Wayne,
Actually, we found LTCC to be very good from a mechanical robustness point of view. There were certain metal combinations and re-fire profiles that one could not do however, but all in all it came out as a monolithic structure - just as it was advertised.
Have used it from large cryogenic apps, Mil, to implantable medical, and beyond ... I have seen some pretty poorly fab'd stuff too ... Like most things in our industry - attention to detail is important.
There were a few conductors that had minimal adhesion, but from what Ingemar wrote, I assumed that at one time, all was well ... and that something had changed.
Knowing Ingemar, he probably is into some strange stuff [circuits] over there :-)
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Friday, March 30, 2007 6:18 AM
To: [log in to unmask]
Subject: Re: [TN] Cofired gold
Hi Inge!
I always prefer destruction. Something about the finality of the job.
Anyway, I think you are probably going in the wrong direction. If the deposits are not porous, then the coverage is probably OK, and as Steve points out, the print process doesn't have a big window for thickness you can deposit, anyway--certainly not the factor of 3 you are suggesting you may be thin by.
If you are ball bonding to LTCC, generally the ball goes on the part you've mounted to it and the stitch goes on the LTCC. Are you just doing some kind of qualification test?
LTCC is magical stuff! It's a miracle that the stuff holds together during processing at all, especially during the densification phase of the firing process. I'd be looking for a chemistry cause for your trouble, not print thickness. If the firing environment got just a tad low in oxygen, other phases of the process may have stolen the oxygen molecules necessary for the bond between the metal and the dielectric. That's exactly what happened to me several years ago with a big LTCC job: I fired on low temperature copper on one side, and on the other side the previously fired and well-adhered gold actually fell off!
So it may be that the LTCC manufacturer was lying about the adhesion you should have, or it may be that the substrate manufacturer messed up the firing environment slightly.
Wayne
>>> [log in to unmask] >>>
Ingemar,
Hmmm. 2 microns huh! Do you think this is a 'direct write' process? Sounds awful thin for a stencil don't you think?
I was using a Sonix acoustic microscope. With a 200+ MHz transducer [think it was 280 MHz] I could reasonably well pick out the outline of major circuitry features on the underside of a flip chip, if I had it gated properly.
Seeing internal traces within an LTCC body was always dependent upon how many internal ground planes and RF shields were placed inside the body as well. Vias above the target region were always bad news. You are on the surface. You may have a chance, but am not so sure you will be able to differentiate the top and bottom surface of the conductor when it is that thin. Definitely go for the reflected, high-frequency image.
Give the challenge to a really good acoustic microscope guy with a good machine and decent software :-)
Steve
-----Original Message-----
From: Hfjord [mailto:[log in to unmask]]
Sent: Thursday, March 29, 2007 12:34 PM
To: 'TechNet E-Mail Forum'; Creswick, Steven
Subject: SV: [TN] Cofired gold
24 layer stack with cofired AuAg top. Lifts at ball bond pull tests,
insufficient adhesion, I would say. Should take 4,000 psi. Smashed ball
approx. 2 mil diameter. If the gold lifts at 10 grams, then you are >
one magnitude below 4,000 psi. SEM'd edges of lifted lines= 2
micrometers thick. Paste maker recommends >7 micrometers as fired.
Substrate maker denies, that's why I'm asking for method to check all
incoming parts without destruction of the gold.
Abcd recommended confocal laser, don't think that will work. VHF
transducer, not bad idea! Good. Will test that next week. Thanks.
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Creswick, Steven
Skickat: den 29 mars 2007 16:14
Till: [log in to unmask]
Ämne: Re: [TN] Cofired gold
Ingemar,
Are you speaking of a standard, single print of gold which has been
co-fired into the body of the part, or possibly of a stack of inks which
has been co-fired in at one time?
XRF comes to mind, but it will require special 'standards', and the prep
of the standards would result in the destruction of a sample of parts to
construct them ... and even then I am not sure how successful you will
be.
It sounds as though a simple profilometer won't get you what you desire
- especially since the bulk of the conductor is below the surface. I
occasionally did internal inspection of LTCC parts back in the olden
days - but that was looking more for omissions and commissions.
I believe that acoustically, with a very high frequencurface of the conductor.
The margin of error [due to surface imperfections] may be quite large,
however.
Can you get them to tell you what their stencil thickness was and based
on solids content have some idea what you may have ended up with?
Will ponder further
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Thursday, March 29, 2007 9:39 AM
To: [log in to unmask]
Subject: [TN] Cofired gold
Trikeman or other gnu,
Anyone know of a non destructive method to check cofired topgold
thickness (Au over LTCC)?
Inge
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