Having trouble soldering headphone wires

Question

I was fixing my friend’s headphones today and I’m having trouble soldering the tiny wires. I know they have a tiny coating layer that works as an isolation, I have stripped this layer using a gas burner (not sure if that’s the right word, I’m talking about Dremel Versatip, a kind of a lighter on steroids).

There was one wire that wasn’t isolated and the solder sticks to it perfectly, flowing all over the wire and creating a nice top layer. However the other wires don’t seem to like the solder at all, it’s far away from the situation on the first wire. What am I doing wrong?

Answer 1

You should use a wire stripper or a pair of pliers to strip insulation from wire. Small wires can be stripped with your bare hands: just pinch the insulation with a fingernail and yank it off. If you catch it just right, the insulation tears and slides off, but the conductor is left intact.

It sounds like by burning the insulation, you have fouled the metal with the products of combustion of the plastic. This layer will prevent the solder from flowing onto the metal and bonding with it. Even if the plastic was completely vaporized, it's likely that the metal oxidized. Solder does not like oxidized surfaces. To solder, you need clean surfaces free of oxidation or other residue. (Other than soldering flux. Speaking of which, you should have some and use it!)

One of the functions of soldering flux is that, being mildly acidic, when it gets hot, it etches away the thin layer of oxidation from copper. It will probably not eat through the carbonated remains of burnt insulation, however. Still, it is worth a try. If flux doesn't cut through, then just cut a piece off the end of the wire and try again, and this time strip it mechanically rather than, err, pyrognostically. :)

Update: if the wire insulation is just a fine lacquer (similar to what is used on wire used for winding voice coils or transformers), it may be possible to solder with the lacquer in place, with better results than torching the lacquer. Some hobbyists spray lacquer on DIY circuit boards, without covering the solder pads, and solder right through it anyway. It may also be possible to remove the lacquer by using a solvent such as acetone.

Answer 2

Usually what I do is to hold the iron tip with the solder on the tip of the wire for a while, until I see that the solder melts on the wire. That means that the protection layer is gone, and then I put some new solder on.

It takes a few seconds before the coating melts, be careful to have some headroom to avoid melting the rubber coating.

After soldering, remember that you removed the insulating layer, and you have to replace it with heat shrinking pipe or something else.

Answer 3

Transformer manufacturers use some fluid into which to dip the wire ends.

Methylene Chloride - a chemical that deserves respect - works for some materials.

One source suggested using a Bic lighter to make the wire red hot meaning that the insulation is a goner.

Answer 4

You don't use flame for reaching bare metal - what covered the metal will burn and combustion products will coat the surface. You have to mechanically clean the surface by using something like a knife or a piece of finest sandpaper.

Answer 5

Some wires have (in addition to insulation) a strand of high-tensile string wound with the wires, to reduce breakage. If you do not trim back such a string, it might melt, wet the strands, and prevent solder from adhering.

Usually, fine strands aren't individually insulated, but if they WERE, one could use a small solder pot (dipping the wire ends can burn through polyester varnish and the varnish acts as flux).Solder Pot