Hi Dave-
Copper and Indium have IMC issues. Enough nickel should do the job of keeping them apart in this case. If I have to jump from a gold based system to a copper based system I use nickel wires (have to be real thin because the wire is very stiff).
Wayne
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Friday, March 29, 2013 10:17 AM
To: TechNet E-Mail Forum; Wayne Thayer
Subject: Re: [TN] solder which does ot scavenge gold
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]<mailto:[log in to unmask]>>
To: <[log in to unmask]<mailto:[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]<mailto:[log in to unmask]>>
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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]<mailto:[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
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From: "Guy Ramsey" <[log in to unmask]<mailto:[log in to unmask]>>
To: "'TechNet E-Mail Forum'" <[log in to unmask]<mailto:[log in to unmask]>>,
<[log in to unmask]<mailto:[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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