Clean, hot, quick. That's the secret to a good solder joint.
Soldering joins wires with a soft metal alloy melted around them. Done well, it gives a strong, low-resistance joint.
1. What you need
- A temperature-controlled soldering iron (around 30 to 60W is plenty for most wire).
- Electronics solder with a flux core.
- A damp sponge or brass wool to clean the tip.
- Heatshrink tubing and a heat gun or lighter.
- Something to hold the work, such as helping hands.
2. Step by step
- Slide the heatshrink onto the wire first. It's easy to forget.
- Strip about 10mm of insulation.
- Twist the strands, then join the wires (side by side or a twist splice).
- Clean the iron tip and put a little fresh solder on it.
- Heat the wire, not the solder. Touch the solder to the wire on the far side of the iron. It should flow into the strands.
- Remove the solder, then the iron. Don't move the joint while it cools.
- Slide the heatshrink over and shrink it.
3. What a good joint looks like
- Smooth and shiny, with the strands still visible under the solder.
- Solder has flowed right through the joint.
A dull, grainy or lumpy joint is a "cold" joint. Reheat it with a touch of fresh flux.
4. Where not to rely on solder
In vehicles, solder can make the wire stiff right at the joint and it can crack with vibration. Support the wire, or use a quality crimp. See Crimping Terminals Properly.
5. Safety
- The tip is hot enough to burn badly. Always put the iron back in its stand.
- Work with ventilation and keep your face out of the flux smoke.
- Wash your hands after handling solder, especially if it contains lead.
- Never solder on a live circuit.
6. Quick summary
- Heat the wire, feed the solder.
- Shiny and flowed is good. Dull and lumpy is bad.
- Heatshrink on first.
240V AC mains can kill. In Australia, work on household mains wiring must be carried out by a licensed electrician. Low-voltage DC can still deliver very high current, so treat batteries with respect.
