I am aware of the Dexter Dexcoat FST process. This is an organometallic tin
that places a barrier layer on the copper surface that greatly retards the
copper tin migration to a point that shelf life well in excess of one year is
easily achievable. This process produces a lead free planar surface that has
better solderability than Ni/AU. Some OEM's have claimed that this process is
the only alternative finish that they can run on the same assembly line with
HASL product and no changes need to be made. The key here is that the process
in not an ordinary immersion tin which would have a very limited shelf life.

Regards,
Chris McGary

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