Hardware Assembly, Soldering and PCB Basics
The Hardware Assembly, Soldering and PCB Basics domain tests hand skill plus the reasoning behind it: choosing solder and flux, setting iron temperature, recognising a good joint, and knowing how a board is laid out and populated. Examiners look for sequence, cleanliness and heat control.
Rules of a good joint
- 60/40 Sn-Pb solder melts near 183–190 °C; lead-free SAC305 near 217–220 °C and needs a hotter tip.
- Tip temperature: about 320–350 °C for leaded work, 350–380 °C for lead-free. Above 400 °C the flux burns, pads lift and tips oxidise.
- Keep the tip tinned; clean on brass wool rather than a dry sponge alone.
- Heat pad and lead together for 1–2 s, feed solder into the joint (not onto the tip), remove solder then iron.
- A good joint is bright, concave and smooth; a cold joint is dull, blobby and grainy - reheat with fresh flux.
- Through-hole parts are inserted, bent, soldered and trimmed; SMD parts need tweezers, flux and often hot air.
- Use an ESD mat and wrist strap for CMOS and microcontroller boards.
Worked example
A joint on a thick ground plane stays dull after 2 s with a 25 W iron. Reasoning: the plane sinks heat faster than a small iron can deliver it, so the alloy never reaches flow temperature. Fix: use a 40–60 W iron with a larger chisel tip, pre-tin the pad and preheat the board. Raising the set temperature is wrong - it damages the laminate without solving the heat-transfer problem.
Traps that cost marks
- Carrying solder on the tip and dabbing it, producing a cold non-wetting joint.
- Bridging adjacent pads and failing to check with magnification or continuity.
- Desoldering with a pump while the joint is still solid.