Practical Work, Soldering and PCB Familiarisation
10. Practical Work, Soldering and PCB Familiarisation
This domain tests workshop skills: identifying components, making sound solder joints, handling PCBs and using test equipment safely.
Soldering
- Use a 25–40 W iron for PCB work; a higher-wattage iron is for heavier joints, not for longer contact.
- Solder is usually 60/40 or 63/37 tin-lead with a rosin (non-corrosive) flux core; lead-free solder needs a higher temperature.
- Procedure: clean and tin the tip, heat the pad and lead together for 2–3 seconds, apply solder to the joint (not the iron), let it flow, remove solder then iron, and do not move the joint while it cools.
- A good joint is bright, smooth and cone-shaped. A cold joint is dull, lumpy and intermittently conductive; a bridge is unwanted solder linking two pads.
- Use a solder sucker or desoldering braid to remove solder; never use acid flux on electronics.
PCB work
- A PCB is an insulating board with copper tracks; layout keeps tracks short, avoids sharp angles and separates high-current from signal paths.
- Etching removes unwanted copper; drilling takes component leads; the silk screen shows component positions.
- Check polarity (diodes, electrolytic capacitors, ICs) before soldering, and fit ICs in sockets.
- Test with a multimeter for continuity and for shorts between adjacent tracks.
Worked example
An amplifier works when prodded but fails when the board is flexed. Inspection shows a dull, ball-shaped joint on a resistor lead. Reheat it with a little fresh flux-cored solder until it wets and forms a shiny fillet; the intermittent fault disappears. A bridge between two adjacent pads would instead show continuity where none should exist.
Traps
- Applying the iron too long and lifting the copper pad.
- Using acid flux or excessive solder.
- Soldering a live circuit or a charged capacitor.
- Assuming joints are good because they look big.