Electronic – Soldering Magnet Wire

soldering

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?

Best Answer

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.