Proper way to contact this coax feedthrough's 2.4 mm pin - Page 1
Forum section: electronics / general

author: felisnigripes
author_rank: Newbie
timestamp: April 12, 2023, 11:10:37 am
content: Dear Forum,a high vacuum chamber in our lab utilizes these coax connectors with a standard BNC connector on the outside and a 2.4 mm pin on the inside:https://shop.allectra.com/product-images/4aa99f3b37fb4dcac0bf7e3cf79a52e7.jpeg/largeWhat is the intended and proper way to contact it on the inside?A low-cost option is desired. It should connect to enameled copper wire in our setup.Best regards

author: tooki
author_rank: Super Contributor
timestamp: April 12, 2023, 11:47:48 pm
content: It took me a whole two minutes of browsing the manufacturer’s website to find the cables they sell to use with them (380-IVCX-500 and -1000).Anyhow, a size 10 solder contact should work too. For example, Mill-Max 0712-0-33-15-08-27-10-0. You’d just need to verify whether the materials used are acceptable for your vacuum-side application. (I wouldn’t crimp to solid wire if it can be avoided. I also would avoid solid wire entirely if it can be avoided. Especially really thin solid wire.)Also, are you sure your wire is enamel? For vacuum it should be Kapton or Teflon. (I work in a physics lab that does lots of vacuum and cryogenic stuff, so I’ve crimped literally hundreds of vacuum-side contacts onto Kapton and Teflon wires and cables…)

author: felisnigripes
author_rank: Newbie
timestamp: April 14, 2023, 10:40:23 am
content: Thank you for finding the cable that goes with the connector. It is however a bit expensive for my case, especially since I will need 16 of them. The solder contact however looks exactly like what I was looking for!Why is solid wire to be avoided?Since cost is a factor in my case and this is the wire we have, I am inclined to try it first. I have seen some high vacuum setups using soldered enameled wire reach a vacuum level of 1e-5 mbar (which I also aim for). But I can imagine that this would be unthinkable for UHV.I am building a small setup for readout of RTDs on silicon. Working with vacuum- and electronics related stuff is all actually quite new for me so I am grateful for this kind of information!

author: tooki
author_rank: Super Contributor
timestamp: April 15, 2023, 12:53:16 pm
content: Quote from: felisnigripes on April 14, 2023, 10:40:23 amQuote from: felisnigripes on April 14, 2023, 10:40:23 amBecause it is much more prone to breaking. If the wire is nicked even the tiniest bit when stripping, you now have a weak spot (German has a nice word for this: eine Sollbruchstelle), and that weak spot is exactly where the most flexing will happen: at the connector. With stranded wire, if you damage a conductor and it breaks, there are at least 6 more conductors still left connected. With solid wire, a few bends back and forth and your only conductor just snaps right off. And I’m not kidding when I say just a few bends back and forth: with thin wire, like 28AWG (about 0.3mm), it can take just 5-10 sharp bends to break off an intact conductor, and even less for a damaged one.In the project I did a bunch of Kapton wiring for, we insisted that they switch to stranded wire, because the thin solid wire they used before kept breaking, resulting in having to re-terminate many wires in-situ, where it was much harder to do. We haven’t run into any problems with the stranded wire.(Kapton is somewhat difficult to strip, so you have to get really close to the wire inside, so it’s really easy to damage the conductor. I ended up doing the stripping on the $$$ micro-coax stripping machine, whose spinning blades’ depth can be set in 0.01mm increments, because the much faster v-cut stripping machine kept damaging the conductor if set deep enough to actually get the Kapton off.)How thick is your wire? Can its enamel be removed with heat or chemicals, rather than mechanically? Is the wire subject to vibration? Movement? How well can the wires be mechanically supported in your vacuum chamber?FYI, the wire we use is from Accu-Glass:https://www.accuglassproducts.com/wire-and-cable,mostly the 26AWG UHV Kapton stranded wire. (Thinner wires would be enough for the currents involved, but thinner wire didn’t have the mechanical robustness needed.) Aside from that, lots of RG196 coaxial cable (Teflon).The vacuum level you mention (1e-5 mbar) is well within what Teflon insulation works at, so I would recommend that over Kapton if possible. Teflon is both cheaper and easier to work with.

author: felisnigripes
author_rank: Newbie
timestamp: April 20, 2023, 08:29:16 am
content: QuoteQuoteThis is all helpful information, thank you.QuoteWe have enameled wire in all thinkable diameters, I was thinking of using something between 0.2 to 0.4 mm. The enamel can be stripped by heat. No vibration or movement, unless I want to unplug them.

author: tooki
author_rank: Super Contributor
timestamp: April 21, 2023, 06:00:09 pm
content: I wouldn’t even dream of using solid wire that thin unless saving space is absolutely essential!! That’s exactly the range of thickness that caused us huge headaches. If you absolutely must go with your enameled wire, I’d go for 0.8mm, or a bundle of 0.5mm as sort of DIY stranded. Wrap Kapton tape around the end of the wire as strain relief. (We’ve done that a lot with thinner wires.)You should actually take a look at Mill-Max 4274-0-19-15-08-27-10, which is for thinner wire than the one I mentioned above. Even so, the 4824-… is for 24-20AWG wire (~0.25-0.65mm2, or about 0.6-0.9mm diameterstrandedwire). It’s a crimp terminal but you can solder into it.And I’d still double check the datasheet for your enameled wire to see exactly what it is insulated with, then verify that material’s outgassing properties.But in your position I’d get some Teflon insulated stranded wire.It’s not that expensive.

author: tooki
author_rank: Super Contributor
timestamp: April 21, 2023, 06:05:57 pm
content: P.S. Just in case you didn’t know: metric enameled wire is measured by diameter, not cross-sectional area. So for example 0.25mm2wire is about 0.6mm diameter. 0.2mm diameter is only 0.032mm2! That has practically no mechanical robustness.