Contaminated Pads

I am sure that this is a problem that will never be solved, but something that I am seeing in increased numbers. Often refered to as "Black Pads". After production, returns from the field coming back with contaminated pads under various components. They have a blackish tinge to them and solder will not adhere to them. I know that I am being very vague, but there is so much information surrounding this I hardly know where to start. Any ideas? Have already looked into the PCB manufacturing process, masking, baths, etc.

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