PCB substrate for ultrahigh vacuum - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 03, 2021, 02:02:18 pm
content: Hi!Does anybody know any PCB substrates that are compatible with very high vacuum without outgassing like crazy?I've finished my high vacuum setup (reached 10^-7 Torr!) and would like to experiment with building some electron guns and faraday cups, and I'm toying with the idea of having a ground plane as a strip to solder stuff like ion lenses, accelerating electrodes etc. to.Not sure it's realistic, but if so, It would be nice to have the PCB material at hand.I'd like to avoid purely ceramic PCB's, as they are fairly costly, but they may be the only option.Any experiences?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 03, 2021, 02:07:03 pm
content: No experience but aluminium might be worth a try as it's pretty cheap

author: coppercone2
author_rank: Super Contributor
timestamp: January 03, 2021, 02:20:32 pm
content: I think aluminum has a bad reputation for vacuums

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 03, 2021, 02:27:57 pm
content: Aluminium is fine for vacuum, but the whole idea is I wanted something solderable. Plastic and resins like epoxy is a no-go in vacuum, except really expensive stuff, so being able to "manhattan" some structural or mechanical prototypes by solder and heavy copper wire would be very useful. Perhaps thin copper or brass sheet would be better than PCB.Brass is not that good either, as zinc has a low enough vapor pressure to cause outgassing. Tin should also be lead free tin/silver alloys.

author: coppercone2
author_rank: Super Contributor
timestamp: January 03, 2021, 02:30:04 pm
content: you can dead bug it and leave it on a ceramic, the PCB itself is not that imporant unless you need vibration resistance and stuffit will just be fragilenot sure if missile compatability is necessary for a scientific instrument

author: coppercone2
author_rank: Super Contributor
timestamp: January 03, 2021, 02:36:47 pm
content: if you make good fixtures for positoning parts (nice custom stuff) you can make any weird structure out of SMD parts you want for simple circuits, so long you stay away from no leads packages with many pins. Then you figure out how to support and it strategically glue it to a ceramic in only a few points. when you are done you can cut the glue and gently sand it with diamonds to clean up the substrate and remove the circuit you carefully elevated over it, using a minimum of glue, allowing you to reuse a substrate.this should at least get you some where without multi layer ceramic PCB, bit like model making or ship in a bottleDid you look at kapton ? not sure how flex pcb ofgasses in comparison to FR4. or teflon flex or just teflon

author: octillion
author_rank: Contributor
timestamp: January 03, 2021, 02:44:31 pm
content: I think the previous post might have been suggesting alumina substrates, not aluminum.I work regularly with all sorts of PCBs around the 1e-6 mBar range without issue, but would hardly consider this UHV. Are you planning to go well below 1e-7 Torr?Perhaps polyimide flex PCBs could have good specs (avoid soldermask/coverlay and adhesive to other layers or stiffeners).

author: LaserSteve
author_rank: Super Contributor
timestamp: January 03, 2021, 02:49:24 pm
content: Whatever you do, watch the internal heating on the components. You have no convection cooling. Even little things can get hot fast.Steve

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: January 03, 2021, 03:02:26 pm
content: Quote from: ChristofferB on January 03, 2021, 02:02:18 pmCouldn't you just use a thin sheet of copper? (I'm assuming you just want to use the material to hold pieces in place rather than build an electronic circuit).

author: coppercone2
author_rank: Super Contributor
timestamp: January 03, 2021, 03:14:37 pm
content: I would recommend strategically placing parts over a copper strip elevated by glue in select places but it does not let you do a DAC or ADC or other complex elements near the circuitry in question, you kind of need a substrate for something like a modern high resolution DAC

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 03, 2021, 04:15:15 pm
content: Quote from: ChristofferB on January 03, 2021, 02:27:57 pmI meant aluminium-clad PCB (IMS). You need a powerful iron due to the thermal load, but perfectly solderable

author: joeqsmith
author_rank: Super Contributor
timestamp: January 03, 2021, 06:36:34 pm
content: Sounds like fun. Bake out the material before installing it.https://www.arlonemd.com/

author: daqq
author_rank: Super Contributor
timestamp: January 03, 2021, 07:54:15 pm
content: Check:http://www.magazines007.com/pdf/High-Voltage-PCDesign.pdf, has some advice on vacuum PCB design and some generally good tips on weird PCB design.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 05, 2021, 10:40:19 pm
content: Thanks for the ideas!Aluminium PCB's looks very neat! I might try and get some of that!What I need is not complex circuits, but for example, for a faraday cup it would be nice to be able to dead bug an electrometer chip right at the detector.

author: coppercone2
author_rank: Super Contributor
timestamp: January 05, 2021, 10:44:14 pm
content: if you are dead bugging why not just let it float on mechanical supports? the circuits are not that fragile its better then glass. I would only bother if you needed complex SMD packs and stuff (hitttie parts would be hard on dead bug lol, but really that stuff is designed with strip line in mind). pure dead bug is better then manhattan in some cases because you don't have capacitors forming to ground.it would be the fastest way to get into this for anyone else tryingnot really sure where pads are better... it seems to be purely a mechanical consideration.And I am not getting something about the aluminum here, is the idea that you just reduce surface area of exposed dielectric by giving it an aluminum backing? The cutaways I see online still have a membrane backing in most cases. Or was it a consideration for vacuum heat dissipation?this is a surprisingly easy to read NASA experiment resulthttps://en.wikipedia.org/wiki/Materials_for_use_in_vacuumIt seems aluminum is a good choice if welding and heat is not an issue (reason why I was confused because I only ever saw stainless steel for this stuff). In an aluminum PCB unless the aluminum is of a known kind, you might run into problems, it seems vacuum compatability is alloy and treatment dependent. Using the conversion coats on the back of a PCB might also be very difficult, they are sensitive to temperatures and very difficult to apply at home from experience.Kapton is a type of polyimide film, has very low outgassing. Kapton is discouraged if a ceramic alternative can be used.so kapton flex might be the best option