Lawrence,

If you are unfamiliar with the process of soldering, you might consider
using a conductive epoxy.  Since the power dissipation is not high, you have
a lot of options.  Epotek and Diemat are companies that make epoxies for
this specific application.  Some are more "elastic" than others, which
allows for a mismatch of CTE between your chip and your carrier.  But for
low power, either should be fine.

If you have trouble with it sticking down, you can try gold plating the
copper heat sink.  There are companies who will do this for a fee, just a
very thin "flash" gold layer is all that's needed (probably with a nickel
layer underneath?)

Soldering down GaAs die is typically done with 80/20 Au/Sn, but it's tricky
to get it right.  People tend to use conductive epoxy, except for very high
power application.

You shouldn't have any issues if the backside of the die is gold plated and
the carrier is gold plated.  It might work without plating on one or the
other or both, but I've never tried it.

David Nemeth


-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of X.P. Zhu
Sent: Thursday, March 27, 2008 10:49 PM
To: [email protected]
Subject: [mems-talk] About Die Bonding

Hi, everyone,

    I want to bond GaAs chips onto oxide free copper
heat sink. (The heat power of the chips is not very
high, so I don't choose ceramic or W-Cu as heat sink
material.) I wonder which solder I should use:
eutectic AuSn or just Indium? Can you give me some
advice? Thank you very much.