Soldering
What this domain tests
Tool selection, correct joint formation, recognition of defects, and safe handling of a soldering iron. Practical assessment looks for shiny concave fillets; theory asks about temperatures, fluxes and defect names.
Core rules
- Use an electronics-grade iron, typically 25–40 W, with a clean tinned tip; typical tip temperature is 300–350 °C for 60/40 tin-lead solder. Lead-free solder needs a higher temperature and wets less easily.
- Rosin (colophony) flux is for electronics; acid flux is for plumbing and corrodes joints.
- Heat the pad and lead together, then feed solder into the joint, not onto the iron tip. Aim for 2–3 seconds, then remove solder and iron.
- A good joint is smooth, shiny and concave. A cold joint is dull, rounded and grainy; a bridge shorts adjacent pads; a spike is excess solder.
- Clean flux residue with isopropyl alcohol and inspect with magnification.
Worked example
A through-hole resistor lead is loose in its pad. Clean and tin the tip, touch it to the pad and lead for about one second, feed 60/40 solder until a small fillet forms, remove the solder then the iron, and hold the lead still for a second. The result should be a shiny cone. If the joint is dull, add a little rosin flux and reheat briefly; do not simply pile on more solder.
Traps
- Melting solder on the iron and carrying it to the joint, which produces a cold joint.
- Overheating a pad until it lifts.
- Using too much solder and bridging tracks.
- Failing to tin a new tip or clean it between joints.