Inge
I'll reply point by point:
1) "Pinning metal crystals"- Metal grains will slide by other grains
understress unless
something irregular (e.g., an impurity precipitate) gets in the way. These
irregularities "pin" the crystals in place. This is what occurs in work
hardening of copper pipe, strengthening steel, etc. A thin film will have
more irregularities because its lattice structure is trying to match the
substrate. This can strengthen it overall under the right conditions-
example is thin films can have stresses 1e3 times higher than the bulk
material before falling apart.
2) "Deeper wire bond is not better"- Thicker pads doesn't mean you will
increase your contact area with the wire because the bonder has a fixed
pressure pad width. The contact along the "depth" (i.e., sides) part is
minimal. Also, wire bonding is a thermo-compression process. Changing the
coating thickness changes the compression parameters and will confuse the
original problem of why it is not bonding in the first place. So, you may
get a weaker "pressure" (used to illustrate and not imply anything) bond in
joining the wire to the pad.
3) "Larger wire diameters"- Bigger diameters mean lower throughput in the
bonding process. Otherwise, you can solder leads on directly if you really
want strength!!!!
4) "Blue dielectric"- Is that Dupont product thick and blue, or is the blue
a result of it being transparent and ~1um thick? I'm not familiar with it.
5) "using a thin film guy"- I was assuming you'd find one that has packaging
experience as well. You are at Ericsson and must have an army of people
that know wire bonding. Tap them.
6) Overall, the problem is hard to diagnose long distance. It's like
instructing one to sail over the phone without knowing the conditions. But
the solutions I'm hearing are taking you farther from determining the
sources of your problem. Look, I'm not Mr. Bond, so I can't tell you off my
head what you can do. Unless your contaminants are outragious, I'm not sure
you'll get anything from Auger. If you have epoxy, the pad surface wont be
conductive....hmmm... which means it would look real weird in SEM (without a
coating). You'd probably see that coating burn off under the SEM e-beam.
Now, that's an easy way to see contamination.
Carey
ps- How did you know I have big hands????
-----Original Message-----
From: Ingemar Hernefjord (EMW) <[log in to unmask]>
To: 'Carey Pico' <[log in to unmask]>; TechNet E-Mail Forum. <[log in to unmask]>;
Ingemar Hernefjord (EMW) <[log in to unmask]>
Date: Monday, October 11, 1999 8:22 AM
Subject: Running: Wire bonding question
>
>Hi Doc,
>feel that I need to tell you what's going on. We have not come clear yet
through this passage. Please, se below for my "reporting". / cu / Ingemar
>
>
>
>
>------------------------------------ september mail------------------------
>Ingemar
>Your problem is in basic metallurgy. The bond/bond strength failures are
>not occurring in the bulk of the pad, but rather either at 1) the
>interface of the Au-pad/substrate or 2) poor sonic-bonding parameters.
>
>
>
>About the latter, thicker does not usually help for a variety of reasons.
>One is that thinner films actually are stronger than thicker ones (although
>25 um is nearly bulk quality depending on deposition methods). In
>metallurgy speak and generallizations, there are more dislocations pinning
>metal crystals into place in thin films.
>
>RE: What do you mean with 'pinning metal crystals' ?? Please, explain.
>
>
>
>
> Another is the bonding process is
>mixing the interface of the wire with the bond pad along the side of the
>wire onto the pad. It's not like a shaft getting pushed down. So, deeper
>is not better since you are effectively cratering out the side support that
>you think you are getting. Also, your bonding parameters using the
>"springiness" of the film to break the surface (if pure Au, then its to
>break through the surface impurities that lodged into the surface).
>
>RE: No, deeper is not better. My purpose was to get more microcontacts.
Carey, hold your enormous hands softly against each other and look against
the azur blue sky outside, you will see some lighe passing through, because
your hands are not very flat (or are they?). Then, still looking against the
sky, increase the pressure and you will find that the light can't pass. What
you have created are many more contacts, but the skin on your left hand did
not go deeper into the right one. Not world's best metaphor, but you
certainly understand. So...if you have a rough gold, maybe you can create
MORE contacts by using a thicker wire. That was my thought.
>
>
>
>
>Thicker wire means you have to recalibrate your
>whole system (i.e., pad thickness) since you need to change the bond
>strength that flattens the wire. High surface roughness (your Ra=5um is
>20%... very high) gets in the way of
>having a reproduceable process since one can't control uniformity.
>
>RE: you are right. There are companies, however, that use thicker wire as
standard, for avoiding the trouble I speak about. Works, seemingly, but you
then have to accept more space between pads.
>
>
>
>I don't understand what you mean by Au/blue dielectric/Au. Are these
>capacitors? The combination doesn't make sense from an IC point of view.
>And what is the blue dielectric?
>
>RE: blue dielectric is a duPont standard dielectric paste.
>
>
>
>However, before embarking on this, you need to determine where the failure
>is occuring. Dirty interfaces are usually the culprits. Burnishing makes
>me real uncomfortable in this regard.
>
>RE: Yes, "dirt" is unwanted, but according to Harman smoothness (surface
microtopography) is also one very important factor to observe. We have
studied the gold surface in fluorescent UV light microscope and found no
evidence of epoxy contaminations, next step is microprobing with Auger or
what method will be recommended. Also, O2/Ar plasma cleaning will be tried
next day. If we still get stuck, one of the labs that came up in this forum
will be contacted. One of our own thinfilm drs have been involved on my
order, he has little experience from wire bonding, but knows (as you) a lot
of the "few-atomlayer" world, which is controversial and special, as you
say.
>
>
>END OF MONOLOGUE
>
>
>Carey
>ps- thin films/interfaces are my expertise (Ph.D)
>
>
>
>
>
>
>-----Original Message-----
>From: Ingemar Hernefjord (EMW) <[log in to unmask]>
>To: [log in to unmask] <[log in to unmask]>
>Date: Friday, September 24, 1999 5:33 AM
>Subject: [TN] Wire bonding question
>
>
>>Is there a wire bond specialist listening?
>>I am frustrated about a MCM package, bottom Al2O3, seal frame Kovar,
>leadframe Kovar and conductors/mounting pads consisting of either a)one
>layer Au thickfilm b) Au thickfilm/blue dielectric/Au thickfilm. Problem:
>bondlift or bad bond yield. Automatic 25micron U/S ball bond K&S. The Ra
is
>from 1-5 microns. Conductor/bondpad thickness 15-20um. Surface roughness
>eliminated somewhat by burnishing. Recommended (duPont) Ra-value is 0.5um
>for good yield. Lots of Si, Bi and Cd particles but <10um large. Of course
>these oxides and binders are not wanted, Au should be 100% pure, but as all
>know that does not exist in practice for thickfilm. Now to my question,
dear
>Mr/Mrs Bond : if a gold thickfilm is rough, wouldn't an increase from 25
>microns to 50 microns make the bond conditions a lot better? My simple
>logic: rougher surface>thicker wire. With all admiration for Harman's
>"Bible", there is nothing mentioned this way.
>>Manipulating the force, amplitude and capillary face are all tried with
>minor improvement, at least far from 6Sigma goal. Plasma cleaning or any
>other cleaning does not help either.
>>All answers are welcome, even from the small island (forgotten the name)
>where Aussilek tries to live.
>>Ingemar Hernefjord
>>Ericsson Microwave Systems
>>
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