Soldering, Desoldering and PCB Assembly
Domain 3: Soldering, Desoldering and PCB Assembly
This domain tests hand soldering and rework: tip selection and temperature, flux, joint quality, through-hole and SMD techniques, desoldering, and PCB handling. The exam favours observable acceptance criteria over memorised slogans.
Core rules
- Set an iron temperature suited to the alloy and board: roughly 315–360 °C for leaded solder, 340–380 °C for lead-free; use the lowest reliable temperature and short contact time.
- Tin the tip, clean it, and apply heat to the pad and lead together before feeding solder. A good joint is shiny or satin, concave, wets both surfaces, and does not ball up.
- Flux aids wetting; clean residue if the board or customer requires it. Do not rely on flux to fix a dirty or oxidised surface.
- Through-hole: insert, bend leads slightly if allowed, solder, trim, inspect. SMD: tack one corner, align, solder remaining pins, use drag soldering or hot air for fine pitch.
- Desoldering: add fresh solder and flux, then use a pump, wick, or hot air. Do not lever pads; overheated pads delaminate.
Worked example
A 0.8 mm through-hole resistor lead and pad need soldering with lead-free solder. Choose a 1.0 mm chisel tip at 360 °C, clean and tin it, heat pad and lead for about 1 second, feed 1–2 mm of solder, remove solder then iron, and let the joint cool undisturbed. Inspect: the fillet should slope to the lead, wet the pad, and show no cracks. If the joint is dull and ball-shaped, the iron was too cool or the joint moved; reheat with flux rather than adding more solder.
Traps
- Applying solder to the iron and dropping it onto the joint, which creates a cold joint.
- Too much solder hiding a poor fillet or bridging pins.
- Using a fine tip for a large ground plane and then overheating the pad.
- Mixing leaded and lead-free solder without adjusting temperature.
- Pulling a component before all joints are molten.