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We are attempting to electrically connect an SMA connector to two pieces of stainless steel to measure the capacitance between them for a school project.

In the picture below, you can see the two pieces of stainless steel, grey and blue, respectively, with a red insulator between them. The outer barrel of the SMA connector simply screws down to Stainless 1. However, the inner conductor can't simply connect to Stainless 2.

We have considered directly soldering the inner conductor to Stainless 2 using a wire and a blowtorch or machining some type of plug/outlet, but it isn't elegant. We are also worried about contact resistance introducing noise to our measurements. Have we missed any ideal way to interface these conductors?

Note: It's possible to reduce the thickness of Stainless 1 or add material to Stainless 2 if needed. The parts haven't been machined yet.

enter image description here

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    \$\begingroup\$ pogo pins may work \$\endgroup\$
    – jsotola
    Commented Jan 30 at 6:04
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    \$\begingroup\$ At what frequency (and what dimension scale)? What accuracy? Even at microwaves, it strains belief that contact resistance would dominate; as long as the measurement is done correctly (e.g. measure out-of-phase reactance directly, not just assuming |Z| ~= X). \$\endgroup\$
    – Tim Williams
    Commented Jan 30 at 7:01
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    \$\begingroup\$ If you can easily machine the stainless steel 1 part down to match the centre pin height to your soldering part would seem the better way of doing it. \$\endgroup\$
    – citizen
    Commented Jan 30 at 11:54
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    \$\begingroup\$ Maybe, this type of conenctor could be interesting: digikey.de/de/products/detail/amphenol-rf/132169/1011919 ... You could screwn it to stainless (1) to connect the shields and still add a sma cable to it. Stainless (2) could be connected with some sort of pressurefit (undersized drill to press the internal connector pin while screwing it in). Isolation could be maintained by machining the parts correctly sized. \$\endgroup\$
    – ElectronicsStudent
    Commented Jan 30 at 14:48
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    \$\begingroup\$ Could you use a different material like brass? Silver soldering (brazing) to stainless requires a nasty flux, but it can be cleaned with water. I do like the pogo pin idea. \$\endgroup\$
    – qrk
    Commented Jan 31 at 0:58

3 Answers 3

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Have you bought the SMAs yet?

You can get types intended for easy assembly/removal, with a flush female sprung socket instead of a protruding centre pin. This is intended to accept a pin that's been soldered or otherwise fixed to the board.

You could solder a pin, using 'aluminum' solder with its very aggressive flux to first tin the stainless. Or you could drill a small hole into stainless 2, and screw or interference-fit a pin into it.

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Solder designed for aluminium (with organo-fluoride flux) can tin stainless steel. Once tinned, you can then solder as normal.

There are various brands available, but I've used Alu-Sol from Multicore/Loctite with great success.

I've used it numerous times to make joints to materials that are otherwise unsolderable.

(Don't breath the flux fumes, which are much less pleasant than rosin.)

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    \$\begingroup\$ If this works then the same should be applied to both surfaces, using some suitable RG coax, which can then be attached to a SMA or whatever RF connector that would be suitable. Soldering ought to be much better in terms of contact resistance than some toothed washer, let alone just screwing the connector flat against the surface. \$\endgroup\$
    – Lundin
    Commented Jan 30 at 12:57
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    \$\begingroup\$ Interesting; I've never seen a solder for aluminum that doesn't contain zinc. I'd always assumed the zinc was necessary for some metallurgical purpose or other. \$\endgroup\$
    – Hearth
    Commented Jan 30 at 15:49
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    \$\begingroup\$ @Hearth It surprised me when I discovered Alu-Sol about 20y ago, but it turns out that the fluoride flux is the key to soldering Al (and Stainless) with what is effectively a normal soft solder. I should say, however, that the fumes are best not inhaled - much worse than rosin fumes. \$\endgroup\$
    – colintd
    Commented Jan 30 at 16:53
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    \$\begingroup\$ @colintd I imagine it's not the best idea to leave the flux residue on things, either. The datasheet says it's not very corrosive, but "not very corrosive" can still be pretty corrosive on these fluxes apparently intended for non-electronics use. \$\endgroup\$
    – Hearth
    Commented Jan 30 at 19:51
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    \$\begingroup\$ @Hearth I had that worry initially and cleaned with IPA, but since then I've done jobs without cleaning and I've had no obvious corrosion even years later. My impression is the active ingredients burn off very quickly during the soldering, and the residue is dark, "crisp" and inactive. \$\endgroup\$
    – colintd
    Commented Jan 30 at 20:30
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A drop of phosphoric acid is an excellent flux for soldering to stainless steel.

Because of the very poor thermal conductivity stainless steel doesn't need to be really thin.

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