Background info: 
We were asked to populate and assembly with a Ma-Com part, MA46H120, a GaAs
Constant Gamma Flip-Chip Varactor Diode. The data sheet says: 

Mounting Techniques - These chips were designed to be inserted onto hard or
soft substrates with the junction side down. They can be mounted with
conductive epoxy or with a low temperature solder preform. The die can also
be assembled with the junction side up, and wire or ribbon bonds made to the
pads.

Solder Die Attachment - Solder which does not scavenge gold, such as
Indalloy #2 (80In-15Pb-5Ag) is recommended. Sn-Pb based solders are not
recommended due to solder Embrittlement.  Do not expose die to a temperature
greater than 235C, or greater than 200C for longer than 10 seconds.

The Indalloy #2 cost $2,222.00 for 100gm or type five solder paste. 

We now have another customer asking for us to solder to thick gold. They
don't have a low temp requirement and asked for SnAu solder (Indalloy #182)
80Au-20Sn. . . I shudder to think what that will cost, and I don't think the
part, an inductor, will survive the 300C reflow. 

Question: 
Does SAC 305 "scavenge gold"? Would it form a brittle solder connection on a
thick soft gold pad? 

Guy






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