Soldering Basics for Electrical and Plumbing Work

What soldering does

Soldering joins metal parts using a filler metal, called solder, that melts at a lower temperature than the parts. The parts are heated until the solder melts on contact and flows into the joint. When it cools and hardens, it bonds the parts. The parts themselves never melt, which is the main difference from welding.

Three things have to work together: heat, flux and solder.

Heat

The parts need to be hot enough to melt the solder, not the solder alone. This is the single most important idea. If you melt solder with the flame or the iron and let it drip onto cold metal, it tends to ball up and fail to bond, sometimes leaving what is called a cold joint. Instead, heat the parts, then touch the solder to the joint so the heated metal melts it.

• Electrical work typically uses a soldering iron, whose tip is heated electrically. A small iron suits wires and circuit boards, and a larger one is needed for heavier connections. Adjustable temperature helps with different jobs, and choosing the right soldering iron depends on what you will solder.
• Plumbing work on copper pipe uses a gas torch because the pipe is large and conducts heat away quickly.

Overheating can damage components and insulation, burn flux and weaken the joint. Too little heat gives dull, lumpy joints.

Flux

Metal surfaces oxidize, especially when hot, and solder will not stick to oxide. Flux is a chemical that cleans oxides away and prevents new ones from forming while you work. It also helps solder flow.

Different jobs use different types of flux, and the distinction matters:

• Electronics commonly use solder with a rosin-type flux core. Corrosive acid fluxes should not be used on electrical connections, since residue can damage wiring over time.
• Plumbing uses a paste flux made for pipe joints, brushed onto the cleaned surface before assembly. Use the product made for potable water lines where relevant.

Follow the product label for each, and do not swap one for the other.

Solder

Solder is sold as wire, sometimes with flux in the core. Traditional solders contained lead, and many electronics products used a tin-lead mix. Lead-free alternatives are now common, especially because lead-free solder is generally required for drinking water plumbing in the United States. Lead-free solder often melts at a somewhat higher temperature and can be slightly harder to work. Check the label and any local rules, and do not use lead-bearing solder on potable water lines.

The wire diameter should suit the job: thin wire for small electronics and thicker wire for plumbing and heavier connections.

Soldering a wire connection

1. Strip the insulation and twist or hook the wires so they hold together mechanically.
2. Clean and, if needed, tin the wires, which means coating them lightly with solder.
3. Heat the connection with a clean, tinned iron tip.
4. Touch the solder to the heated wire, not to the iron, and let it flow.
5. Remove the solder, then the iron, and keep the joint still until the solder sets.
6. Inspect it. A good joint tends to look smooth and shiny, with the solder flowed evenly, rather than dull or balled up.

Electrical connections that carry significant current or are part of building wiring may need to follow electrical codes, which often specify approved connectors rather than solder. Check the rules that apply, and use the tools in the guide to wire connection tools where appropriate.

Soldering a copper plumbing joint

1. Cut the pipe square and remove the burrs inside and out.
2. Clean the pipe end and the fitting socket until bright, using abrasive cloth or a purpose-made brush.
3. Apply a thin layer of flux to both.
4. Push the pipe fully into the fitting and wipe off excess flux.
5. Heat the fitting, moving the flame evenly, until the flux sizzles and the solder melts when touched to the joint edge. Capillary action draws the solder into the gap.
6. Let the joint cool without disturbing it and wipe it clean.

Water inside the pipe prevents it from heating properly, so lines must be completely drained and dry. Pipe fitting tools such as cutters and cleaning brushes make the prep easier.

Safety

• Soldering tools get very hot. Use a stand for the iron and never leave a hot iron or torch unattended.
• With a torch, keep flammable materials away, use a heat shield behind joints near wood or insulation, and keep a fire extinguisher or water source close.
• Ventilate the area. Flux fumes can irritate the eyes and lungs, and a small fan or fume extractor can draw them away from your face.
• Wear safety glasses, since solder can spatter.
• Wash your hands after handling solder, especially solder containing lead, and keep it away from food.
• Disconnect power and make sure circuits are dead before soldering anything electrical.

This article is general information. Plumbing and electrical work in buildings may be regulated, so check local requirements.