Hi Charlie -

A couple of thoughts, for what they're worth:

First, I'm assuming that you're assembling these components to one of the
conventional glass supported polymer laminates, not ceramic or one of the
exotics, right?

A good convection reflow oven should allow you to set up enough differential
that you can solder one side, then the other without dropping parts off on the
conveyor, and stay with Sn63 solder paste;

Consider having your J leaded ASIC pretinned, if necessary, to get the gold
out of the way prior to assembly;

Note that soldering with Sn96/Ag3.5 on a circuit board that is coated with
Sn63 will yield a variant alloy in the final soldered connection.  If the part
is tinned in Sn63, this will skew the alloy ratio further;

Are you sure gold embrittlement is really a problem?  I don't recall having
ever seen any metallurgical reports that say what Au does in Sn96/Ag3.5, so
I'm not sure that its limitations are the same as those of Sn63 and similar.
I do recall having worked with the Sn96/Ag3.5 and similar solders years ago
and producing some of the ugliest solder joints ever conceived, in a cosmetic
sense.  The solder joints were quite strong and did the job intended.


Good luck - Kelly

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