PCB Board Parts: Names & Identification
A resistor is never just 'R12'. On a busy line, the gap between a clear reference designator and a vague label is the difference between a clean build and a rework nightmare. This guide keeps your documentation honest.
Reference designators are a contract
Every component on a board should carry a unique reference designator — R for resistors, C for capacitors, U for ICs, Q for transistors, D for diodes, J for connectors, L for inductors, Y for crystals. When the silk, schematic and BOM all agree, the pick-and-place file is trustworthy.
- R / RN — resistors and resistor networks
- C — capacitors (note polarity for electrolytics)
- U / IC — integrated circuits
- Q — transistors and MOSFETs
- D — diodes and LEDs
- J / P — connectors and headers
- L — inductors and chokes
- Y / X — crystals and oscillators
Decoding passive markings
Through-hole resistors use color bands; surface-mount use a three- or four-digit code where '103' means 10 kΩ. Capacitors rarely show value on small parts, so the BOM and centroid file must carry it. Never rely on the package alone — a 0603 can be any value.
Polarity you cannot guess
Electrolytic capacitors, tantalums, diodes and some connectors have polarity. The silk screen should show the cathode band or pin-1 dot. A reversed electrolytic is a board-level hazard, not a minor defect.
Key takeaways
Key Takeaways
- Unique reference designators keep silk, schematic and BOM in lockstep.
- SMD passive codes must be backed by the BOM — the package shows no value.
- Mark polarity explicitly; reversed electrolytes are a safety hazard.
