the same time.

Any help deeply appreciated.

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