Connecting Stainless Steel Electrodes to a PCB

Phoenix or Weidmuller barrier strip?

John

If you silver solder coat the end of the wire, then it can be soldered with ordinary soft solder. I use tiny stainless steel 'aircraft' type cable as glove heating elements and connect it this way. I have also used crimp terminals to connect to it. But it is stranded, not solid conductor.

Choose the correct flux, and your understanding becomes wrong. You WILL need to clean carefully, as any flux that will flux stainless will be highly corrosive - but you could possibly flux the wire and tin it, then clean, then use a milder flux for connecting the tinned wire to the board.

For soft soldering stainless at the high-volt lab, as best I recall, we used Eutetic Castolin 157, or perhaps it was 157A or 157B, back in the

In the AgEng metalworking lab, for stainless steel sink soldering, we used 95-5 solder and plain old acid (Zinc dichloride?) flux, with more attention paid to physical cleaning preparation as follows. Put on a glove, dip a hunk of scotchbrite in flux, scrub the wire for mechanical removal of as much oxide as possible (leaving a coating of flux on the part).

However, depending on the mechanical stresses involved (if any), you might do better to solder down a screw-type terminal strip.

Are you saying that you're unable to make solder stick to the 316, or is it more like you're not allowed to solder directly to the board?

You could "tin" the end of the SS wire with some acid flux and silver solder, clean it, and solder should stick to it.

If you're not _allowed_ to solder it to the board, I'd look into some kind of crimp connector.

Good Luck! Rich

Maybe you could plate or tin the wire (eg. with stainless solder) and then it would be solderable.

Best regards, Spehro Pefhany

I am allowed to attach the stainless to the board any way I can. I didn't realize that "tinning" was an option. I am molding these electrodes into a piece of plastic and then I need to attach the board on top of the encased electrodes. I am unfamiliar with the acid flux- silver solder technique. Will I still be able to tin when the stainless is in plastic.

Thanks, James

What makes stainless a soldering nightmare is what makes it stainless, a hard adherent Chromium-oxide &/or nickel-oxide based surface layer.

If you disrupt that layer with an agressive flux and make metal to metal contact with a tinning layer that is solderable then soldering to the tinned layer is no problem. However, do not depend upon such for mechanical strength.

Since the disruption of the passivating layer is chemical (e.g., agressive flux) so long as the plastic you use can take normal soldering temps, for perhaps longer than soldering usually takes, since even an agressive flux can take time on stainless, then no problem.

As for purely mechanical connections, while doable, they can suffer the same issues as connecting aluminum wiring to things. For the same reasons, the surface oxide layer spontaneously grows when exposed to atmosphere. So something that connects now is not necessarily going to stay connected unless the mechanical bonding is tight and avoids any motion, even temperature induced creeps.

The same kinds of connections used to connect aluminum wiring to copper will probably be OK for stainless.

Me, I'd tin. All your problems are up front, and you can deal with them then. Tin and lead oxides are semiconducting and soft and readily displaceable. They don't cause the kinds of problems that stainless or aluminum surface films (hard, dureable, adherent and INSULATING) do.

But, it is your nickle, so your call. (Pun is accidental)

According to some other responders, with the right flux, you won't even need silver solder - someone mentioned another alloy, which sounds like they have more experience in these things.

Obviously, you can't tin _through_ the plastic, but if you're worried about the pin heating up and melting the plastic housing, that depends on a lot of other factors.

Can you draw a picture or something of what you're actually trying to accomplish? i.e., what's the end result you're looking for?

Thanks, Rich

I have tinned stainless steel with silver solder and some nasty flux but that kind of solder needs to be really hot (just about red hot) so doing it after plastic is in contact with the wire is likely to be troublesome. You can then solder to the silver solder using ordinary soft solder. Often silver solder contains cadmium, especially the stuff that melts at a lower temperature than red-hot. This cadmium would be a RoHS problem in some countries and probably isn't good for you either.

Once I worked at a place where one of the guys had an unlabelled jar of some kind of grey goo that was a very fancy solder paste (nearly all flux and almost no solder, and a very fine consistency). That stuff would solder stainless but the process was slow and not easy.

For mass production, I suggest you investigate spot-welding the stainless steel to some ordinary mild steel. Both stainless and mild steel can be spot welded very easily due to the poor electrical conductivity and poor thermal conductivity. You could spot-weld on a tab of ordinary steel that is already tinned with soft solder in the place where it needs to be, or you could spot weld a stainless steel spade terminal onto the stainless wire. I have seen something like this on the end of the element in electric hotplates and irons and water heaters.

I think that the key to reliable spot welding is very high contact pressure and very high current, many many thousands of amps. You can find a lot of home made spot welding devices on the internet but most of these have insufficient current, and therefore would not work if the electrodes were pressed together with a proper amount of pressure. They make these things sort-of work (sometimes) by not pressing hard on the electrodes, so then the contact resistance is higher and the heating is then usually enough to weld even with a low current, sometimes. I think that this is not very reliable though, because it relies on the poor contact between the metal being just poor enough to get hot, but not too poor where the current would be reduced. Therefore I would start by trying out a professional spot welding machine, and only improvise if necessary and with great caution.

You might also find that it is possible to TIG-weld something onto the end of the wire. This might be especially easy if you can make your terminal in the form of a tube that slips over the stainles wire, where it would only be necessary to melt the end of the tube and the wire inside it, with a very short burst of current. This could be done before the heat has a chance to spread to the plastic. Ask on sci.engr.joining.welding, they might have some ideas.

Chris

Direct soldering is a mediocre idea, because the stiff 316 wire will just crack the tin/lead and work its way loose if any stress is applied. It also won't crimp well (soft crimp lugs won't deform the SS wire, so the crimp can rotate and come loose).

But, you can splice the stainless to a stranded-copper-wire pigtail, then solder or crimp-lug-and-bolt the copper. Lap the stainless with the copper wire, wrap the pair with a short length of fine copper wire, apply Nokorode or other acid flux (the stuff recommended for stainless) and solder. Wash off any remaining flux.

Hard solder (silver solder) is preferred for this, of course, but it requires at least a propane torch (better with acetylene/air). It can be tricky to keep from melting the copper. Small quantities of silver solder are jewelry-making supplies at art stores.

There's a few options.

Soldering is quite easy with the correct flux, I have used a Lonco product with good results. I think it's "431" I recall reading the MSDS and it's not that toxic, it's a clear yellowish liquid with a sweet smell. It must be washed off with hot water after soldering.

Spot weld it to copper wire.

Secure them into a screw-down terminal board.

Barry

Also take a look at this page :

There are many fluxes available that enable "tinning" stainless steel with ordinary tin-laed solder. After the "tinning' is performed, you can solder the tinned ss wire to the board. See, for example:

If you go with soldering tinned stainless to the board (certainly the cheapest and simplest option) it's probably worthwhile to try the following non-standard pain-in-the-behind method of mechanically connecting the wire (which is stiff, and could tear pads off if "mechanically" held in by solder, if subject to any force).

For each wire, drill two holes perhaps 1 cm apart with pads (a connecting trace that's free of soldermask is optional, but could only help). Tin the wire further than that (plain old low-temp solder is fine with the right flux). Put wire through the board, bend once at 90 degrees, bend again at 90 degrees to match the spacing between holes, and insert the tip in the second hole - pull tight to board. Bend the tip over on the top of the board, and bend the body down flat against the top of the board, so it's completely and solidly held in place by bends. Then solder it.

Jon wrote in news:1182450970.213216.272550 @p77g2000hsh.googlegroups.com:

Jon,

I'm not sure how heavy the SS wire is, but if you coat(braze) the part to be soldered (tin/lead) with "silver solder" first, it will solder easily to a PCB. see the section on Brazing in your link.

Ken