Joe,
Like many others, we have been experimenting with gold ball bonding on
nickel with a thin gold plate. With 6 sigma in mind, we were not
satisfied with the results. Reason seemed to be that there was a bad
match between the sof gold (HV~80) and the very surface of nickel
(HV~300). Tests on matte nickel performed well, but the board maker
could not plate matte nickel at that time. We tried doped gold wire with
various hardness, and got better results, but still not obtaining zero
bond failures we abandoned the project. Wedge gold wire bonding,
however, is different. We have made some products with this method on
0.05 um gold over nickel, and with good results.
Ingemar

-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Joe Fjelstad
Skickat: den 18 oktober 2005 22:58
Till: [log in to unmask]
Ämne: Re: [TN] SV: wire bondable immersion gold

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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