Soldering, PCB and Assembly Practices
What this domain tests
Soldering, PCB and Assembly Practices tests hand soldering, desoldering, component mounting, PCB handling and inspection. Questions often show a joint description or assembly fault and ask for the cause or correction.
Core rules
- Common tin-lead solder is 60/40 or 63/37 Sn-Pb, melting near 183–190 °C. A controlled iron tip is usually set around 300–350 °C for tin-lead work; lead-free solder needs a higher temperature.
- Flux removes oxide and helps wetting. Use rosin or no-clean flux for electronics; acid flux is for plumbing and causes corrosion.
- A good joint is shiny, smooth and concave, with the solder feathered to pad and lead. Heat the pad and lead together, then feed solder.
- PCB assembly: clean copper, etch with ferric chloride, drill, deburr, insert components at correct height and polarity, solder, trim leads, inspect and clean flux residue.
- Use an ESD wrist strap or grounded mat for CMOS and microcontroller boards.
Worked example
A joint is dull, grainy and blobby, and the component moves when tugged. Diagnosis: a cold joint caused by insufficient heat or a contaminated surface. Correction: remove old solder with a desoldering pump or wick, clean the pad, apply flux, place the tip against pad and lead for one to two seconds, feed solder, remove solder first, then remove the iron, and let the joint cool without movement. Reheat once if needed, but do not keep the iron on the pad for many seconds.
Traps
Using too much solder and bridging adjacent pads; moving a joint before it solidifies; overheating and lifting copper tracks; soldering a polarised component backwards; and ignoring flux residue. A shiny joint is not proof of electrical connection if the pad was never heated.