Dave hit the nail on the head. The difficulty of wire bond on gold may have
many root causes. Here are my 2 cents:
(1) Plating problem. Plating condition, e.g.current density, PH, additive
concentration variation will resulting entrapment of impurities
(co-deposite), sometime the Au deposite become porous. A good reference
paper for Au plating is:
High-Performance Gold Plating for Microdevices - by A. Gemmler
et.al. Plating and Surface Finishing, Aug. 1994, p.52
If your vendor perform the similar analysis as indicated in the paper, you
would not have any worry about the wire bonding for a long time (You will
notice the change on the "wire bonding yield chart" if someone changed job
on your vendor side. for example, an engineer or QA...or someone decided to
go "out sourcing", cut cost to "extend Au bath life", etc...No kidding!)
(2) Surface contamination and surface diffusion. Poor coverage at the edge
of the gold pads (exposed Cu or Ni) sometime will result Cu/Ni migrated to
Au surface under humidity condition. (surface diffusion is 2 order of
magnitude high than the bulk diffusion).
Insufficient final rinse (or delay of the final rinse, e.g. "went for
lunch") after the plating may cause residue on the gold pad (oxidation of
residue...e.g.brownish look Au for KCN based plating). Good reference:
The Auger Analysis of Contaminants that Influence the
Thermocompression Bonding of Gold. - by G.E. McGuire et.al, Thin Solid Films
Vol. 45, 1977. p. 59
(3) Plasma cleaning does have some draw backs: You will activated PWB
surface too. Plasma clean is a standard cleaning method used for ceramic
thin/thick film product prior to wirebond. The work time after the clean is
limited to few hours (depend upon what class of clean room you are using).
It is costly to do surface analysis (Auger, SIMS, XPS) to define the working
time after plasma clean. I usually specify on the low side (ASAP=less than
0.5 hr). UV-Ozone may be faster, but PWB damage have to be assessed if you
are using FR4 or polyimide.
(4) Hardness of Au is a quick and cheap way to define high quality Au (it
does not tell you much about surface contaminant). However, it is not easy
to do micro hardness (you need special machine! $$$!). Particularly, when
thickness is on the low side (< 30 micro-inches).
(5) It is difficult to do cost-benefit assessment for plasma equipment.
Quality improvement normally take about half a year to show some measurable
result. I face the same issue too. Please accept my condonlence.
my 2 cents...Good luck.
a pupil (jk)
p.s. regarding Au quality, you may want take a look of "The use of SIMS to
investigate wire bonding yield problem on gold contact" by G.R. Mount,
et.al. MRS Symp. Proc. Vol. 390 Electronic Packaging Material Science VIII,
1995 p.245
At 07:55 AM 4/21/99 -0700, you wrote:
>All this back and forth conversation is great regarding how to bond to Au.
>Remember the first rule of thumb. GET TO THE ROOT CAUSE!!!!
>
>A eraser is simply abrading the surface, removing "crud" but also removing
>precious Au that you need for acceptable bonds. The eraser does not
>miraculously act as a vacuum but a tool to smear the crud, not remove it.
>If it is an organic problem that you have, plasma may work but plasma will
>only remove ~200-300 angstroms, not a whole lot being removed. Plasma also
>depends on the gas, flow, temp, electrode configuration, chamber size. You
>just can't buy a plasma unit and plug it in.
>
>Good Luck and if more help is needed, I can be reached at
>[log in to unmask]
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of
>[log in to unmask]
>Sent: Wednesday, April 21, 1999 2:45 AM
>To: [log in to unmask]
>Subject: Re: [TN] Wire Bonding On Gold
>
> Hi, sorry to get in between. You seem to discuss what is hot also here
>at our company to and from. My first question would be: please,
>gentlemen, what are you from Chuck and pupil? I have got some good
>advice from Dave at Rockwell, maybe we have something good to learn
>from you too. I have tried for one year to buy an american plasma
>cleaner, but when the economists ask "what is the payback time" , then
>what can I answer? Oh, my adrenaline...! /Regards
>
> Ingemar Hernefjord
> Ericsson Microwave Systems
>
>
>
>
>> >Date: Tue, 20 Apr 1999 19:09:41 -0400
>> >To: [log in to unmask]
>> >From: joyce <[log in to unmask]>
>> >Subject: Re: [TN] Wire Bonding On Gold
>> >
>> >(1) If you have good plating, you don't need any physical abrasion.
>> >(2) you can use eraser if the surface of Au is not clean, but make
>sure it
>> has no/low ionics (such as Sulphur- common additive for rubber).
>> >(3) you can use brush
>> >(4) depend on your process and product, you may be able to use plasma
>> too...However, if your plating surface required eraser to remove the
>top
>> surface, the plasma normally would not do the trick.. Plasma is great
>to
>> remove MINOR oragnic residue.
>> >(5) if your gold is too hard (impurity or voids concentration are
>high),
>> you will have problem to bond and have reliability problem. (eraser,
>brush,
>> plasma are not going to help you).
>> >good luck.
>> >jk
>> >At 06:29 AM 4/20/99 EDT, you wrote:
>> >>We are presently wire bonding aluminum wire to gold lands on circuit
>boards.
>> >>I have heard many times that wire bonding goes sour to clean them
>with an
>> >>eraser. This seems a little primitive. Is there any other way that
>this can
>> >>be accomplished.
>> >>If the eraser works why hasn't someone designed something to take
>care of
>> >>this.
>> >>Chuck Garth
>> >>
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