Tips for a bill of materials that EMS partners love: complete, unambiguous, with alternatives.
What is a BOM?
The BOM (Bill of Materials) is the parts list – the most important document for PCB assembly. It contains all electronic components that are to be mounted on the circuit board.
A good BOM saves time, avoids queries, and speeds up quoting. A poor BOM leads to delays, incorrect orders, and in the worst case, defective assemblies.
Required Fields
Every professional BOM should contain at least these fields:
| Field | Example | Why important? |
|---|---|---|
| Reference | R1, R2, C5, U3 | Unique assignment on the board |
| Quantity | 1, 2, 10 | Count per board |
| Description | Resistor 10kΩ 0402 1% | Quick identification |
| Manufacturer | Yageo, TDK, Texas Instruments | Exact component specification |
| Manufacturer P/N (MPN) | RC0402FR-0710KL | Unique order number |
| Package/Footprint | 0402, SOT-23, TQFP-48 | Does the component fit the footprint? |
| DNP marking | DNP / Do Not Place | Component is NOT to be placed |
Optional but Helpful Fields
- Alternative MPN: Second manufacturer as a fallback option
- Distributor + order number: e.g., Mouser 603-RC0402FR0710KL
- Value: 10kΩ, 100nF, 4.7µF
- Voltage/Power: 16V, 0.25W
- Tolerance: 1%, 5%, 10%
- Notes: Special requirements (e.g., "Automotive Grade")
The 5 Most Common BOM Mistakes
- Missing MPN: Without a manufacturer part number, the EMS partner cannot uniquely identify the component
- Obsolete components: End-of-life (EOL) parts lead to delays – check availability in advance!
- Wrong package size: 0402 imperial ≠ 0402 metric (= 01005 imperial)
- DNP not marked: Non-placed positions must be clearly identified
- Multiple values per row: "R1-R10" instead of individual rows – complicates processing
BOM Format and Delivery
Preferred formats for BOM delivery:
- Excel (.xlsx): Most widely used, easy to edit
- CSV (.csv): Machine-readable, ideal for automatic processing
- PDF: Only as a supplement – not as a primary format (not machine-readable)
Tip: Export the BOM directly from your EDA tool (KiCad, Altium, Eagle) – this minimizes transcription errors.
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