PCB Assembly and Wiring Practices
What this domain tests
Board anatomy, assembly sequence, and correct wire preparation and termination. Expect questions on through-hole versus SMD parts, solder mask and silkscreen, track continuity, wire stripping and crimping.
Rules
- A PCB has a substrate, copper tracks, solder mask and silkscreen. The mask prevents bridging; the silkscreen shows component references.
- Assemble low-profile parts first: resistors, diodes, IC sockets, then capacitors, then connectors and tall parts. Observe polarity on diodes, electrolytics and ICs; the notch or dot marks pin 1.
- Keep leads short on the solder side and avoid force that lifts pads.
- Strip about 6 mm of insulation without nicking the conductor. Twist stranded wire before tinning. Use the correct crimp terminal and tool; a bad crimp is a common cause of intermittent faults.
- Route wires away from heat and moving parts; use looms, grommets and strain relief. Colour conventions help, but continuity testing is the proof.
- SMD rework needs tweezers, flux and controlled heat, not a plumbing torch.
Worked example
You fit an LED and resistor on a board. The LED cathode (flat side, short lead) goes to the resistor, which goes to the negative supply; the anode goes to the positive side through the switch. After soldering, set the multimeter to continuity, confirm the track from switch to anode, and check there is no beep between adjacent pads. Then power up with a current-limited supply and measure the resistor drop to confirm the expected current.
Traps
- Soldering an IC directly when a socket is specified, or fitting it backwards.
- Trusting colour codes instead of a continuity check.
- Over-stripping wire and cutting strands.
- Bridging tracks with excess solder.