The ball bonder we have is a general wire soldering machine that uses High voltage to form the ball, then ultrasound to weld it in place. The standard wire is 25mil gold.
Edit: Also from my experience on decapping a 6560, it was bonded with gold wires of approximately the same size.
I also found a MOS 2050 SRAM wafer on ebay:
Attachment:
File comment: MOS2050 wafer
034B5E2B-1B67-4C39-B979-D638081CC479.jpeg [ 204.46 KiB | Viewed 1864 times ]
Too expensive for me, but the seller claims that its a 2inch wafer from 1970.