PCB BOM: How to Fill Each Field and Check for Errors

A PCB Bill of Materials (BOM) is the master component list that tells your assembler which parts to buy, where to place them, and how many are needed per board. A production-ready BOM ties every component to a complete Manufacturer Part Number (MPN), an unambiguous per-board quantity, an explicit fitted status (Fitted vs. DNP), and the exact PCB revision and assembly variant.

What Is a PCB BOM—and How Does It Work with Other Assembly Files?

Your assembly BOM defines what goes on the board; your fabrication files, placement file, and assembly drawing define where and how it is built. Keeping each document focused prevents conflicting instructions across engineering, purchasing, and the SMT line.

Format note on Gerber X3 and IPC-2581: Traditional Gerber RS-274X carries 2D board layer images (such as copper, solder mask, and silkscreen) without structured BOM data, whereas Gerber X3 and IPC-2581 can embed component and BOM data in the CAD export. Even when exporting those formats, confirm whether your contract manufacturer still requires a standalone .xlsx or .csv BOM for quoting and ERP import.

Why placement row counts rarely match BOM row counts: Depending on exporter settings and assembler preferences, placement exporters (such as KiCad 9.0 PCB Editor) often filter out DNP parts and through-hole (THT) components while adding fiducials. A grouped BOM row like R1, R2 (1 row) also expands into 2 rows in the placement file. Always compare expanded reference designator sets within the same mounting scope rather than raw row counts.

How to Fill Each PCB BOM Field

We recommend a 5-item header block plus 10 core columns (and 1 optional purchasing column) so purchasing and production can verify each row without guessing.

Header Metadata (Place at the Top of the Sheet or Cover Tab)

Include these five identifiers on every release so your BOM cannot be paired with the wrong bare board:

Field-by-Field Specification Table

Why Complete MPN Suffixes Matter

Truncating an MPN breaks automated quoting and risks receiving the wrong package, temperature grade, or reel orientation.

• In YAGEO’s RC0603FR-0710KL, 0603 is the size, F is ±1% tolerance, R is paper tape reel, 07 is 7-inch reel, 10K is resistance, and L indicates lead-free/RoHS termination.
• In Texas Instruments’ LM358DR, D specifies the 8-pin SOIC package and R specifies Tape & Reel (whereas P is PDIP through-hole and DGK is VSSOP—completely incompatible footprints).

Exact MPN vs. Specification-Based Passives

Use exact MPNs by default; use specification-based lines only when your assembler explicitly supports generic passives. Some prototype assemblers allow generic resistor/capacitor selection from their house reel library. Even then, “10k” is insufficient—specify value, tolerance, power/voltage rating, dielectric (X7R vs. Y5V), and Imperial/Metric package so circuit margins are protected.

When to Group vs. Split Identical Parts

Group identical MPNs onto one row only when every build instruction matches. In KiCad 9.0’s BOM exporter, grouping depends on the selected fields. Always split rows if components share an MPN but differ in:

• Fitted status: R1, R2 are Fitted, while R3 is DNP.
• Supply responsibility: Assembler buys C1, customer consigns C2.
• Approved alternate scope: Alternate approved for R1 pull-up, but not R4 precision divider.

Annotated PCB BOM Example: Read and Check Every Column

Follow how each row below links designator, quantity, identity, status, and supply ownership. (Note: DEMO-BOM-01 is a fictional teaching example built with real manufacturer part numbers.)

Column-by-Column Check Notes:

1. R1, R2 vs. R3 (Same MPN, Split Rows): R3 uses the same resistor MPN as R1, R2, but stays on its own row because it is DNP in the BASE variant.
2. C1 (Critical Specs Beyond Capacitance): 100 nF equals 0.1 µF, and choosing 50 V with X7R dielectric provides better margin than low-voltage or Y5V parts—though X7R still experiences DC bias derating, so verify effective capacitance against the part’s DC bias curve at your operating voltage.
3. U1 (Exact Package Suffix): Keeping DR ensures the assembler quotes the 8-SOIC (D) footprint in Tape & Reel rather than PDIP or VSSOP.
4. J1 (Consigned + THT): J1 is customer-supplied and through-hole. It remains Fitted (it must be hand- or wave-soldered) and may be absent from an SMD-only placement file.
5. R3 (DNP Quantity Convention): Under Positions per Board, R3 shows Qty = 1 with Fitted Status = DNP (0 fitted demand). If your assembler’s template uses Fitted Qty per Board, set R3 to 0 or omit it per their variant rules.

Resolve Three Frequent BOM Mix-ups

1. Manufacturer Part Number (MPN) vs. Distributor SKU vs. Internal PN

An MPN identifies the physical component; a distributor SKU identifies only one vendor’s listing. Never paste a DigiKey, Mouser, or LCSC code into the MPN column. If that distributor runs out of stock, the assembler cannot cross-reference an alternative supplier without the original manufacturer name and MPN. Keep MPN, Distributor SKU, and your Internal Company PN in three separate columns (matching how KiCad’s IPC-2581 exporter separates manufacturer and distributor part fields).

2. Quantity per Board vs. Purchase Quantity (Attrition & Overages)

The BOM quantity column must always equal the per-board design count—never multiply it by your build volume or add attrition inside the BOM.

• Design demand example: For 100 BASE boards, R1, R2 require 2 × 100 = 200 fitted resistors, while R3 (DNP) requires 0.
• Why purchase quantity is higher: SMT pick-and-place feeders require empty leader tape (or extra components) to thread into the feeder, alongside mispick attrition and Minimum Order Quantities (MOQ). If you consign parts, confirm the exact feeder leader-tape, overage, and continuous cut-tape requirements with your assembler based on their feeder type, packaging, and build volume.

3. DNP vs. Assembly Variant vs. PCB Revision

DNP is a per-position instruction, a Variant is a build configuration, and a Revision is a controlled design change.

• DNP (Do Not Populate): The pads exist on the bare PCB, but no component is placed in this build.
• Assembly Variant: A named build option (e.g., BASE vs. PRO) that toggles Fitted/DNP or swaps an MPN on the same bare PCB.
• Revision (Rev A → Rev B): A formal change to copper, schematic, or BOM documentation. Always specify both PCB Revision and Assembly Variant in your release header.

Incorrect vs. Correct BOM Rows—and How to Fix Them

Never “fix” a conflicting BOM row by guessing which column is right—always check the schematic and datasheet first.

Pre-Submission Checklist: Match BOM, Placement File, and Assembly Drawing

Run these 8 checks in order on your final exported files—not inside your CAD tool—before sending the RFQ or production package.

1. 1. Lock the Release Package: Confirm your release manifest maps the BOM, Gerbers/ODB++, placement file, and assembly drawing to the exact same PCB Revision and Assembly Variant.
2. 2. Audit Designators & Grouping: Expand any hyphenated ranges (R1-R4), confirm zero duplicate designators, and verify each row’s Qty matches its designator count.
3. 3. Verify Fitted vs. DNP Counts: Confirm every DNP slot is isolated from Fitted rows and uses the assembler’s expected quantity rule (Positions vs. 0).
4. 4. Cross-Check MPN, Specs, Lifecycle & Footprint: Confirm every MPN is Active (not NRND/Obsolete), matches its description, and matches the PCB land pattern (especially Imperial vs. Metric chip sizes and exposed thermal pads).
5. 5. Confirm Supply Ownership & Alternates: Ensure every Fitted row is marked Assembler or Customer, and every alternate names an exact MPN with approved reference scope.
6. 6. Reconcile BOM vs. Placement File by Scope: Compare expanded Fitted SMD designators in the BOM against the placement file. Verify THT parts, mechanical hardware, and fiducials separately.
7. 7. Check Polarity & Orientation on the Assembly Drawing: Verify Pin-1 markers, diode/capacitor polarity, and DNP callouts on the assembly drawing match the BOM and placement rotation.
8. 8. Reopen Exported Files & Inspect Portal Mapping: Open the final .xlsx/.csv in a text editor or spreadsheet to confirm zero truncated MPNs or shifted columns. Inspect online portal previews to verify DNP and Customer Supplied flags mapped cleanly.

For the broader RFQ package, see the PCB assembly quote checklist and PCB DFM checklist.

PCB BOM FAQ