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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