As  you know, gold dissolves extremely rapidly in molten solder compared 
with other metals. Some of you might have experienced the effect of solder 
splash or balls landing on gold plated surfaces which  contaminate the gold 
surface finish. Typically, tape or sheath is used to protect the gold plated 
surface (gold fingers) from solder contamination.  Recently, we found some 
gold plated assemblies that exhibited no contamination on the gold surface 
even when solder balls or splash landed on the surface.  The strange thing 
is that no solderability problem has been reported on any gold plated SMT 
pads. Can someone comment on this phenomenon!!

Thanks

Blue Smurf
Here is some additional info.
Electroplated Ni thickness: ~ 100 micro inch
Electroplated  Gold thickness ~40 micro inch