Hardware Assembly, Soldering and PCB Basics
Hardware Assembly, Soldering and PCB Basics tests whether you can build and inspect a small electronic assembly safely: component placement, soldering quality, PCB track and pad care, and basic continuity checks.
Core rules
- Use a temperature-controlled iron around 300–350 °C for leaded solder; higher for lead-free. Tin the tip and keep it clean.
- Solder joints should be shiny, concave and smooth. A dull, lumpy or ball-shaped joint is a cold joint.
- Heat the pad and lead together, then apply solder to the joint, not to the iron tip.
- Flux removes oxide; do not use acid flux on electronics.
- Observe polarity: electrolytic capacitors, diodes, LEDs and ICs. Match the PCB silkscreen.
- Through-hole parts are inserted and soldered on the solder side; SMD parts are placed on pads and reflowed or hand-soldered.
- Avoid ESD damage: wrist strap, grounded mat, and handle ICs by the body.
Worked example
A 5 V supply drives a red LED (forward drop about 2 V) with a 220 Ω series resistor. Current = (5 − 2)/220 = 3/220 ≈ 13.6 mA, a safe value for a standard 20 mA LED. The resistor can go either way, but the LED must be oriented with anode to the positive side and cathode (flat side/bar) to the resistor and ground. Check the joint with a continuity test before powering.
Traps
- Holding the iron on a pad too long and lifting the copper track.
- Using too little heat and making a cold joint, then blaming the component.
- Bridging adjacent pads with excess solder.
- Testing continuity on a powered board; always isolate power first.