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