IPC-4554
Specification for Immersion Tin Plating for Printed Circuit Boards
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.
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.
Recommended Drawing Call-Out
Immersion Tin final finish per IPC-4554
Additional Recommendation
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 an XRF, but is not solderable.
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