Jamey,

You do not have to worry about intemetallics, but if you are using the Sn96
to raise  the melting temperature of your solder joint, you are defeatiing
the purpoose because once the joint contains 2.5% lead you form a eutectic
mixture and you have chunks of tin/silver solder being held together with a
mortar of Sn 63.  The Sn 63 of course has a melting point of near 180 Deg C
and the higher melting point of 221 deg C for the Sn 96 is no longer doing
you any good.  Even 1% lead in the joint will lower the point where some
liquid solder may be present by about 10 degrees.

If you are only soldering the components/ boards in a wave solder and trying
to keep the lead low to be environmentally friendly, there is not an
intermetallic or melting temperature problem except now you joints will look
very grainy and not cosmetically happy, because you will no longer have a
eutectic and some portions will solidify before others.  Eventually you whole
wave solder operation will look grainy and like you have a rash of cold
solder joints.

It is suggested that if you want to maintain the integrity of the Sn96, that
you use OSP or electroless nickle/immersion gold as a final finish.

Phil Hinton 

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