Recently there was a question about Leaded Solder on ROHS Compliant Parts. One of the experts said "No, using leaded solders on a lead free application will work just fine. You might notice a small difference in the wetting characteristics of the assembly but just remember to keep the temperatures low and if possible use a variable power soldering iron."

It's kind of a side note but, one thing they did not mention is BGA parts in a leaded process. In theory, ROHS parts shouldn't have any problems with leaded solder. But in the case of ROHS BGA's, the ROHS balls do not collapse during a leaded solder reflow profile. We just had this problem and we found that we have to find leaded versions of these parts so they will reflow properly.