Soldering, Desoldering and PCB Assembly
This domain tests hand-soldering technique, joint inspection, desoldering and safe PCB assembly order. The exam rewards correct temperature, flux use and recognition of defective joints.
Technique
- Tin the iron tip and keep it clean with a damp sponge or brass wool.
- Typical tip temperature is about 300–350 °C for tin-lead solder and 350–380 °C for lead-free; higher is not better.
- Heat the pad and lead together, then feed solder into the joint, not onto the iron tip.
- A good joint is shiny, concave and smooth. A cold joint is dull, grainy or ball-shaped.
- Use flux to improve wetting; clean residue after soldering.
- For desoldering, add fresh solder and flux, then use a solder sucker or wick; do not overheat the pad.
Assembly order
Fit low-profile parts first (resistors, diodes, IC sockets), then taller parts, then connectors and heat-sensitive devices. Check polarity before soldering. Keep ICs in anti-static foam and fit them last where possible.
Worked example
A resistor joint looks dull and has a visible gap at the pad. The fault is a cold joint, not a faulty resistor. Set the iron to about 330 °C, clean the tip, apply flux, reheat the pad and lead for 2–3 seconds, feed a small amount of solder, remove the iron and hold the part still until the joint solidifies. Re-inspect for a shiny fillet.
Traps that cost marks
Applying the iron for too long and lifting a copper pad; using acid flux on electronics; moving the joint while it cools; adding solder only to the iron tip; assuming a dull joint is a component fault.
Exam focus
Be able to identify cold, bridged, excess-solder and insufficient-solder joints and state the corrective action.