Soldering, PCB Assembly and Wiring Practices
The Soldering, PCB Assembly and Wiring Practices domain tests hand soldering, component placement, PCB terminology and safe wiring. Marks are lost through poor joint quality, wrong polarity and failure to control heat.
Core rules and concepts
- Use a clean, tinned iron tip at roughly 350–380 °C for leaded solder; higher temperatures damage pads and components. Sn63/Pb37 is eutectic (183 °C); lead-free SAC305 melts near 217–220 °C.
- Heat the pad and lead together, apply solder, then remove solder first and iron second. A good joint is shiny, concave and smooth; a cold joint is dull, grainy and unreliable.
- PCB terms: pad, track, via, silkscreen, solder mask. Through-hole parts are inserted and soldered on the underside; SMD parts sit on pads and are reflowed or hand-soldered.
- Observe polarity for electrolytic capacitors, diodes, LEDs and ICs; match pin 1 and the notch/dot.
- In Indian wiring practice, IS 732 uses brown for live, blue for neutral and green-yellow for earth. Never solder live wiring.
Worked example
Soldering a 1 kΩ resistor (brown-black-red-gold) into a PCB: confirm the value and bend the leads to match the hole spacing, insert the part, flip the board, heat pad and lead for about 1–2 s, feed solder until it forms a small fillet, remove solder, then iron. Trim the lead and inspect under light. A dull, ball-shaped joint should be reflowed with flux.
Traps that cost marks
Holding the iron on a pad too long and lifting copper; using acid plumbing flux on electronics; moving the joint before it freezes; bridging adjacent pads; forgetting ESD precautions; using the wrong wire colour for earth.