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









______________________________________________________________________
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] 
______________________________________________________________________