Hi Guy - it appears that the component datasheet suggestions are due to a 
concern with two issues: (1) gold embrittlement of the resulting solder 
joint; (2) gold leaching of the plating resulting in a bad solder 
connection. Tin and gold form an intermetallic phase - AuSn4 - which 
typical causes the solder joint to be very brittle and fail. This occurs 
when the solder joint content exceeds 3- 5 wt. % gold in the solder and 
since the component is heavily gold plated, there is a likelihood this 
would occur. Using the 80/20 AuSn eutectic alloy or an high % In alloy 
would eliminate this issue. The second concern is that  the dissolution 
rate of gold into tin solder alloys is 100 microinches per second - so if 
the soldering process  isn't very well controlled then you will leach all 
of the gold off the component into the solder and be left with a 
mechanical (not a metallurgical) connection. Unfortunately, the use of the 
recommended alloys is good solution to avoiding those two issues but a 
costly solution as you pointed out. Since SAC305 is a high % tin alloy, it 
would be a problem. I think you could probably control the soldering 
process to avoid the gold leaching issue, just don't know if the gold 
embrittlement  issue would be avoidable. I would favor the use of  the 
80/20 AuSn eutectic alloy over the In alloy - its not too bad to work with 
and it should be a little bit cheaper. Several of the TechNet folks have 
extensive experience with the 80/20 alloy  so maybe they can offer some 
advice/suggestions. Good  luck.

Dave Hillman
Rockwell Collins
[log in to unmask]



From:   "Guy Ramsey" <[log in to unmask]>
To:     "'TechNet E-Mail Forum'" <[log in to unmask]>, 
<[log in to unmask]>
Date:   03/29/2013 08:26 AM
Subject:        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] 
______________________________________________________________________