Hi Wayne - yea, I didn't think about that aspect. I am not a fan of the 
Indium solder alloys but based on some of the responses, it looks like it 
may be the best option. What is the incompatibility with Cu and In? Is 
there an IMC issue or a wetting issue? I have some experience with Indium 
alloys and corrosion problems but have not heard of a copper 
incompatibility.

Dave



From:   Wayne Thayer <[log in to unmask]>
To:     <[log in to unmask]>
Date:   03/29/2013 08:54 AM
Subject:        Re: [TN] solder which does ot scavenge gold
Sent by:        TechNet <[log in to unmask]>



Hi Dave-

AuSn eutectic may not an option because of the temperature restrictions on 
the part.  The eutectic point is very near 270C.

Wayne

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Friday, March 29, 2013 9:46 AM
To: [log in to unmask]
Subject: Re: [TN] solder which does ot scavenge gold

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









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