Hi Guy,
 Uve already got guru's advice, so maybe I don't add much of interest. Only
tell you that we had a nightmare with Indium soldering to gold. The result
was rhat we banned Indium for the next coming 1,000 years.  Why? I'll brief
you and if you want deeper insight, you should contact the old english
Indium man. Nobody beats him, natural after half life at Indium.

So what...

Independent of what Indium alloy you use, the first phase when soldering
with SAC or other high-Tin solders, is the formation of a AuIn2 layer. This
takes only seconds. The  process eats at least 1 micrometer of gold. If the
gold is thinner, there will not be a correct IMC and the component will
loosen when thermomechanically exposed. So, remember: the gold that you
solder to, must be thicker than 1 um!

Furhermore, if your total solder joint is thin, let's say 15 um or thinner,
AND you have a thick gold, there is a risk of a second IMC, namely Au9In2.
It's not ternary, just one single phase and is known for its brittlement
and having overall lower data than AuIn2. If the soldering top temperature
is too long, you may even get Kirkendall voids,which makes Au9In2 even more
brittle.

I hope I remember the above correctly. I worked with tat problem about 30
years ago!

Inge

PS there is a an old paper in my Dropbox under  'Soldering....' The
headline is " Aspects on Indium Soldering...'  I have better reports, but
don't know where they are..


On 29 March 2013 06:27, Guy Ramsey <[log in to unmask]> wrote:

> 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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