Te handbook of metal echants, page 610.

They list:
Aqua Regia:
3:1 HCL:HNO3, room temp

You can also use KCN (potassium cyanide), or a Tri iodide etch (400g KI,
200G I2, 1000ml H2O)

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Benyimin Hadad
Sent: Sunday, February 04, 2007 5:00 PM
To: General MEMS discussion
Subject: [mems-talk] etching AuSn 80-20

Hello all

I want to etch AuSn 80-20, with wet etching.
How can I etch it?