Hi, The most common reason for solderability issues with the white tin surface coating during multiple thermal excursions is a low white tin deposit. We recomend .65 microns, that has proven to be sufficient for at least one year shelf life and up to four passes in assembly. The "tin whiskers" do not grow on the white tin surface because the grain structure is very dense. There is documentation of all this on the Florida CirTech website, www.floridacirtech.com In almost every case I've seen, when the first pass or two solders fine and then problems arise, it is an insufficient thickness of white tin that was on the board. Steve Wentz Florida CirTech