Interesting.  I always think gold is the functional layer and the nickel is
there to prevent the copper migration to gold layers. 

Regards,
 
Jason Zhao
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
Sent: Thursday, October 29, 2009 7:13 AM
To: [log in to unmask]
Subject: Re: [TN] Al wire bonding

Anat,

One can bond to the nickel [without the gold]!! This become especially easy
with heavier wire sizes [>2 mil].

Typically, one uses an ENIG plating.  The immersion gold results in a gold
flash of ~2-5µinches
The gold is merely there to protect the nickel from oxidizing too much.  One
actually bonds 'through' the gold layer to get to the underlying nickel.

If given a choice, I like pure soft gold instead of a hardened gold.  If you
use ENIG, you may not get much choice in your gold.

It is my opinion that the thermal history & chronology of the board has more
of an inpact on wire bond reliability than simple plating.  Eg - if one were
to reflow the board, [maybe] clean it, then let it set in que for multiple
weeks in a shop environment - I virtually guarantee you that you will note
differences in bonding characteristics as compared to a virgin board.

ENIG, or gold flash, is my answer.  Keep them moving - don't let them set
around, is my recommendation.

Steve Creswick





On Thu, Oct 29, 2009 at 3:38 AM, Anat Douek <[log in to unmask]> wrote:

>
>
> Hi,
> What is the min gold thickness required for reliable aluminum wire bonding
> ?
> Which gold type is prefered for this process?
> It is needed  for a military application.
>
> Regards,
> Anat Douek
> Process Engineer
> Herley General Microwave Israel
> tel. 972-25689455
>
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