On boards requiring immersion nickel /gold plating we are seeing a condition
of several random surfacemount pads not plating at all. These pads appear to
be free of contamination after chemical pre-clean. At times adjacent via
holes and connected pads on the opposite side of the board also do not
plate, indicating some kind of galvanic reaction.
Some pads do plate but have a darker appearance after nickel /gold plating.
Analysis of those pads show sufficient gold thickness but only 25% of the
nickel thickness that the "normal" looking pads have.        Some LPI
soldermasks show this condition more than others.
Has anyone seen this phenomenon before, what is causing it, and what can be
done to correct or overcome it?