IPC-4554 details the requirements for immersion tin (ImSn) final finish.  The specification calls out for a minimum usable (free) tin thickness of 0.4 micron (15 µin) if the PCBs are to be soldered within 30 days.  We would recommended a minimum free tin thickness of 30 µin, especially if there is a double sided SMT reflow requirement.  Further, mid-assembly dwell time should be minimized since the tin is constantly forming an intermetallic with the underlying copper layer.  The intermetallic layer will measure as tin when using XRF, but is not solderable.