John,
Good morning!
Just to add a little bit more to what has already been posted.
Yes, keep the gold thin. I typically spec 5-8 microinches MAXIMUM. The typical purpose of the thin gold layer is to prevent the underlying nickel from oxidizing.
With aluminum wedge bonding, one typically is bonding TO the underlying nickel and bonding THROUGH the gold - so keep it thin. Keep the cost down, and the reliablitiy up.
If you apply thick gold [say greater than 10-15 microinches], you may find yourself bonding to the gold. This will be a BAD thing. Initial wire pulls may look fine, but after slight aging the wires will almost literally fall off with failures within the weak Al-Au intermetallic. Save the thick, pure, soft gold for gold wire bonding - there, we are bonding TO the gold.
I ditto what Ingemar has indicated on bonding to Nickel. It is a 'piece of cake' [almost] when dealing with >=5 mil [127 um] dia wire. And, you can quite nicely bond directly to relatively clean copper too! .... just keep it in a non-hermetic environment! I don't think you will want to bring this up to your customer, however.
I am a little bit concerned about the portion of your post - "Our customer has a part on which they spec the finish of some edge connector fingers as "Gold Plated for Wire Bonding Al Wire"".
In my mind, edge connector gold is always HARD [containing cobalt, or some other such hardener], and wear resistant.
From my experience, hard gold is not easy to bond to - or should I say, not a reliable surface to wire bond to. Definitely not with gold wire, and certainly not something I could sleep with at night with aluminum wire.
What would they like? A hard, wear resistant surface for connector interface, or something to wire bond to? I see this as mutually exclusive.
You can plate a hard gold for the interconnect, then over-plate with an immersion gold! However, this would only be suitable for gold wire bonding. Even then, I guess that I come from the old school because I have not yet encountered an immersion gold that was anywhere near as gold wire bondable, as a pure soft electrolytic gold [Rudy may give me some flak on that one :-) ] although I have bonded many products to immersion golds. The available process window for the wire bonders has alway so tight, and relatively unforgiving with the immersion golds. Am spoiled by the good ol' electrolytics.
Bottom line, keep the gold thin for aluminum wedge bonding - but I would not use a hard, edge card connector gold.
See what Ingemar and Rudy have to say as well. Maybe Dave H will chime in.
Steve Creswick
-----Original Message-----
From: John Parsons [mailto:[log in to unmask]]
Sent: Wednesday, January 12, 2005 6:32 PM
To: [log in to unmask]
Subject: [TN] FAB - Al Wire Bonding
Greetings all,
I am trying to learn something about the wire bonding process. Our customer
has a part on which they spec the finish of some edge connector fingers as
"Gold Plated for Wire Bonding Al Wire". As we do not provide a wire
bondable gold finish in-house we have subcontracted the service. The
plating service tells me that typically for Al wire bonding all that is
required is a typical ENIG process (5-10u" Au over 200u" Ni). I believe
that in past verbal communications with the customer they have said that an
ENIG finish would not suffice and that they require 15u" of bondable gold
(electroless gold or electroplated??) .
What say you? Or does "it depend" :-)
Regards
John Parsons
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