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Was wandering whether anyone could suggest how to solder stainless steel. I know hydrochloric acid flux is suggested to remove oxidation. Anyone could suggest a supplier in UK or who would be able to deliver to UK.

The part we have is a folded miniature box (approx. 2x4x3mm). We require to close up 2 gaps of stainless steel material by soldering the surfaces together, however conventional flux does not seem to work...

If anyone has any suggestions on technique or super aggressive flux supplier suitable for this procedure I would be most grateful for advice.

Thanks in advance.

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  • \$\begingroup\$ My intuition is that you could apply copper vitriol (hope it's called the same in English) to a region to have a copper surface on there \$\endgroup\$
    – Marcus Müller
    Commented Feb 27, 2017 at 16:49
  • \$\begingroup\$ That must be cupric sulphate. \$\endgroup\$
    – Leon Heller
    Commented Feb 27, 2017 at 16:56
  • \$\begingroup\$ If all else fails, silver brazing (with borax flux) works very well on stainless steel - but at about 650C. \$\endgroup\$
    – user16324
    Commented Feb 27, 2017 at 17:09
  • \$\begingroup\$ Our soldering irons go only up to 400C and this is a micro part required to be soldered under microscope, so ideally would prefer a flux. Copper sulphate is interesting idea. Any resources how to use it? \$\endgroup\$
    – Tad Vaas
    Commented Feb 27, 2017 at 17:13
  • \$\begingroup\$ @TadVaas 1. dissolve in water. 2. apply to steel. 3. Wait for copper to deposit within a couple hours. 4. wash off residue 5. solder onto copper surface \$\endgroup\$
    – Marcus Müller
    Commented Feb 27, 2017 at 19:33

4 Answers 4

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I used Ruby Fluid flux to solder #430 stainless steel and it worked pretty well.

I tried using two types of solder

  1. Chipquick RASWLF.015 and RASWLF.031 (SN-96.5% Ag-3.0% Cu-0.5%)
  2. Radioshack 60/40 rosin core solder (Sn-60% Pb-40%)

My process was as follows.

  1. Scratch up the surface a bit. I used the stainless-steel tweezers at my workstation, but a bit of sandpaper would also work. Please note that I did some tests without scratching the surface and I couldn't get the solder to adhere. Scratching the surface appears to be essential.

  2. Preheat the metal (either using a heat gun or the soldering iron)

  3. Using a cotton swab apply the flux. This was Ruby-Fluid in my case.

  4. Apply the solder using the iron.

enter image description here

The test samples appeared to come out good. I got good adhesion with both the silver-bearing solder as well as the regular tin-lead stuff. Once things cooled down, I could pull on the solder-samples quite a bit with my tweezers and the joints did not loosen at all.

I was actually surprised that the regular tin-lead solder worked. When I did some web searches on this in the past people were always saying that using silver solder was essential. But my test proved that non-silver-bearing solder can work also.

Also, if anyone was wondering, I made sure to clean off the soldering iron tip between soldering each test sample. Therefore, I am sure there was no significant amount of leftover silver on the iron when I soldered the tin-lead sample.

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Stainless steel solders very well with nothing more than dilute HCl. Special flux is not required. I'm not sure this is true of all alloys, but it is of the ones I've used. Use all appropriate safety precautions and clean the soldered surface with water.

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I used to do a fair amount of soldering of stainless with this flux -

EutecTor 157

I would imagine it's available world-wide, but I don't know for sure.

If I remember correctly, I have also used "standard" zinc chloride flux (ie, zinc and HCl) successfully, though some mechanical abrasion (brushing with a stainless steel brush) with the flux film in place is helpful in that application. It's not quite as active as the 157.

Clean up well after using either one.

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It might be worth it to mechanically attach a contact. What about you bore a small hole, tap it, and screw a miniature brass screw into the same with enough force to ensure contact by tension?

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  • \$\begingroup\$ We have no real estate on the part to attach a screw. The soldering perimeter needs to be less than 2mm square. \$\endgroup\$
    – Tad Vaas
    Commented Feb 27, 2017 at 19:47
  • \$\begingroup\$ you seem to be underestimating mechanical engineer's abilities to produce tiny screws. \$\endgroup\$
    – Marcus Müller
    Commented Feb 27, 2017 at 19:48
  • \$\begingroup\$ alternatively, what about bolting something on? \$\endgroup\$
    – Marcus Müller
    Commented Feb 27, 2017 at 19:57
  • \$\begingroup\$ The are that needs to be joined together is approx 2mm. Not sure bolting is viable. \$\endgroup\$
    – Tad Vaas
    Commented Mar 17, 2017 at 22:16

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