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: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org] On Behalf Of X.P. Zhu Sent: Thursday, March 27, 2008 10:49 PM To: mems-talk@memsnet.org 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.