See "IPC-4554 Specification for Immersion Tin Plating for Printed Circuit Boards" 3.2.1 Immersion Tin Thickness.
The most common reason for solderability issues with the white tin surface coating during multiple thermal excursions is a thin white tin deposit. Florida Cirtech recommends 0.65 microns, that has proven to be sufficient for at least one year shelf life and up to four passes in assembly. 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. [S Wentz Florida Cirtech ]
Additional points: * Heat cycles deplete the pure tin layer. Each solder cycle reduces the tin thickness by ~0.1 micron (4 uin). * A highly activated flux will begin to corrode the imm tin coating after 10 minutes. Corroded tin has poor solderability.
Finally, white tin thickness has substantial variability, depending on the process used by the licensee [your fabricator]. So, that is where you need to start, with the process of the licensor.
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