label: pcb,printed circuit board
RoHS became Euro-law more than a year ago but it seems like the Industry still hasn't agreed on what an ideal RoHS pcb finish might be. Part of the problem is that we aren't just dealing with RoHS issues these days. The advanced packaging just adds to pcb-grief.
More and more components are showing up in QFN, micro BGA and wafer-scale packages. All of these are more sensitive to surface flatness. Lead-free HASL (Hot Air Surface Leveling) is decent for solderability, but it can have a bumpy surface which those little packages don't like.
ENIG (Electroless Nickle Immersion Gold) gives a nice flat and level surface with good solderability and high resistance to tarnish. But if the processing at the board fab house isn't spot-on, it can suffer from black pad problems - especially with BGA and QFN packages. Silver, again has a flat surface and good solderability, but it can corrode in salty or high sulfur environments (like near the coast or in industrial facilities) and is susceptible to tarnish and oxidation.
I ran across an interesting article on the subject in the online Lead-Free Magazine. The first half is interesting but mostly about pot soldering. If you don't find that interesting, scroll down to about paragraph 12 for their take on the board surface issue.
