The hard parts

A project log for DIY Space Grade PCBs

3D print circuits using silver ink and ceramic substrates to make high performance, multilayer, fine line < 3mil, PCBs

chuck-glasserChuck Glasser 09/23/2016 at 05:580 Comments

After the ceramic green tape is blanked and via punched via a laser, printed with inks, the various layers stacked on a fixture it is necessary to laminate the layers together via the isostatic press.

To laminate the tape, the various stacked layers are placed in a pouch and vacuum sealed. So, I thought to myself, why go to the trouble, "not really that much trouble", of vacuum bagging when I could just manually press the pouch free of most of the air and rely on the extreme pressure of the hydraulics to overcome whatever air is present within the pouch? Wouldn't that be enough? I think the answer is yes. However, there is a problem.

A few layers of tape were cut out using the template at the lower right,


and sealed in a bag which was then sealed.

The pouch is trimmed.

The pouch is then placed within the isostatic press

The tape stack is inside the piston cavity along with the hydraulic fluid at 0 PSI and heated to 70 C for a hour or so until everything has had time to get to the pressing temperature.

Then pressure is applied for around 10 minutes. You can see on the gauge that the pressure is around 4,000 PSI.

Here is what I found after taking the tape stack out of the press.

Hydraulic oil in the bag. Dang, and double dang. Note that the tape does not appear flat. That has nothing to do with the failure of the pouch, it just the nature of the details of this step.

The stack was cleaned with a towel and placed on a quartz substrate. Not even close to being flat!

The tape stack was fired anyway.

You'll note that the quartz fused to the tape. That's a surprise! More importantly, note that the tape did not flatten out under the influence of of the firing temperature of 850 C.

Brittle, fragile, useless junk!

Oh, what to do?

Fundamentally, the reason for vacuum bagging the contents is that a bag that has a good seal is visibly different than one that has a leak. Now I know.

Alumina, Al2O3, otherwise know as aluminum oxide is a fundamental industrial material. Almost as hard as diamond. Very, very refractory! In one of it's many forms it comes in sheets. Very flat sheets. Along with making up 50% of composition of the LTCC green tape.

Considering the materials extreme hardness, how exactly is it conveniently cut?

The waterjet works brilliantly! It takes about 55 seconds to cut a 25 mil thick plate with a 2.25" circle.