Blackpad occurs during the immersion gold deposit and not during assembly which is responsible only for revealing the defect. The primary factors are a very aggressive I Au reaction with the electroless nickel whereby there is severe hyper corrosion of the nickel surface that leaves an elevated phosphorus content at the interface of the gold to the nickel. Phosphorus is nonsolderable and once the Au is dissolved by the assembly solder, this underlying non wettable phos rich surface is exposed resulting in black pad.Gold concentration and copper contamination in the gold bath have a direct impact on the aggressiveness of the reaction and have been found to contribute to black pad. This is obviously a simple explanation of a very complex problem.
This link provides good explanation about ENIG PCB: https://en.wikipedia.org/wiki/Electroless_nickel_immersion_gold
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