Air-Tightness of PC-Board (i.e. PCB as Vacuum Seal) - Page 1
Forum section: electronics / general

author: jbe
author_rank: Contributor
timestamp: January 20, 2016, 02:47:52 pm
content: Hi There!I'm currently working on a new connection-panel for a samplestick of a cryostat; therefore I need to bring about 24-30 signals (coax) from a "vacuum" (~0.1-10mbar or 1ATM of He) to the outside world.As I would like to keep grounds seperate between the signals (-> star-ground next to the sample, down in the cryostat) the Connectors' ground should be isolated from its mount, however i've not seen vacuum-sealed isolated connectors. The other option is to use standard connectors (i.e. SMA) and have a isolating panel, but This option is not the greatest in terms of shielding.So I had the following idea: Why not just use a PCB as interface panel, solder in PCB-mount connectors and connect the cabling on the inside. (here also a two-layer PCB with a groundplane on one side would serve as rather nice rf-shield)Now my question:Is a standard FR-4 PCB air/He-tight? (at room temperature)Any comments or questions are appreciated

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 20, 2016, 03:10:47 pm
content: I don't think so, and I understand standard FR-4 is atrocious at outgassing, so, good luck with that too.You should probably ask a reputable PCB manufacturer, what would they would recommend for vacuum and sealing applications, if anything.The other side is, if you have any through holes or through-board components, will they leak? And, if not, but you have SMT parts, then will they be too strained and crack under the flex due to withstanding air pressure?Don't be surprised if vacuum sealing is hard. As far as I know, all that stuff is expensive, and probably with moderately good reason, too.Tim

author: jbe
author_rank: Contributor
timestamp: January 20, 2016, 03:34:02 pm
content: Thank you for you comment!The outgassing should not be too big of a problem, as we already have smaller boards mounted inside of the enclosed volume.This works surpsingly well. People always talk about the material sucking up water, so I was a little bit worried in the beginning,but so far I never had a problem even at temperatueres as low as 1.5-1.7 Kelvin.Also in the reasonably high vacuum of an electron microscope (about 2E-6 mbar) I never encountered and problem.The system in question will never be operated at pressures lower than 1E-2 to 1E-3 mbar, as only a rotary vane pump is used.Vias and through-holes of course have to be sealed with solder, which should provide a nice seal.Leaky components also should be no problem, as only theis legs will pertrude the board (a.ka. "the seal").And as long as these are properly soldered in and aren't hollow this should also be fine.So remaining would be a possible leakage though the FR4 laminate, that was basically my question.-Jonathan

author: Koen
author_rank: Frequent Contributor
timestamp: January 20, 2016, 04:01:31 pm
content: Hello, not sure my answer will help but I do use stacks of FR4 boards covered in copper as vacuum suction cups for various things. It's easier, cheaper, faster, planer than having other materials milled.

author: TassiloH
author_rank: Regular Contributor
timestamp: January 20, 2016, 07:08:50 pm
content: Have a look at these:http://www.connectpositronic.com/?media_dl=2302The XAVAC BNC series with option S1400 has isolated BNCs, up to 4 per block. These feedthroughs are not as good as welded glass-insulated feedthroughs, but mounting/re-using is easy. I have used their Sub-D series in an application at <10E-6 mbar without a problem.At 10E-1 mbar, your PCB solution will work. But I think you will have to make several smaller boards for mechanical reasons (inward bending of the board). You will have to machine an o-ring groove into the counterpart of the board so that the O-ring stays in place.At 10E-3 mbar the leak rate might be too high. Coating the whole PCB and all vias not only with solder, but with also epoxy resin might help. Of course it depends on the available pumping speed.It will be pretty leaky for Helium, though. I mean, even the Buna/Viton O-rings leak He to a certain amount. I'd think the epoxy boards will be worse.

author: HAL-42b
author_rank: Frequent Contributor
timestamp: January 20, 2016, 07:29:48 pm
content: This might help

author: dmills
author_rank: Super Contributor
timestamp: January 20, 2016, 10:47:45 pm
content: Pretty much **ANYTHING** leaks with He!10e-1 mbar is a very pony sort of vacuum so many things my instincts say will not work probably will if you have a sufficiently fast pump (and fat enough lines to avoid choking the pump).I would leave the maximum possible amount of bare copper on the inside face of the board and maybe get it gold plated, at 10e-1 mechanical issues are likely as much of a problem as outgassing I would have thought, and push comes to shove you can always paint torr seal all over the thing!It wont cost much to try anyway.Regards, Dan.

author: jbe
author_rank: Contributor
timestamp: March 01, 2016, 08:49:30 pm
content: So, I tried it and it seems to work quite nicely for my applicationHere are some images of how it went down. I could detect any additional leakage in the vacuum system after introducing the PCB-Panel.Now it's time to hunt the 50Hz-ghost down

author: HAL-42b
author_rank: Frequent Contributor
timestamp: March 01, 2016, 10:12:23 pm
content: It is a surprisingly large area. I'm surprised it worked without caving in. Good job.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: March 02, 2016, 04:48:46 pm
content: Quote from: jbe on March 01, 2016, 08:49:30 pmI might have a use for this at some point soon. Is that just a regular PCB, nothing special?

author: jbe
author_rank: Contributor
timestamp: March 02, 2016, 05:14:39 pm
content: Quote from: PointyOintment on March 02, 2016, 04:48:46 pmYep, it's just a standard double-sided PCB (hand-etched, even). I left as much copper on as possible and tried to keep the pads reasonably large, to prevent leakage. I also intentionally left the copper raw, to provide a better seal with the O-ring of my box (so, no chemical tin-plating, HASL, solder-mask .. etc. gold-plating should be fine though imho).Plated through-holes would have been ideal, but in the timeframe where I needed to get this done and up&running this was unfortunately not an option. I tested the seal beforehand on the same system with a bare piece of FR4 with all the copper stripped and this already seemed to be reasonably tight.My next test was the completed PCB, etched but instead of putting in 100EUR worth of connectors I just plugged all the holes with solder.Keep in mind, that my requirements on the vacuum are quite low (as stated above), so by now I would not recommend to use this for any vacuum below 1E-3mbar as I have not tested it.