Rudy 

The unfused tin/lead finish was used very frequently in the 60's and 70's and
if cleaned and stored properly had good solderabability for wave solder
applications using RMA fluxes.  I used the finish for at least 10 years with
good results.  The only problem was tin/lead slivers and if you use tin, a
major problem is also tin slivers.  The factory where I worked had a avg, RH
of 28% and both unfused tin/lead and unfused tin were used and boards were
stored an average of 6 months.  The bright or semi bright unfused tin plates
have a good shelf life and In test that I have run were very satisfactory
after two years of storage.  The sliver problem was usually alleviated by
building a slight mushroom with the copper and then plating the tin/lead
covering. over the mushroom so that there was no overhang of the tin/lead
that was not supported by copper.

As David mentioned, the use of a thin 0.0002-0.0003 in. of tin/lead plate as
a final finish is not infrequently used for surface mount to provide a flat
land and they are soldered with standard no-clean and RMA fluxes.  Good
storage conditions, again, are desirable.

Phi Hinton 
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