Hello Inge and Joyce, 

You are right, Inge, that folks have been using very thin gold for aluminum 
wedge bonding for many years (and have even in some cases bonding directly to 
nickel as you mention). 

The importance of thin gold with aluminum wire (and conversly thin aluminum 
with gold wire) is a hard won lesson left over from the days of the "Purple 
plague".  

However, there actually is a link within the link that shows images of ball 
bonds not wedge bonds. 

There is no process detail but the text reads (company name removed):

"Wire-bond process for high-temperature applications produces bonds that are 
stable at high temperatures with a thinner gold layer. These special 
high-temperature wire bonds are especially useful in avionics and automotive apps, and 
require no additional lead time for production. The wire bond process 
eliminates the Kirkendall voiding that takes place in an Al / Au interface at 200°C 
thermal exposures."

It is obvious from the wording that they have concerns of Kirkendall voiding 
and they are finding benefit in not making the bond directly to aluminum. Even 
so that is mighty thin gold for gold ball bonding to be sure. It is 
impressive if they are pulling it off repeatedly. 

Kind regards, 
Joe 

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