PCB Design, Soldering and Assembly Practice
PCB Design, Soldering and Assembly Practice tests schematic capture, footprint selection, placement, routing, manufacturability, hand soldering, ESD control, inspection, and rework. Expect practical judgments about current capacity, clearance, and joint quality.
Core rules
- The netlist drives connectivity; footprints must match the physical package and pin numbering.
- Place decoupling capacitors close to IC power pins, keep high-current loops short, and use a ground plane where possible.
- Trace width is set by current and allowable temperature rise; wider traces reduce resistance and voltage drop. Clearance must satisfy the voltage and fabrication process.
- Keep sensitive analog and noisy digital returns separated and join them at a defined point.
- For hand soldering, use a temperature-controlled iron, adequate flux, and a tip that contacts both pad and lead. Typical leaded solder work is around 300–320 °C; lead-free often needs 350–380 °C.
- ESD wrist straps and grounded mats protect MOSFETs and ICs. Inspect for bridges, cold joints, tombstoning, and polarity errors.
Worked example
A 0.1 m copper trace is 1 mm wide and 35 µm thick. Resistivity of copper ≈ 1.72 × 10^−8 Ω·m, so R = ρL/(wt) = 1.72×10^−8 × 0.1/(0.001 × 35×10^−6) ≈ 0.049 Ω. At 1 A, drop ≈ 49 mV and dissipation ≈ 49 mW. That is acceptable for a signal but check temperature rise and return path for a power rail; narrow the trace and the drop rises proportionally.
Traps
Using a datasheet footprint without checking pad numbering mirrors the part. Long decoupling traces add inductance and defeat the capacitor. Excessive iron temperature or time lifts pads and damages plastic. Solder bridges between fine-pitch pins are caused by too much solder, not too little flux. Ignoring annular ring and drill tolerance causes breakout.