David, I have heard that epoxy does not stick to gold very well. If X.P.
Zhu uses solder I can understand the gold flash. But this may not be the
best backside coating if he uses epoxy. What do you think?

Also he is using copper. That is going to have a huge TCE compared to a
ceramic. Depending on the temperature excursion and his bonding method
he could run into die cracking problems. However I agree with you that
Epoxy is probably his best bet.

Regards,

Steven J. Adamson
President IMAPS
Market Manager, Asymtek

-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of David Nemeth
Sent: Friday, March 28, 2008 7:07 AM
To: 'General MEMS discussion'
Subject: RE: [mems-talk] About Die Bonding

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