PCBs in UHV

On 04/27/2009 08:19 PM, John Larkin sent:

Hello John:

I'm not tuned in to the latest PCB materials these days. However, out gassing of everything is part of the startup process. Put the cleaned/washed board in and cook with externally applied artificial heat for days if necessary. As your pumps achieve -9 to -10, you know you got there.

HTH

Pete

--
1PW  @?6A62?FEH9:DE=6o2@=]4@> [r4o7t]

Talk to one of the mass spectrometer makers to find out what is available now. And how reliable it is. It will also have to survive baking to whatever temperature is needed to enable a hard vacuum to be achieved.

When I was in that game I think they used PEEK for the internal plastic supports and copper pins. Not exactly mass production stuff. They put the least electronics they could into exposed hard vacuum because of the difficulties.

And not all PEEKs were created equal so they had to test every batch.

Regards, Martin Brown

At those 'pressures', only a little outgassing can be a real show stopper. If you can get away with putting the electronics outside of the vacuum, that should be the way. Otherwise, you might try ceramic substrates. Everything should be thoroughly clean and dry. Outgassing of individual components, or any crud stuck underneath is also likely to be an issue.

Regards, Jeroen Belleman

Thick film hybrid construction on the standard alumina substrate?

If you can find anybody who still does this ...

-- Bill Sloman, Nijmegen

I've seen them done on a ceramic substrate.

Have Fun! Rich

It can also depend on how absurdly large/small your pumps are - a massively overpumped system can tolerate all sorts of crap that an underpumped system would never manage (10e-10 with a plexiglass window? with a cryopump designed for a lower pressure and a larger connected system, it can be done - with no baking!) - but I would 4th or 5th the "minimize what's in there" advice. Any component with guts, or plastic etc. that can offgas will offgas. Ceramic and metal packaging is the order of the day, and a ceramic substrate is your best bet if you have to put a board in there.

You might (depends on how this will be used, or what some idiot can do wrong) need to take precautions against it being powered up as the system is pumped down, as there's a delightful range well above where you want to work that makes pretty plasma (I worked during college in a plasma lab.) Could make any powered up electronics go a bit whacky, to say the least - also IIRC tends to help lay down carbon tracks, if there's any carbon to work with.

Use feedthroughs and get out in the air as soon as possible.

--
Cats, coffee, chocolate...vices to live by

I've used PC board materials (fiberglass, high glass content) in high vacuum with no problems, BUT it was on the cold side of a cryostat.

Most components (the paint on resistors, the epoxy on ICs, the vinyl sheath on electrolytic capacitors) aren't good in vacuum, and aren't bakeable enough anyhow. If it's a simple circuit, you might be able to weld one up, or if it's more complex, consider welding a box for it that has glass/metal, or ceramic feedthroughs, so that only some external wiring comes out to the vacuum.

For quick/dirty fabrication, a spark plug is a good ceramic feedthrough, and the base of a metal vacuum tube is an array of feedthroughs.

Notoriously horrible. Use alumina.

Cheers

Phil Hobbs

Maybe in production; or even better, AlN, to keep the chips cool.

But we need to test a prototype, so some sort of polyimide or Rogers laminate will have to do for now.

If I copper-pour most of the top and bottom, and gold plate, that might cover a bunch of the surface, and help a bit.

John

Yes, PEEK is god, but wouldn't glass be better?

[snip]

Glass is somewhat limited in how it can be machined ...

Rene

Have you ever tried to drill a lot of holes in glass?

My recollections are that they did have trouble with batch to batch variation that only really showed up when they tried to bake the machine down to full hard vacuum working pressure. So you should be pretty careful and do tests to avoid making boards that cannot be pumped down. It may be better now, but I wouldn't hold your breath.

Regards, Martin Brown

But Macor glass-ceramic is easily machined and suitable for UHV bake-out.

I postdoced in a lab that did a lot of UHV. (I was/am far from an expert and just did what they told me to do.) But they did not want any lead or tin in the UHV system. You'd have to look up the vapor pressure of these metals. But they can be a problem... especially during the bakeout process.

George Herold

It may actually do the opposite - whatever is in there to offgas _will_ offgas - if you make it harder for it to get out, it will offgas for a longer period of time. You might do better to minimize surface coverage and/or swiss-cheese the board with non-plated holes to allow trapped material to get out and be pumped away.

If the system is overpumped, and the location of the board is nearer the pump than the location that really needs to be UHV, "sealing" it might be OK. If the board is closer to the location that really needs to be UHV, making it as easy as possible to offgas will be better, as you'll get the material that needs to leave out of the system faster.

At low pressures, you don't have "mass flow" and pumping as we are used to thinking of pumping with normal fluids - it becomes a game of individual molecules bouncing about - so even if you have a massive pump, if your gassy board is near the area of interest, it will be raising the local pressure as long as it's offgassing.

As a crappy and possibly useless analogy, consider a wet sponge in an area you are trying to keep bone dry with a dehumidifier. If the sponge is just exposed, it will dry out fairly fast. If you put plastic wrap over the sponge, it will take much longer to dry out.

--
Cats, coffee, chocolate...vices to live by

Yes, the diffusion situation gets complex if the board is partially covered with copper. If I cover 95%, the crud rate drops by (almost?)

20:1 but long diffusion-type exponentials are introduced, some of which may have decades-long tails if the escape distances are long. Messy math.

Seems like it may be better to cover as much as I can; the fast stuff will get out near the openings, and then maybe I can get my testing done. I suppose I'll have to talk to a real UHV jock before I finalize things.

John

It sounds like you don't like the alumina substrate idea; there are also (in mass production, I've seen 'em) steel circuit board options. Porcelainized steel, with additive copper wiring, can support a lot of circuit complexity, and it's heatsink-friendly (which could be important; vacuum DOES act as an insulator).

I'd suspect that with any of today's boards, you'd never achieve UHV in the first place.

Just bite the bullet and use ceramic.

Or, if you've got a UHV-capable bell jar, just have one of your techs do the experiment.

I'd also suspect that after the test, the bell jar and pumps would have to be thoroughly baked out.

You don't use oil diffusion pumps, do you?

Thanks, Rich

But it can, if absolutely required, be moulded int complex shapes. And wires can be integrated for power and signal connections as long as one chooses metals and glasses that have rather similar expansion vs temp coefficents.