SAC305 will suck up gold instantly.  The SnAu eutectic is apparently a pretty low energy state, so it really wants to be made.  The alloy is 80% Au, 20% Sn by weight, so the gold will disappear until that equation is satisfied.  And I do mean instantly:  While soldering near gold wirebonds (for a prototype), a solder ball about 3mils in diameter went flying and went clean through two wirebonds in line, leaving the remaining wire ends just hanging in space as if they were connected.  It lodged in the 3rd bond!

I realize that the Indium is pricey, and as paste it doesn't have a long shelf life.  Usually better to use preforms, or shave some wire solder yourself.  Depends on your throughput.

Do keep in mind that Indium is basically incompatible with copper, so it probably isn't a good choice for use with a copper plated circuit board unless you're quite sure the nickel barrier will keep the metals separated during reflow.

Conductive epoxy may sound pretty good by now.... The world standard is Epotek H20E which is available as a two part mix.


Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Friday, March 29, 2013 9:28 AM
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






______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________

______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________