Soldering a Wire Joint: Not the Same Job as Soldering a Board

Solder is a conductor and an adhesive, not a structural material. A joint relying on solder for its strength will fail, and it will fail at the worst moment. Make the connection hold on its own first. Hook each stripped end into a small J, link them, and squeeze flat. Or lay the two ends alongside each other and wrap the strands together — the Western Union splice, which was designed for telegraph wire that had to survive weather and tension. Then solder it, and the solder's only job is to fill the gaps and exclude air. Tug the joint before soldering: if it comes apart in your fingers, solder will not save it.

Put the iron on one side of the twisted joint and feed solder in from the OTHER side. When the joint is hot enough, the solder melts on contact with the wire rather than the iron and flows right through the strands. Melting solder onto the iron and carrying it over gives a joint that looks filled and is not — the solder sits on top of cold copper. That is the same cold joint the board rung warns about, and on a wire it is harder to spot because there is no shiny fillet to judge. Hold everything still until the solder has visibly gone dull and solid. A joint moved while freezing has a fractured crystal structure: it is grey and grainy rather than smooth, and it is weak. Helping hands or a vice are worth using for this reason alone.

Solder wicks along stranded wire under the insulation, and wherever it goes the wire becomes rigid. The boundary between the stiff soldered section and the flexible wire is then a hinge, and the wire breaks exactly there. Keep the joint short, use just enough solder, and do not let it creep back under the insulation. A little flux and quick work help; a big blob and a long dwell guarantee wicking. This is the same reason the crimp rung says never to solder a crimp. Wherever a wire must flex, it must be copper doing the flexing — not a stiff soldered section with a stress concentration at its end.

Slide the heat shrink on BEFORE you solder — far enough down the wire that the iron will not shrink it prematurely — then bring it over the finished joint and warm it evenly. Heat shrink grips the wire on both sides of the joint, so it adds real strain relief as well as insulation. Adhesive-lined shrink seals against moisture too, which matters for anything outdoors or in an engine bay. Electrical tape is a temporary answer. It unwinds in heat, its adhesive creeps, and a taped joint in a loom will be found hanging open years later. Use it for a test, not for a build.