is a 24k, soft gold that is being sought when wirebondable gold
is speicified. This is as opposed to hard gold which is "alloyed" with nickel
or cobalt  and commonly used in PCB manufacture for separable edge card
contacts. 
The thickness requirement will vary with the type of bonding to be done.
Aluminum wire wedge bonding normally requires  ~ 0.1 to 0.25 microns ( 6-10
microinches) over nickel while gold thermosonic wire commonly requires 0.75
to 1.25  microns (30 to 50 microinches). Control of the bath is critical as
impurities can cause problems. Also surface cleanliness is very important to
achieving  good joints. Beware of the fact that wirebond schedules may need
to be adjusted for the different base materials used in PCB manfacture.

Cheers,
J. Fjelstad