Soldering is one of those skills that seem hard until you understand the principle: you don't "glue" the wire with molten solder, you heat the two metal parts until the solder, touching them, flows on its own and bonds them. Almost every ugly joint comes from ignoring this principle. With the right tools and a few rules, a few hours of practice are enough to make reliable joints.
The essential tools
- Temperature-controlled soldering iron: this is the most important purchase. Fixed-power irons without control run too hot or too cold and oxidise the tip quickly.
- Interchangeable tips: a screwdriver or chisel tip a few millimetres wide covers most jobs; a fine conical one is for small components.
- Flux-core solder, in thin wire (0.5-0.8 mm for electronics).
- Extra flux, as paste or pen, for surface-mount work and repairs.
- Brass wool or a damp sponge for cleaning the tip.
- A stand for the iron, a helping hand with clips and a magnifier.
- A desoldering pump and copper braid for removing solder.
Leaded or lead-free solder
Traditional tin-lead alloys melt at a lower temperature and flow easily: they are more forgiving for learners. Lead-free alloys, mandatory in the production of most consumer electronics in Europe, need higher temperatures and wet a little less easily.
| Tin-lead (63/37) | Lead-free (SAC type) | |
|---|---|---|
| Melting point | about 183 °C | around 217-220 °C |
| Typical tip temperature | 300-340 °C | 340-380 °C |
| Joint appearance | shiny | duller |
With leaded alloys a simple hygiene rule applies: wash your hands after work and don't eat at the bench. In any case, the fumes you see are mostly burnt flux and they irritate the airways: work in a ventilated room, preferably with a small extractor with a filter.
Preparing the tip
A new tip must be tinned as soon as it reaches temperature: melt a little solder on the working surface until it becomes shiny. A tinned tip transfers heat; an oxidised tip, grey and dull, does not. Clean it often on the brass while working and, before putting the iron away, leave a layer of fresh solder to protect it.
Technique for a through-hole component
- Insert the component and bend the leads slightly so it doesn't fall out.
- Place the tip so that it touches the pad and the lead at the same time.
- After one or two seconds, bring the solder wire to the joint, on the side opposite the tip, not onto the tip.
- Let enough flow to form a small cone around the lead.
- Remove the solder first, then the tip, and don't move the component until the joint has solidified.
- Trim the excess lead.
The whole thing takes a few seconds. Holding the tip on too long overheats the component and can lift the pad off the printed circuit board.
Recognising a good joint
A good joint is concave, smooth, and the solder blends smoothly into both the pad and the lead. Typical defects:
- cold joint: grainy or blob-like, the solder did not wet properly; reheat it adding a little flux;
- too much solder: a ball hiding the lead, at risk of touching nearby traces;
- bridge: solder joining two adjacent pads; remove it with braid;
- lifted pad: a sign of too much heat or mechanical force.
Soldering wires together
To join two wires, strip a few millimetres, pre-tin them separately, then bring them together and heat them as one. Before soldering, remember to slide heat-shrink tubing onto one of the wires: forgetting it is the mistake everyone makes at least once.
Desoldering
- Pump: heat the joint, place the cocked pump on it and trigger it. It works well on through-holes.
- Braid: lay it on the solder and press it with the tip; the copper soaks up the alloy by capillary action. Adding flux helps a lot.
- For components with many pins, it often pays to add fresh solder before removing it: the new alloy lowers the melting temperature of the old one and makes it easier to remove.
Safety at the bench
- The tip exceeds 300 °C: the iron always goes in its stand, never on the table.
- Safety glasses when trimming leads or desoldering: fragments and solder can spatter.
- Never solder on powered circuits.
- Switch off the station when you get up: it's the rule that prevents the most trivial accidents.
In short
- Heat pad and lead together; the solder goes on the joint, not on the tip.
- A temperature-controlled iron and a well-tinned tip do half the work.
- Lead-free alloys need more temperature and a bit more flux.
- A good joint is smooth and concave; a cold one is grainy.
- Ventilation, an iron stand and glasses are the minimum safety.
