Guy,

I don't have a paper or data sheet to reference but since the main gold embrittlement structure is Au-Sn and SAC solders are mainly Sn, I would think that SAC solders would form a brittle solder connection on a thick soft gold pad.
 
Regards,
Thomas A. Carroll, CQA, CIT, FAA ODA Unit Member
Supplier Quality Engineer
Boeing Defense, Space and Security - East Region Supplier Quality
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Friday, March 29, 2013 2:28 PM
To: [log in to unmask]
Subject: [TN] solder which does ot scavenge gold

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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