Hello,
We need to get a small quantity of NON-fire retardant PC board material for an ion chamber project. This will be under high vacuum conditions, so fire is impossible. We need to avoid contamination of the very low-pressure gas in the chamber due to outgassing of the fire retardant. I've looked around, I can't find anybody that seems to carry this material anymore.
We also need to get some etched boards made with the same stuff, so links to PCB fabricators that work with non-FR boards would also be appreciated.
So, just to not confuse people, we will be making some of the simplest boards by manual techniques, and having the more complex ones etched by a fab, as it needs plated through holes.
Thanks,
Jon
Are you specifically having problems with FR-4, or with anything that doesn't burn?
There's a popular material I see mentioned for RF stuff -- "Rogers mumble mumble". I believe that it's teflon, or teflon-impregnated glass. You may have a better chance getting it, and if it's compatible with your ion chamber then your problem is solved.
--
Tim Wescott Wescott Design Services
Tim Wescott wrote:
We don't even KNOW! My boss said we need to get PC board material without the FR stuff, because in the "old days" the PBB would outgas. PBB (polybrominated biphenyl) is no longer made, now they mostly use TBBA, but that is at least remotely similar to PBB. What we do need is a material which does not outgas in a high vacuum system.
Yup, Teflon board should be pretty vacuum-compatible.
Rogers Duroid... and their puff sheet says that it's a low-outgassing material, approved for spacecraft applications.
It's pricey - even at surplus it's running around $100 for a 12-by-18 sheet - but might be worth it for a one-shot project.
I have a LOT of experience with this! electron beam stuff 10^-9 to 10^-12 torrs
Do NOT use FR4
for cheap, use polyimide, not to be confused with polyamide.
Also called 'flex PCB' so should be readily available. most PCB houses can work with it.
Be VERY careful in your construction. You can end up capturing outgassing materials. Most important think like being in a thermos, you have ABSOLUTELY no convection cooling. OpAmps with fairly light power into them without enough metal to pull the heat out will unsolder themselves and fall off your board. give or take 40 seconds.
sadly I can't recall how I handled the vias now. whther had to leave unplated, or filled sorry will check notes.
RobertMacy wrote:
We need rigid boards to be self-supporting.
Yes, we are looking at Rogers materials. We don't need much, just a few square inches.
I think we want them un-tented, but plated should be fine. In this particular case, there will be NO electronics in the vacuum. We have run up to 180 W in hard vacuum in a past project, it had water cooling to a big plate and then conduction cooling from all the heat sources to that plate. Certainly a nightmare, but they HAD to do it that way!
In this particular project, there will be a very low pressure gas flowing all the time through the chamber, so I'm really not so sure why outgassing is a problem. I haven't figured out how many replacements/hour of gas are being provided, though.
Maybe it would help if you made/proffered a list of candidates. Then go to the manufacturers or data.
I use Panasonic's Megtron-6 for downhole work,up to 200C. My source has been Matrix Electronics. Could supply you some if low volume and slicing sheets not required.
Technically speaking FR4 is a glass-fibre-reinforced composite material where the glass-fibres are bonded with a fire-retardant (FR) epoxy resin.
You can get poly-imide bonded glass-fibre substrates, which tolerates high temperatures better (which is why we used for a specific application at Cambridge Instruments UK back in 1984). What we got back in 1984 was just as stiff as regular FR4-bonded glass-fibre reinforeced boards.
Try ringing around the printed circuit board manufactures you know. People like Roger supply in quite large sheets.
We had a six-layer board made with the two out layers made with isocyanate resin-bonded Teflon cloth (for low dielectric constant) and the manufacturer charged us 1000 UK pounds per tripple-extended Eurocard board, primarily to cover the cost of buying a single sheet of the substrate.
If the printed circuit board shop has to buy in a sheet of substrate for your application, they'll be both unethusiastic and expensive.
Think about heat-sinking paths through the board and it's mechanical supports. At one point we clamped a heavy strip of copper braid to the board, and the other end of the braid to the metal wall of the vacuum chamber. It didn't look elegant, but it worked.
Unplated vias wouldn't work very well. Buried blind vias could be a menace.
Contamination?
-- Bill Sloman, Sydney
Robert Baer wrote:
Rogers publishes a list of outgassing specs (Total Mass Loss) at 125 C for 24 hours in hard vacuum. Their specs are far better than FR4. So, I think their RO4003C will be acceptable.
There are some people selling bits of Rogers material on eBay, we may pick up some for our hand-made pieces. We will only need about 10 square inches, I think. Thanks for the offer, we'll see what develops.
If you have a relationship with a PCB supplier they may be able to sample you some.
-- John Devereux
