A PCB BOM, or bill of materials, is the component list used to quote, source, and assemble a specific printed circuit board assembly. It identifies the parts, their locations on the board, and the quantity fitted per board. For a build release, it also needs to make clear which design revision and assembly variant the list belongs to.
A useful BOM lets an engineer, buyer, and assembler reach the same conclusion about what should be purchased and installed. A description such as “10k resistor” rarely provides enough information on its own: the package, tolerance, exact part or approved specification, and fitted status still need to be resolved.
What does a PCB BOM tell the manufacturer?
The BOM connects the design to the physical components. Reference designators such as R1, C3, and U2 connect a part to its location in the design; part numbers identify what should be sourced; quantities explain how many are needed for each assembly.
It works alongside other production files:
- Gerber and drill files describe the bare-board manufacturing data.
- The BOM identifies the components and their build requirements.
- The pick-and-place file, also called a CPL or centroid file, provides placement data for the components within its scope.
- Assembly drawings and notes communicate orientation, special instructions, and details that a spreadsheet cannot explain adequately.
These files answer different questions. A BOM does not replace placement coordinates, and Gerber artwork does not identify every component that should be purchased. For the fabrication side, see this explanation of PCB Gerber files.
What fields should a PCB BOM include?
Start with the information needed to identify and fit the intended parts. Your assembler may require particular column names or a template, but the underlying decisions remain important even when the layout changes.
| Field | What to record | What to check |
|---|---|---|
| Reference designators | Locations such as R1, R2, and U1 | Each location belongs to the intended part and build variant |
| Fitted quantity per board | Number installed on one assembly | Count matches the fitted reference designators |
| Manufacturer | Component manufacturer | It agrees with the specified manufacturer part number |
| Manufacturer part number, or MPN | Complete orderable part number | Meaningful suffixes have not been removed |
| Description and value | Component type and relevant ratings | Description agrees with the MPN and design requirement |
| Package or footprint | Package identification and relevant PCB footprint | Physical dimensions and pin arrangement fit the design |
| Fitted status | Fitted, DNP, or another clearly defined state | Optional and unpopulated positions are unambiguous |
| Sourcing and substitution notes | Supply responsibility and approved alternatives, when applicable | Purchasing permission and substitution approval are clear |
Record the board or assembly identifier, revision, selected variant, and release date in the file header, a cover sheet, or another clearly associated release record. Those details do not need to be repeated in every row, but they must remain identifiable after the file is exported or forwarded.
Why use an MPN instead of only a value?
The MPN identifies a manufacturer’s specific part. A distributor’s stock number identifies that seller’s catalog entry. Keep them in separate, labeled columns when both are used.
For a resistor, value alone does not specify tolerance, power rating, or package. For an IC, a shortened family name may leave the package or device option unresolved. Retain the full MPN and check relevant ordering information rather than trimming a suffix because it looks unimportant.
A part can also have the expected package name and still require a footprint check. Package dimensions, pin numbering, and the recommended land pattern matter. If component selection itself is unresolved, review these PCB component selection checks before freezing the BOM.
A PCB BOM example with fitted and DNP parts
The following is a teaching example, not a complete circuit or a production recommendation. The board identifier, revision, variant, and reference designators are fictional. The component number is real so its identity can be checked against the manufacturer’s data.
Example release: DEMO-BOM, PCB revision B, assembly revision B, Base variant.
Both rows use YAGEO RC0603FR-0710KL, a 10 kΩ, 1% resistor in the EIA 0603 / metric 1608 package. These details are listed in the YAGEO specification sheet.
| References | Fitted qty/board | Status | Description | Manufacturer part number |
|---|---|---|---|---|
| R1, R2 | 2 | Fitted | 10 kΩ, 1%, EIA 0603 resistor | RC0603FR-0710KL |
| R3 | 0 | DNP | Same resistor, not fitted in Base variant | RC0603FR-0710KL |
The manufacturer is YAGEO for both lines. It is stated above the table to keep the example readable; a working BOM can use a dedicated manufacturer column.
R1 and R2 share one line because they use the same part and fitted status. R3 remains separate because it is not installed in this variant. Its zero fitted quantity is intentional, even though one footprint exists on the board.
Here, the build quantity means individual assembled boards, not panels. If an order is counted in panels, state the number of boards per panel so purchasing and assembly use the same basis.
For 50 boards, the theoretical fitted requirement is:
2 resistors per board × 50 boards = 100 fitted resistors.
R3 adds no fitted parts to that Base-variant build. Any extra material for setup or attrition, minimum purchase quantity, or packaging requirement belongs in a separately agreed procurement quantity. Increasing the BOM’s per-board fitted quantity to hide extra purchasing material makes the assembly instructions ambiguous.
The package notation also matters. In this example, EIA 0603 corresponds to metric 1608. Record the size system clearly rather than assuming every use of “0603” means the same dimensions.
How should DNP and DNI components be handled?
DNP means “do not populate”; DNI means “do not install.” In an assembly BOM, they normally identify positions that should remain unpopulated for the specified build. Define the notation in your release package and use it consistently.
There are two workable ways to communicate this:
- Retain the positions in the BOM with an explicit not-fitted status and clearly defined quantity convention, as in the example.
- Exclude them from the fitted-parts BOM while identifying the intended unpopulated positions in the released variant documentation or assembly drawing.
Check the exported file, not only the schematic view. Software settings can change what reaches the manufacturer. For example, the KiCad 9.0 manual describes an option to exclude DNP components from the BOM export. Altium also supports fitted and not-fitted variants with configurable output documentation, as explained in its design variant documentation.
An empty MPN cell is not a reliable substitute for a DNP instruction. It could mean the part is intentionally omitted, still being selected, or simply missing from the spreadsheet. Resolve that distinction before release.
How do approved alternatives belong in the BOM?
If alternatives are permitted, identify the exact approved manufacturer and MPN, where they may be used, and any conditions attached to approval. A separate approved-parts list is useful when several alternatives apply to one internal part number; link it to the released BOM revision.
“Any equivalent” leaves too much to interpretation. Matching a nominal value does not establish electrical, mechanical, thermal, firmware, or application compatibility. Availability is a sourcing observation; suitability and approval are design decisions.
Likewise, permission for a supplier to purchase components does not automatically authorize substitutions. State who may approve changes and how the accepted replacement will be recorded. If the original MPN becomes unavailable after release, preserve the original requirement and record the approved change instead of silently overwriting the file.
How do you create and check a PCB BOM?
Generate the initial list from the released design data where possible. Then review the output as a manufacturing document rather than assuming the export is complete.
- Select the correct design and variant. Confirm the board and assembly identifiers before exporting.
- Include the necessary fields. Check that manufacturers, MPNs, references, quantities, and fitted states appear in the output.
- Review grouping. Combine references only when the specified part and relevant build requirements match. Grouping by value alone can combine parts with different tolerances, packages, or fitted states.
- Verify part identity. Compare the MPN, description, package, and design requirements with the manufacturer’s data.
- Cross-check the file set. Compare fitted references and variant information across the BOM, placement data, and assembly drawing.
- Release one controlled package. Identify the approved revision and remove superseded files from the submission package while retaining them in your internal history.
The cross-check needs to respect each file’s scope. An SMT placement file may not include through-hole components or mechanical hardware listed in the assembly BOM. Investigate unexplained differences, but do not delete legitimate BOM items simply to make every file contain the same references.
Common PCB BOM errors and how to catch them
| Error | Why it matters | Practical check |
|---|---|---|
| Value provided without an exact part or approved specification | Purchasing may select a different component | Resolve manufacturer/MPN or the authorized specification before procurement |
| Quantity differs from fitted references | Material demand and assembly intent disagree | Count the fitted positions for each line; handle DNP separately |
| Same reference appears on conflicting fitted lines | One location has incompatible instructions | Check the selected variant and remove unintended duplicates |
| MPN disagrees with the description or footprint | A recognizable part number can still be wrong for the design | Compare ordering data, dimensions, and pin arrangement |
| Optional parts are represented by blank cells | Missing data can be mistaken for an omission instruction | Use an explicit fitted state or controlled variant documentation |
| Spreadsheet changes an identifier | Leading zeros, long numbers, or punctuation can be altered | Treat identifiers as text and reopen the exported file |
| BOM and placement data come from different releases | Correct individual files can describe different builds | Compare the release manifest, variant, and references together |
A familiar filename is not proof of a valid release. If a late component change affects a footprint, orientation, or fitted state, review the related outputs as well as the edited BOM row.
What should you check before sending the BOM?
Before requesting a quote or authorizing a build, confirm that:
- The assembly identifier, revision, variant, and requested board quantity are clear.
- Fitted parts have unambiguous identities and per-board quantities.
- DNP positions and approved alternatives are consistently documented.
- Customer-supplied and manufacturer-sourced parts are identified where responsibility is split.
- The associated fabrication, placement, and drawing files describe the approved build.
- Open questions are listed explicitly rather than hidden in blank cells or informal comments.
For an early estimate, some part selections may still be provisional. Label them as unresolved and ask which assumptions the estimate uses. Before purchasing or production, obtain agreement on those selections and the release package. That keeps a preliminary component list from being mistaken for authorization to build.
Frequently asked questions about PCB BOMs
Does a PCB BOM include the bare PCB and mechanical parts?
It can, depending on the agreed assembly scope. If the bare board, fasteners, or other mechanical items are included, identify them with the relevant part number, revision, quantity, or specification. Keep detailed fabrication and assembly requirements in the associated drawings. An SMT placement file will not necessarily contain these items.
Which file format should I use for a PCB BOM?
Use the format and column names accepted by your assembler. CSV and Excel workbooks are common, but acceptance and required fields vary. Check the exported file for altered part numbers, missing columns, and lost fitted-status information; a readable PDF can be a supporting reference rather than an assumed substitute for the requested data file.
Do different assembly variants need separate BOMs?
Each build needs an unambiguous component list for its selected variant. This can be communicated through separate released exports or a controlled master BOM with clearly identified variant quantities and fitted states. Confirm which output the assembler will use, and keep the placement data and assembly drawings aligned with that same build.
Should component prices and stock levels be included in the BOM?
They may be useful for procurement, but they are time-dependent information rather than permanent component identity. Record the supplier, pricing quantity, currency, and check date in a separate sourcing view or clearly labeled columns. A past stock check does not confirm availability when purchasing begins.
Can I change the BOM after requesting a quote?
Yes, but identify the changed lines and issue a revised file rather than silently replacing the previous version. Ask whether the change affects price, availability, placement data, inspection, or lead-time assumptions. If purchasing or production has already been authorized, agree on the change and its consequences before treating the new BOM as approved.
Prepare your BOM for a PCB assembly quote
Before requesting a quote, identify the assembly revision and variant, mark DNP positions, and list any approved alternatives. For a Thindry project inquiry, send your BOM with the relevant PCB manufacturing files, placement data, assembly drawings, build quantity, and sourcing or test requirements.
Contact Thindry about your PCB assembly requirements or email sales@pcbtry.com. If any part selections are still provisional, identify them and ask which assumptions will be used for the quotation.
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