Some high-P Nickel bath info:

http://www.atotech.com/data/publications/pcb/High_Phosphorus_ENIG_highest_re
sistance_against_corrosive_environment.pdf

I couldn't find the paper on Ni-Sn intermettalic differences between Pb and
Pb-free solders, but the theory is basically that the Ni migrates more
easily into Pb Solder than into Pb-free, further depleting the Ni content
near the surface of the pad, leaving the P behind - a phosphorus rich layer
at the top of the pad.  That P enrichment always occurs, Pb-free slows it
down a tiny bit.  A very minor issue, but I'll latch onto anything that
might make me feel better about Pb-free.

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