GaAs under stress will form disocation network upon soldering. Soft solder, saturated with gold (not leach out the gold metallization) is consider the best choice, that is why Indalloy is better than Au-Sn eutectic. SAC most likely not a good choice.
--------------------------
Sent using BlackBerry
----- Original Message -----
From: David D. Hillman [mailto:[log in to unmask]]
Sent: Friday, March 29, 2013 09:45 AM
To: [log in to unmask] <[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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