We had Au/Ni solderability problems from batch-to-batch from our supplier
which was traced to a maintenance problem by our PCB supplier.

Surface analysis of the pcbs showed a very small layer of copper is being
deposited on top of the gold during the acid clean process. They followed
the routine maintenance program supplied by the chemistry vendor but this
did not include a copper content check. It appears that as little as 100ppm
Cu in the acid cleaner can leave an atomic level layer of copper on the gold
that is sufficient to prevent solder reflow. If the solder does flow then
the joint will be good and there is no risk of joint degredation.

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