Every BOM line item for assembly should include a manufacturer part number, manufacturer name, reference designators, quantity, a clear description, and package details. Add status flags for DNP parts and approved alternates. With these fields complete and consistent with the placement data, an assembler can order, kit, and place components without guessing or sending clarification questions.
A BOM that is incomplete or inconsistent with the placement file is a common source of preparation delays and assembly errors. The sections below explain what each field must contain and how to check the file before it goes to your assembler.
Why do manufacturer part numbers matter more than descriptions?
A manufacturer part number (MPN) identifies exactly one component, while a description only suggests a type. “100nF 0402 capacitor” fits hundreds of parts with different voltage ratings, dielectrics, and tolerances. The MPN removes that ambiguity and lets the assembler verify the exact component you intend to use.
Descriptions still have a role. They help a human reviewer spot an obvious mismatch, such as a resistor value that contradicts the schematic. Treat the description as a cross-check, not as the source of truth. The manufacturer name should always sit next to the MPN, because different manufacturers occasionally use similar part numbers.
Make sure the MPN corresponds to the intended package as well as the intended electrical value. Many part families come in several package variants distinguished only by a suffix, and a wrong suffix can mean a part that physically does not fit your footprint. Distributor order codes are useful for purchasing, but they should supplement the MPN rather than replace it.
How should reference designators and quantities be listed?
List every reference designator for a line on that line, such as C1, C2, C7, and set the quantity to match the number of designators. If a line shows a quantity of five but only four designators, the file contradicts itself and the assembler must stop and ask which one is correct.
Group identical components onto one line, so that all parts sharing the same MPN appear together. Use a consistent separator, either commas or spaces, throughout the whole file, and avoid mixing the two. Ranges such as “R1-R5” are risky because some tools cannot expand them reliably, so write each designator out in full.
Every designator in the BOM must also appear in the placement (centroid) file, and the reverse holds too. Mismatches here are a classic source of errors: a designator renamed late in layout but not updated in the BOM leaves a part either unplaced or unexplained. Components that are not placed by machine, such as mechanical parts or hand-fitted connectors, should still be identified so the assembler knows how to treat them.
What package and footprint details prevent placement errors?
The BOM should state the component package, for example 0402, SOIC-8, or QFN-32, and it must match both the physical part and the footprint on the PCB. A package mismatch is one of the most expensive errors to find late, because the board may already be manufactured with pads that do not suit the part.
Package names are not always standardised. “SOT-23” has several variants, and imperial and metric size codes for chip components (0402 versus 0402M) describe different sizes. Where a package name could be misread, add the dimensions or the manufacturer’s package code. Also check these details:
- Polarity and pin 1: confirm that polarised parts, diodes, LEDs, electrolytic capacitors, and ICs have a consistent orientation between the footprint, the 3D data, and the placement file.
- Through-hole versus SMD: mark which technology each line uses, since this determines the assembly process.
- Height and mounting restrictions: note any part that is unusually tall or needs special handling.
How are DNP parts and approved alternates documented?
Do Not Populate (DNP) parts should stay in the BOM with an explicit DNP marker in a dedicated column, so the assembler knows the footprint is intentionally empty. Approved alternates should be listed against the primary part, each with its own manufacturer and MPN, along with any condition for substitution.
Documenting DNP components
Leaving a DNP part out of the BOM entirely can look like an omission, which triggers a clarification request. Keep the line, keep the designator, and flag it clearly. If a DNP footprint is a variant option, say which variant it belongs to. Make sure the placement file matches, so that the machine does not place a part the BOM says to skip.
Documenting approved alternates
Components go out of stock, and an approved alternate list lets the assembler continue without stopping the job. Only list alternates you have actually verified for electrical, thermal, and mechanical suitability, including package and footprint. State whether the assembler may substitute freely or must ask first. Listing an unverified alternate shifts a technical risk onto the build.
Which BOM file format and column layout work best?
A simple, flat spreadsheet works best: one header row, one component line per row, and no merged cells, colours carrying meaning, or notes in the margins. CSV and XLSX are both widely accepted, but check which format your assembler prefers. Consistent, machine-readable structure matters more than the file type.
A practical column layout looks like this:
- Line number
- Reference designators
- Quantity
- Manufacturer
- Manufacturer part number
- Description (value, tolerance, rating)
- Package or footprint
- Mounting type (SMD or THT)
- DNP flag
- Approved alternate manufacturer and MPN
- Notes
Use one row per unique part, keep header names stable between revisions, and include the BOM revision number and the PCB revision it belongs to. Avoid special characters and line breaks inside cells, since they can corrupt the import into assembly tools.
How can a BOM be checked before submitting it for assembly?
Check the BOM against the placement file, the schematic, and the PCB data before submitting it, looking for missing fields, duplicate or unmatched designators, and MPNs that do not match the intended package. A short, repeatable checklist catches most errors before they reach the assembler.
Use this PCBA BOM checklist:
- Every line has a manufacturer and MPN, with no blank cells.
- Quantity equals the number of listed designators on every line.
- Every BOM designator exists in the placement file, and every placed part appears in the BOM.
- Package and footprint match the physical component and the PCB pads.
- Polarised parts have consistent orientation data.
- DNP parts are flagged in both the BOM and the placement data.
- Approved alternates are verified and clearly labelled.
- Revision numbers on the BOM and the PCB data agree.
- Components are checked for availability and lifecycle status, since an obsolete or out-of-stock part can stall a build.
This type of review is a form of pre-production verification. It reduces avoidable clarification cycles and unnecessary iterations, though it cannot confirm that the electronic design itself will function. That remains a matter for engineering validation and prototype testing.
How Eurocircuits helps with BOM preparation for assembly
Eurocircuits is a European manufacturer of PCB prototypes and low-volume production with its own PCB manufacturing and assembly facilities. Its Digital First approach checks data before physical production starts, so BOM and placement problems can be resolved while changes are still easy to make.
- PCB Visualizer analyses your design data and shows how it will be interpreted for manufacturing before production begins.
- A Verified Component Database and component engineering support help identify component data problems before assembly.
- Checks of component identity, manufacturer part numbers, packages, and consistency with BOM and placement data reduce avoidable preparation delays.
- Manufacturing and assembly under one organisation reduce the number of interfaces between suppliers.
Have a BOM you want reviewed before you order? Contact the Eurocircuits team to discuss your assembly requirements, or visit Eurocircuits to start with your design data.