BOM
Bill of Materials
A BOM (Bill of Materials) is the structured list of every component needed to build a PCB, mapping reference designators on the board to specific manufacturer parts. It is one of the three core files — alongside Gerber and pick and place — required to quote and assemble a PCBA.
What it is
At minimum, a usable BOM contains three fields per component: the reference designator (R1, C5, U3) that matches the silkscreen on the PCB, the quantity per board, and the manufacturer part number (MPN) that uniquely identifies the exact part to source. Without these three, an assembler cannot reliably build the board.
Most production BOMs include additional fields for clarity: a description (e.g. "10k 1% 0603 resistor"), the manufacturer name (since different manufacturers can use overlapping part numbers), the package or footprint, a designator for "do not populate" (DNP) parts that exist on the board but should be left empty, and optionally an alternate or second-source MPN.
BOMs are typically supplied as Excel (.xlsx) or CSV files. PDF should be avoided because it cannot be machine-read and forces manual data entry, introducing transcription errors. A well-formatted BOM speeds up quoting, reduces sourcing errors, and shortens lead time.
When it matters
BOM quality directly affects lead time, cost, and assembly accuracy. Missing or generic part numbers (e.g. "10k resistor" without a specific MPN) force the assembler to make sourcing decisions, which slows quoting and introduces risk. End-of-life or long-lead-time components flagged late in the process can delay an entire build by weeks. A clean BOM with verified MPNs from authorised distributors is one of the most cost-effective ways to protect lead time on any PCBA order.
At Nordic PCB
When you submit a BOM, our suppliers check every line against availability at authorised European distributors before quoting. Components that are end-of-life, long-lead-time, or non-compliant (e.g. non-RoHS) are flagged in the quote response with alternative suggestions where possible. Excel and CSV formats are accepted directly; if your BOM is in another format, we handle the conversion as part of the quote process.
Related terms
- MPN
An MPN (Manufacturer Part Number) is the unique identifier assigned by a component manufacturer to a specific part. It is the only reliable way to specify exactly which component to source — generic descriptions like "10k resistor" are ambiguous. Every line in a BOM should include the MPN alongside the manufacturer name.
- Gerber file
A Gerber file is the industry-standard format that describes each layer of a PCB — copper, solder mask, silkscreen, and drill data — as 2D vector geometry. A complete Gerber package is the minimum a fabricator needs to quote and build your board.
- Pick and place file
A pick and place file (also called centroid file, CPL, or XY data) is a machine-readable list of every component's position, rotation, and board side. It is one of the three core files — alongside Gerber and BOM — needed to quote and run PCB assembly.
- Authorised distributor
An authorised distributor (also called franchised distributor) has a contractual agreement with the original component manufacturer to stock and sell their parts. Examples include Digi-Key, Mouser, Farnell, RS Components, and Arrow. Components sourced from authorised distributors come with full traceability and no counterfeit risk — unlike brokers or independent distributors.
- EOL component
An EOL (End of Life) component is one the manufacturer has announced will no longer be produced. Manufacturers issue Product Change Notices (PCNs) with key dates: Last Time Buy (LTB) and Last Time Ship (LTS). Orders placed in this window are typically Non-Cancellable, Non-Returnable (NCNR). Designs using EOL parts need re-spinning or a last-time buy of lifetime stock.