Sorry readers...yes this is a different reply....
Inge,
The actual wire is pushed relatively deep. I can't say how deep, because
each set up is different. And you're going to kill me, but I tossed all my
wire bonding papers in the recylcing bin when we had to downsize our
offices.
To illustrate that depth is not important, I can point to this: recently I
developed a fluxless laser soldering process for attaching TAB to flat panel
displays. The tin coating on the TAB lead was ~0.5um. The material the TAB
was bonded to was 0.05um Au-on-0.3um Ni (ask me about my non-gold soldering
as well... a true feat!). I guess the bond thickness is ~0.1um. These
bonds were very strong (no different than using 1um Ni).
To Auger: if you get a huge C peak, you probably could see a gray carbon
film anyway. If you have shiny Au, it probably is clean...at least from a
plating perspective.
There has to be packaging folks there.
Good luck
Carey
-----Original Message-----
From: Ingemar Hernefjord (EMW) <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Monday, October 11, 1999 9:35 AM
Subject: Re: [TN] Wire bonding
>Carey, people will get tired, this conversation a bit from PWBs, but as you
are professional and kind, Carey, I feel that the chat may be accepted with
some more Bytes. Comments, see below. / Ingemar
>
>
>Carey's Internet course in "dirt analysis and removal" on bond pads.
Cont'd:
>--------------------------------------------------------------------------
>Ingemar
>Your improvement by sputtering Au onto your pads indicates your surface was
>dirty. Presumably, there was a plasma clean before your deposition but, if
>not, you cleaned it somehow and its not important to discussion. You then
>had a clean, oxide-free (again, Au doesn't form an oxide) surface that
>allowed full contact with the wire. This supports wire bonding is not
deep,
>but along the interface.
>
>RE: How deep do you think,speaking about 25um gold wire and U/S ball
bonding?
>
>
>
>
>Auger analysis (a process that shoots in electrons and looks at core
>electrons in atoms) would NOT have helped you because you always expect
>impurities on the surface, and Auger cannot distinguish chemistry (for the
>most part).
>
>RE: at least you get an idea of what elements you have. An extremly high
Carbon peak could reinforce our assumtions about epoxy contaminations,
couldn't it?
>
>
>
>It does sound like your group needs the regular input of a thin film
>metallurgist/materials scientist. I suggest you connect up with someone in
>the ericsson company arena (I'm thinking on the semiconductor side since
PWB
>shops are not there yet) on a regular basis. Then one can design a series
>of experiments to address issues proactively (it never works in a panic,
>reactive mode). Unfortunately, board shops do not have the mentality to do
>this, and the cost is high. But soon, it will be cost-effective as board
>prices climb higher as does volume/shop.
>
>RE: I have been speaking with our wafer fab, but they had no further
proposals how to indicate "dirty" gold. I said they should because they put
gold on backside of the wafers. But realized that.... backside....not so
interesting for semi people.
>
>
>Good luck
>Carey
>
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