A PCB bill of materials (BOM) is the controlled list of components required to build one assembly. A production-ready BOM identifies exactly what to buy, where each part is placed, how many are needed, which substitutions are approved, and which items are intentionally not fitted.
The best BOM is unambiguous to engineering, purchasing and the assembly line. It should match the schematic, PCB revision and pick-and-place data before a quote is requested.
PCB BOM Quick Answer#
| Required field | What it controls | Typical mistake |
|---|---|---|
| Manufacturer part number | Exact purchasable component | Entering only a generic value |
| Reference designators | Placement locations | Mismatch with centroid data |
| Quantity per assembly | Material demand | Counting a grouped row incorrectly |
| Package or footprint | Physical compatibility | Confusing case size with land pattern |
| Approved alternates | Substitution authority | Allowing an unreviewed equivalent |
| Fit status | Installed versus DNP parts | Silently deleting optional components |
What Is a PCB BOM?#
A PCB BOM is a structured material list for a printed circuit board assembly. It may contain resistors, capacitors, ICs, connectors, fasteners, heat sinks, labels, programmed modules and other items consumed by the assembly process. Bare-board fabrication data is normally controlled separately through Gerber, drill, drawing, netlist or IPC-2581 data.
IPC-2581 is an intelligent manufacturing-data format that can include BOM, approved-vendor, ECAD and change-history information. A spreadsheet BOM remains common, but it should still follow the same principle: every record must be machine-readable, revision-controlled and traceable.
Essential PCB BOM Columns#
Item or line number
Give every row a stable identifier. It makes questions, deviations and purchase records easier to track than describing a row only by its spreadsheet position.
Reference designators
List every location using the same designators as the schematic and placement data: for example, C12, C13 and C18. Group only locations that use the exact same approved part and fit status.
Quantity per assembly
The quantity should equal the number of designators on a fitted row. State whether the BOM describes one PCBA, a panel, a kit or a complete product. Purchasing quantities can then be calculated from build quantity, attrition policy and packaging constraints.
Manufacturer and manufacturer part number
The manufacturer part number (MPN) is the primary identity of a purchased electronic component. Distributor stock codes are useful sourcing references, but they should not replace the MPN because suppliers and stock locations can change.
Description and value
Use a concise engineering description that helps humans detect mistakes: function, nominal value, tolerance, voltage or power rating, technology and package where relevant. Description is a cross-check, not a substitute for the exact MPN.
Package and footprint
Record the physical package and, when useful, the controlled PCB footprint name. Two parts with similar electrical ratings may have different pitch, body dimensions, thermal pads or pin-one orientation.
Approved alternates
Put approved substitutions in dedicated fields or an attached approved-manufacturer list. Do not write “or equivalent” for critical parts. An alternate may require review for electrical limits, pinout, package, lifecycle, regulatory status, firmware and qualification.
Fit and sourcing status
Mark DNP/DNI parts explicitly, and distinguish customer-supplied, assembler-supplied, consigned, no-substitution and programmed items. Never rely on blank cells to communicate intent.
Example PCB BOM Structure#
| Item | References | Qty | Value | MPN | Package | Status |
|---|---|---|---|---|---|---|
| 1 | C1, C4, C9 | 3 | 100 nF, 10%, 50 V, X7R | Exact approved MPN | 0603 | Fit |
| 2 | U1 | 1 | 32-bit MCU | Exact approved MPN | QFN-48 | No substitute |
| 3 | R17 | 0 | 0 ohm option | Approved MPN | 0402 | DNP |
The placeholder part numbers above demonstrate structure only. A released BOM must contain actual orderable identifiers and should never invent or shorten an MPN.
How to Create a PCB BOM#
- Freeze the design revision. Export the BOM, schematic PDF and placement data from the same controlled ECAD revision.
- Normalize the rows. Group only identical components and keep designators sorted so omissions are visible.
- Validate each MPN. Confirm the current manufacturer datasheet, lifecycle state, package and orderable suffix.
- Add sourcing control. Record approved alternates, distributor references, consigned items and no-substitution rules.
- Reconcile quantities. Compare row quantities with the reference-designator count and the placement file.
- Review special handling. Identify moisture sensitivity, programming, bake, traceability, ESD and storage needs where applicable.
- Release with a revision. Use a controlled filename and change record; do not send a file called final-final-new.xlsx.
BOM and Pick-and-Place Reconciliation#
The BOM answers what is installed; the centroid or pick-and-place file answers where and how it is placed. Their reference designators must agree. A component listed in one file but absent from the other should trigger review, not a guess on the line.
Check top/bottom side, rotation convention, footprint, designator and fit status. Visualize the exported copper and outline with the online Gerber viewer, then compare assembly outputs with the approved board revision.
Managing Alternates Without Losing Control#
Alternates reduce shortage risk, but approval must be engineering-led. Passive components may require matching value, tolerance, voltage, temperature coefficient, dielectric, package and pulse behavior. Semiconductors can differ in pinout, startup behavior, thermal performance, firmware support and errata even when their descriptions sound similar.
Separate three decisions: approved before build, proposed by the assembler, and temporarily authorized for a specific lot. Record who approved the change and update the controlled BOM if it becomes permanent.
Common PCB BOM Errors#
- Using descriptions such as “10k resistor” without an MPN and ratings.
- Mixing distributor SKUs with manufacturer part numbers.
- Omitting orderable suffixes that control package, temperature grade or reel format.
- Combining different values or footprints in one row.
- Leaving DNP parts out instead of explicitly identifying them.
- Sending BOM and placement files from different revisions.
- Allowing substitution without a documented approval boundary.
- Forgetting non-electronic items used in the assembled product.
PCB BOM Checklist Before Quotation#
- Every fitted designator appears once.
- Every quantity equals its designator count.
- Every purchased item has an exact MPN.
- Packages and footprints match the PCB library.
- DNP and variant rules are explicit.
- Approved alternates are separated from suggestions.
- Customer-supplied and programmed items are identified.
- BOM, schematic, Gerber and centroid data share one revision.
Send this package with the requirements described in the PCB assembly process guide. For supplier planning, also define inspection and testing expectations using the PCB testing guide.
PCB BOM FAQ#
What format should a PCB BOM use?
XLSX and CSV are common for quoting and purchasing. IPC-2581 can carry BOM and other manufacturing data in a structured exchange format. Use the format your assembler accepts without sacrificing revision control.
Should distributor part numbers be included?
They can be included as sourcing references, but the exact manufacturer part number should remain the main component identity.
How should DNP parts appear in a BOM?
Keep their designators and identify them explicitly as DNP or DNI, normally with zero fitted quantity. This preserves variant intent and prevents accidental installation.
Can an assembler select equivalent parts?
Only within a documented substitution policy. Proposed alternates should be reviewed against electrical, mechanical, regulatory, lifecycle and qualification requirements before use.
Technical Reference#
IPC-2581C defines an intelligent product-data structure that includes BOM, approved-vendor, ECAD and change information. When your files are ready, submit the controlled BOM with a PCB assembly quote request.