PCB design glossary
BOM
AKA: bill of materials
Bill of Materials — the parts list for a PCB, ready for ordering and assembly.
A BOM lists every component on a board: reference designator (R1, C4, U2), value or part number, manufacturer, package, quantity. It's what you hand to a parts distributor or an assembly house.
A good BOM also includes alternate part numbers and supplier links so substitutions are easy when a primary part is out of stock — common at JLCPCB / LCSC.
What a BOM should include
- Reference designators (R1, R2, C4...), grouped when parts are identical.
- Value or description: 10k, 100 nF, 3.3 V LDO.
- Footprint or package, such as 0603 or SOIC-8.
- Manufacturer and manufacturer part number (MPN).
- Quantity per board.
- A supplier part number where the assembler needs one, such as an LCSC number for JLCPCB.
- DNP (do not populate) marks for footprints left empty on purpose.
BOMs for assembly services
Assembly houses each expect their own layout. JLCPCB's template, for example, uses columns for Comment (value), Designator, Footprint and LCSC part number. A BOM that does not match the template usually means manual fixes during ordering, or parts placed wrongly.
Common BOM mistakes
- Identical parts not grouped, so the list is long and error-prone.
- Values without tolerance, voltage or dielectric where it matters, such as capacitors.
- Part numbers that are out of stock, with no alternates.
- Designators in the BOM that do not match the board.
Work it out with a free tool
Tidy a BOM before it goes to the assembler
Paste or upload a parts list and the BOM formatter groups identical parts, merges reference designators and gives back a clean CSV.
Format a BOM