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Connecting Stainless Steel Electrodes to a PCB

Discussion in 'Electronic Design' started by [email protected], Jun 19, 2007.

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  1. Guest

    I want to connect 316 stainless steel wire to a printed circuit board.
    I understand that I cannot solder the stainless steel wire to the
    circuit board so I am wondering if there is another way to connect the
    wire to the board. I am thinking about using a receiving pocket and
    treating the wire as a "pin." Do you have any other suggestions?

    Thanks,
    James
     
  2. John Larkin

    John Larkin Guest

    Phoenix or Weidmuller barrier strip?

    John
     
  3. 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.
     
  4. Ecnerwal

    Ecnerwal Guest

    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
    1980s.

    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.
     
  5. Rich Grise

    Rich Grise Guest

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


    Best regards,
    Spehro Pefhany
     
  7. Guest

    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
     
  8. I am allowed to attach the stainless to the board any way I can. I
    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)
     
  9. Rich Grise

    Rich Grise Guest

    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
     
  10. Chris Jones

    Chris Jones Guest

    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
     
  11. whit3rd

    whit3rd Guest


    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.
     
  12. Barry Lennox

    Barry Lennox Guest

    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
     
  13. Barry Lennox

    Barry Lennox Guest

    Also take a look at this page :

    http://www.azom.com/details.asp?ArticleID=1178#_Soft_Soldering
     
  14. Jon

    Jon Guest

  15. Ecnerwal

    Ecnerwal Guest

    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.
     
  16. Ken Moffett

    Ken Moffett Guest

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