Lawrence,

I agree with David's assessment that if you have low power dissipation, then
a conductive epoxy may be a better option for you.

With regards to soldering, your copper must be coated with gold for the
solder to adhere.  The best solder also depends on the geometry of your
chip, such as the overall size, the aspect ratio and the thickness of the
die.  The thermal expansion coefficient of GaAs and Cu are significantly
different and the higher temperature that is used for the solder, the more
the stress in the chip.  Since Au-Sn eutectic melts at ~280 deg C, compared
to ~150 deg C for In, the stress will be much lower for the In.  Typically,
though, In is not used in production because it tends to aggregate over 100s
or 1000s of hours and cause the formation of voids, which reduce the heat
sinking effectiveness.

Brad Cantos


On 3/27/08 7:48 PM, "X.P. Zhu"  wrote:

> 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?