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I am using low gauge magnet wire, wrapped in cigarette paper soaked in GE varnish, as pegs to solder to inside of a cryostat.

However, I have found it nearly impossible to solder to the magnet wire, at least not at low temperature. I strip the insulation off with a razor blade, apply flux, then try to solder at 400 degrees F. That never works.

If I up the temperature to 450 degrees F, the wire whets as expected, but the insulation changes colors (i.e. it gets darker). It seems to still be insulating, but I am not sure if any harm was done. Additionally, I would prefer not to solder at such a high temperature, as some of the things I need to solder are very fragile and should not be soldered at temperatures greater than 400 degrees F.

I would have thought that the "tinning" process I just describes above would make it possible to solder at 400 degrees F, but that still won't work. The tinned solder wont melt until it gets hotter, despite the fact that fresh solder will melt on the iron's tip.

Am I doing anything obviously wrong here? Does anyone have advice on doing apparently impossible soldering jobs?

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The normal insulation used in cryogenic work is Formvar, not the solder-through polyester or urethane insulation used on common magnet wire. It's also common to use high (electrical) resistance wire because the thermal conductivity of such wire is lower. Such alloys do not soft-solder well.

Suggest you procure some fresh magnet wire with solder-through insulation if you need neither of those characteristics. Since your wire is fat you may need to heat sink the wire to control where the insulation stays.

If you need to use the Formvar insulated wire, it can be removed by special abrasive stripping machines or by very hot caustic chemicals (again, you can buy such an apparatus). You still may need to weld rather than solder if the metal is not copper, or use a more active flux. In the latter case, be sure to clean thoroughly.

Another approach would be to use bare nice shiny pre-tinned wire and add insulation in the form of heat shrink tubing, ceramic tube insulators etc.

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What type of solder are you using? The antimony/tin solder is much more difficult to use than the more traditional lead/tin solder. Sadly, the world is phasing out lead and it is harder to find now days.
Do you use fine sand paper on the wire and well as scraping, you should. I have found that a hotter iron will heat your component less than an iron set at a lower heat. When you place a cool iron on your wire, you have to hold it on while the wire heats up. More heat is conducted along the wire over time to where you don't want it. When your iron is hot, it heats locally quite quickly, you complete the solder joint faster and the undesirable heating is minimized. Also, clip a small alligator clip as a heat-sink (copper works best) between where you are soldering and the component you want to protect. This is a standard soldering technique.

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  • \$\begingroup\$ Thanks for the advice! I will try to sandpaper harder, in addition to scraping. I am glad to say that my lab is still using 60/40 tin/lead solder, which I have (previously) never had any problems with. \$\endgroup\$ – J Ellington Sep 12 '15 at 20:35
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For magnet wire I like to set the iron as hot as possible and get a small pool of solder on a chisel tip. I plunge the end of the wire into the pool, and the insulation burns away in a matter of seconds. I then turn down and cool off the iron to do the actual soldering of the now tinned magnet wire.

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I normally have the soldering iron set to 750°F or so to solder magnet wire with polyurethane insulation and that still requires scraping to get access to the copper.

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