Hi Jerry,
I don't know if this will help or not, but MIL-STD-2000A referred to a
fused electroplated Sn-Pb finish conforming to MIL-P-81728.  I can't
remember the particulars with that spec but I do remember there was quite
a struggle in writing that solderability requirement to include other
lead finishes as an alternate to solder coating.
It only makes sense to me that the lead finish should be compatible with
the eutectic Sn-Pb solder and process temperatures (assuming reflow or
wave) while still providing the ability to maintain solderability.  But
then, component mfr's are more concerned with efforts to reduce/eliminate
lead and qualify for environmental awards and other government kickbacks
(I know what you think of TI's Pd-Ag) than working with the masses.

Steve Sauer
Sr. Mfg. Engineer
Xetron Corporation

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