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  1. #1
    JST's Avatar
    JST
    JST is offline Diamond
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    At work, we have a need to fabricate some thin nichrome "brackets". These will get soldered to another component to add a small amount of resistance to an electronic assembly.

    It turns out that no other "easily available" material is quite the right resistivity, about 60 times more resistive than copper.

    However, there are two problems.

    First, soldering to it seems to be a right b%%%h. I understand that one has to get the oxide off, but..... so far we can't get solder of any sort to actually "do the solder thing" on some sample nichrome wire we have.... We don't have the option of directly brazing, as the component won't take that heat.
    And we haven't the equipment to do a reliable spot weld. This has to hold up, as there will be a couple hundred kW available to burn up parts that fall off from vibration.
    All I can come up with is to possibly braze on a piece of copper which we could then solder to normally.

    Second, that "easily obtainable" thing is not so true for sheet.... Wire we can get, but we really have to have sheet, because the parts must be flat wide pieces for electronic reasons. Even flattened wire is not good enough, we are looking for 0.015" x 0.25" strip, wider where a screw connection is made.

    I checked the web, no joy, aside from an obscure reference to sheet at some UK stove company.

    I realize that I am not giving full information, but I can't, for competitive reasons. The questions I really need answered relate to soldering to nichrome reliably, and source for thin nichrome sheet.

    Any suggestions?

  2. #2
    HelEx is offline Hot Rolled
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    Use phosphoric acid as a flux - Eutectic supply it as a liquid flux for their solders. Try not to get it very hot, it will dry out to a white goo of mostly phosphorous pentoxide on the job. Wash off with hot water.
    Goodfellow have 0.4mm (15.75 thou) as-rolled nichrome foil in 100mm wide rolls. It will be expensive...... Search for Goodfellow + nichrome.

    - Mike -

  3. #3
    Forrest Addy is online now Diamond
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    Silver braze it. Use plenty of proprietory flux.

  4. #4
    macona's Avatar
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    He cant get the part that hot.

    I have seen nichrome spot welded. Should work.

    You might try some of the Harris aluminum solder. It seems to stick to most anything.

    If all else fails there is indium solder. You can solder glass with that stuff... No kidding.

  5. #5
    D.spencer's Avatar
    D.spencer is offline Hot Rolled
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    Is a mechanical connection an option? riveted terminal or machine screw with clamp connection? I like soldered connections too but, is there another way to make the connection? maybee a female spade terminal crimped on the wire would plug directly onto that strip? if it is .250" wide. [img]smile.gif[/img]

  6. #6
    MetalMelter's Avatar
    MetalMelter is offline Cast Iron
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    I have with success used Hydrochloric (muriatic) acid to deoxidize wire that would not allow wetting of any solder no matter how hard I tried. Acid fumes are nasty to breath so take precautions as usual.

    Better yet would be to contact one of the solder supply houses and talk to a tech. There's a lot of chemistry going on here. I'll admit the Hydrochloric acid approach is not the best but that's all I had at the moment yet it did work - and nice too.

  7. #7
    gar
    gar is offline Stainless
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    Can it be laser welded?

    It is amazing what is being done in making automotive body sections today where small flat pieces of different thickness are butted together and laser seam welded. Small meaning a few pieces put together to make a side body section that is later stamped to form the finished section.

    The goal is to reduce weight, but maintain strength were needed.

    A window opening is made by shearing rather than punching.

    .

    .

  8. #8
    SAG 180's Avatar
    SAG 180 is offline Titanium
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    Ezi-weld 801 flux is quite good for soldering stainless steel so it may work on nickel-chrome also. http://www.enco.co.nz/ProductDetails.aspx?id=15698

    Don't know where you'd get it in the States but it's a zinc chloride based fluid with extra additives that really is superior to Baker's soldering fluid which is straight zinc chloride.

    Any sort of high current assemblies I've seen in nichrome such as ammeter shunts, used spot welds as any heat will loosen up screw terminals and once it really gets hot the different expansion coefficients make a solder crack inevitable. 60-40 tin-lead solder will be the least crack prone solder as it's the only solder to cool straight from liquid to solid without any crystals forming and so it ends up as the most flexible solder join.

    You'd have to give some careful consideration how it expands and contract thermally for it to last long. Maybe you can join the materials with an oxy torch with a small nozzle the same way you can with thermocouple heat sensor wires.

  9. #9
    JST's Avatar
    JST
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    Thanks... will try some of the ideas.

    Details of the job include the fact that the part the pieces get soldered to has epoxy end seals at the leads, and the strip must be attached as close to the end as possible. Can't possibly get it to brazing temp, might spot weld if we have to. Mechanical fastening is not so good, too big, generally.

    Hadn't tried phosphoric, muriatic didn't do the job when tried, but there might have been other problems.

    Generally I have found anything with nickel in it to be essentially "teflon" as far as solder is concerned. This is my first try at chrome-containing stuff.

    We'd be happy with anything else that has similar resistivity, we just need 30 to 50 milliohms resistance, in the form of about 2 or 3" of strip max. Nichrome was the best initial bet.

  10. #10
    Nicolas is offline Cast Iron
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    Use "Killed Spirit" as a flux, it's made by adding zinc chips to fuming hydrochloric acid until it stops reacting.
    You can solder directly to any stainless or nickel based alloy with this stuff,
    Regards,
    Nick

  11. #11
    jim rozen is offline Diamond
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    Replace the nichrome alloy with copper-nickel
    or manganin alloy. Both can give you much
    more resisitivity than nichrome and are trivially
    soldered. Stainless steel is another choice
    but you need to use that hot flux (muriatic
    acid based) to solder. But that is still easy
    compared with nichrome.

    If you feel you *must* use nichrome, terminate
    the strips with wrap-around bits of something
    that is solderable and will spot weld. Copper
    won't work well, things have to have decent
    resistivity (larger than zero) for this. Think
    steel, stainless steel, etc.

    Unless you really *need* the high temperature
    capabilities of nichrome (and it sounds like
    it won't be getting hot in use, at all, based
    on the epoxy feed-thrus) then use a different
    alloy. You're just shooting yourself in the
    foot with nichrome.

    Cupronickel,
    Manganin,
    Stainless.

    Jim

  12. #12
    Keith Krome is offline Stainless
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    Tix Solder might work. It is an indium bearing solder alloy used often in the jewelry business. I've used it for gold soldering, but they claim it will solder a broad range of materials.

    It is expensive, but can be bought in small quantities.

  13. #13
    BobWelland is offline Stainless
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    You might give these folks a call - they might be able to help you get ribbon or sheet that meets your needs:

    MWS Wire

    I would expect that they can tell you if what you want exists.

    I have no affiliation with this company nor have I done any business with them.

    Cheers,
    Bob Welland

  14. #14
    tattoomike68 is offline Stainless
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    I have welded nicrome to copper with a spot welder. The nicrome was .004" thick and the copper was .020".

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  16. #16
    JST's Avatar
    JST
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    Jim, the nichrome is for the resistivity..... Not much has "high" resistance in a piece the size we need.

    Iron and other items you mentioned mostly have pretty low resistance, quite close to copper, on a relative scale, while nichrome is about 60X the resistance of copper.

    Manganin is about 1/3 the resistance of nichrome. Never tried to solder to either it or constantan, which is a little higher resistance. I'd have to look up their strengths, to see if they have the mechanical strength for the job.

    Essentially, a strip of the width mentioned needs to go on each end of the component, close to the lead exit. It needs to hold up the component, which weighs maybe an ounce or somewhat less. The component forces the strip to be at least an inch, but there isn't space for longer, and there are other reasons for it to be shorter rather than longer.

    I tried pickling in acid, and the nichrome still laughed at solder. I don't think solder is going to work.

    I HAVE soldered to the heavy flat wire used in large resistors (the open type), it however is a different alloy with a very low temperature coefficient of resistance.

  17. #17
    jim rozen is offline Diamond
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    Manganin is wicked strong. Don't try to snap
    it like you do copper wire! You'll leave a few
    fingers behind.

    There are other alloys besides nichrome that
    are typically used for resistors. Evanohm is
    one that I can think of.

    You're looking for ribbon, right?

    Jim

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