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How to solder electronics: a beginner's guide

Heat the joint, not the solder. A good joint is made by heating both the component leg and the copper pad with the iron, and then feeding solder onto them. Solder that is melted on the tip and dabbed onto cold metal sits on top as a lump and makes a bad joint.

What you need

• A soldering iron. A 25 to 40 W pencil iron is fine for beginners. A temperature-controlled iron is worth buying once you solder often.
• A stand. The iron goes into it every time you put it down.
• Flux-core solder wire, about 0.7 to 1 mm thick.
• Brass wool or a damp sponge for cleaning the tip.
• Side cutters, wire strippers and fine-nose pliers.
• Something to hold the work, such as a small vice or a "helping hands" stand.
• Safety glasses.

Set a temperature-controlled iron to about 330 °C for leaded solder, or 350 to 370 °C for lead-free. Hotter is not better. It burns the flux away and damages parts.

Leaded solder is the easiest to learn with, but it contains lead. Wash your hands afterwards and keep food away from the bench. Work with good ventilation, because the flux makes smoke.

Look after the tip

• Keep it tinned, with a thin coat of fresh solder. A black tip will not transfer heat.
• Wipe it before each joint and add a touch of solder.
• Never file a plated tip.
• Before you switch off, coat the tip generously with solder to protect it.

Making a joint, step by step

1. Prepare. Push the component through the board and bend the legs out slightly so that it stays put.
2. Clean and tin the tip.
3. Heat both surfaces. Touch the tip to the leg and the pad at the same time, for one to two seconds.
4. Feed the solder onto the joint, on the side opposite the iron, not onto the tip. It should flow around the leg and over the pad. Feed just enough to form a small cone.
5. Remove the solder, then the iron, in that order.
6. Keep still for two or three seconds while the joint cools.
7. Trim the leg just above the joint, holding the end so that it does not fly off.

The whole joint takes three to five seconds. If the solder will not flow, the tip is dirty or the joint is not hot enough. Do not hold the iron on for a long time, because LEDs, transistors and chips are damaged by long heating.

Good joints and bad joints

• Good: a smooth cone around the leg, covering the pad.
• Dry or cold: dull, grainy or cracked, or a ball sitting on the pad. Reheat it with a touch of fresh solder.
• Not wetted: solder on the leg but not on the pad. Reheat both together.
• Too much: a blob that hides the leg. Remove the excess with solder wick.
• Bridge: solder joining two neighbouring pads. Remove it with wick and check with a continuity test.

A fault that comes and goes when you tap or flex the board is nearly always a dry joint or a loose wire.

Joining wires

Slide the heat-shrink tubing onto the wire first. Everyone forgets this at least once. Tin both wire ends, hold them together and heat until the solder flows. When the joint is cool, slide the tubing over it and shrink it with hot air.

Removing solder

Lay solder wick on the joint and press the iron on top, or use a desoldering pump. Do not overheat the board, because the copper pads can lift off.

Practise on spare parts before your first real project.

This guide is based on Chapter 9 of Introduction to Electronics, which also covers building circuits on stripboard.