How to Prepare BOM for PCB Assembly

1.   Table of Contents  
2. 1. Why BOM Preparation Is Critical
3. 2. What Is a BOM in PCB Assembly   
4. 3. Step-by-Step BOM Preparation   
5. 4. BOM Format Example
6. 5. Download BOM Template
7. 6. Common BOM Mistakes
8. 7. BOM Checklist
9. 8. BOM vs CPL vs Gerber
10. 9. How to Submit on NextPCB
11. 10. Conclusion

1. Why BOM Preparation Is Critical in PCB Assembly

Among all the files you submit for a PCBA order — Gerbers, CPL, stackup notes — the Bill of Materials causes the most delays. Not because it's the hardest to create, but because it's the easiest to get wrong in ways that aren't obvious until a sourcing engineer flags your order.

A missing Manufacturer Part Number means the factory has to guess what you intended. Inconsistent footprint naming means they might order the right part in the wrong package. An ungrouped BOM with 150 individual rows instead of 40 grouped ones wastes review time and introduces transcription errors during kitting. In each case, the result is a "BOM hold" — your order sits in limbo while you exchange emails to clarify what you meant.

The practical cost of a poor BOM isn't just time. Rework from wrong components, expedited shipping for missed parts, and engineering hours chasing substitutes all add up fast. A correct BOM is not just a list of components — it's a manufacturing document, and it should be treated as one from the first prototype run.

This guide walks through exactly how to build one that won't get held. If you're new to the overall set of files required for a PCBA order, start with our guide on How to Prepare PCB Files for Assembly first, then return here for a deep dive on the BOM.

2. What Is a BOM in PCB Assembly

A Bill of Materials (BOM) is a structured spreadsheet that lists every component that goes onto your board — what it is, how many are needed, which reference designators it corresponds to, and (critically) its exact Manufacturer Part Number.

It's worth distinguishing the BOM from the two other files in a standard PCBA package:

The BOM and CPL are linked through reference designators. If R3 in your BOM calls for a 10kΩ 0402 resistor, the CPL must have a row for R3 with the correct centroid coordinates. Any mismatch between these two files is caught during engineering review — and causes a hold.

3. Step-by-Step: How to Prepare a BOM for PCB Assembly

Step 1: Start from Your Schematic, Not the Layout

The authoritative source for your BOM is the schematic, not the PCB layout. The layout tells you where components sit; the schematic tells you what they are and why. When you export a BOM from your CAD tool, it should pull data that was entered at the schematic stage — component value, footprint assignment, and reference designator.

Before you export anything, confirm every schematic symbol has three things filled in:

• Value — the electrical specification (e.g., 10kΩ, 100nF, LM317)
• Footprint — the physical package assigned in your library (e.g., R_0402, SOT-23)
• Reference designator — the unique identifier on the board (R1, C4, U2)

If any component is missing one of these, fix it in the schematic before exporting. Trying to patch a BOM after export by hand is how errors get introduced.

Step 2: Add Manufacturer Part Numbers (MPN)

The MPN is the single most important field in any BOM intended for manufacturing. It's the unambiguous identifier that tells procurement exactly what to buy — and it eliminates any interpretation on the factory's end.

A value like "10kΩ 1% 0402" describes a resistor, but it doesn't specify a part. Hundreds of manufacturers make that resistor. Without an MPN, the factory either picks one themselves (which may not match your tolerance, temperature coefficient, or qualification tier) or they hold your order and ask. Neither outcome is good.

The correct approach: include the full MPN from the component's datasheet. For example:

• Correct: RC0402FR-0710KL (Yageo 10kΩ 1% 0402 thick film resistor)
• Incorrect: "10k resistor" or "10k 0402"

If you don't have MPNs for all parts yet — common early in a design — flag those rows explicitly. A cell that says "TBD" is better than one that's left blank, because it signals an intentional gap rather than an accidental omission.

Step 3: Group Identical Components

Every component that shares the same value, footprint, and MPN belongs in a single BOM row. Group the reference designators together in one cell, and sum the quantity.

An ungrouped BOM is harder to review, more prone to quantity errors, and slower to kit. Most CAD tools can group components automatically during export — use that feature. If you're cleaning up a manually created BOM, sort by MPN first, then consolidate.

Step 4: Fill In All Required Fields

A complete BOM for PCBA manufacturing needs these six fields at a minimum:                                                                                                                                       

The Manufacturer Name column is often optional but useful — especially when the same MPN root is used by multiple manufacturers. Pairing the MPN with the manufacturer name eliminates any ambiguity.

Step 5: Add Optional Fields That Matter

Depending on your order type and design complexity, these fields can save significant back-and-forth:

• Supplier / Supplier Part Number — if you have preferred distributors (Digi-Key, Mouser, LCSC), including their part numbers speeds up sourcing. For turnkey orders, the factory may or may not use them, but it's a useful fallback.
• Substitution Allowed (Yes/No) — this is more important than most designers realize. If a part goes out of stock, should the factory substitute with an equivalent, or halt and ask you? For generic passives, "Yes" keeps production moving. For any part with a specific qualification, frequency tolerance, or software dependency, "No" is essential.
• DNP (Do Not Place) — if your design has unpopulated variants, mark those components explicitly as DNP. You can also use DNI (Do Not Install) or NC (No Connection) — NextPCB's parser understands all of them. Don't leave them out of the BOM entirely — that creates confusion about whether they were intentionally omitted or accidentally missed.

Step 6: Export from Your CAD Tool

All major EDA tools can generate a BOM automatically. The output quality varies, but using the native exporter is always better than building a BOM by hand.

KiCad: In Eeschema, go to Tools → Generate BOM. KiCad uses Python-based BOM plugins — the built-in "bom_csv_grouped_by_value" plugin exports a grouped CSV sorted by value, which is a solid starting point. If your schematic has MPN fields populated (add them as custom properties in the symbol editor), they'll appear in the output.

Altium Designer: Reports → Bill of Materials opens the BOM dialog. You can configure which columns to include and export to Excel or CSV. Altium's ActiveBOM feature adds live inventory data and pricing directly in the BOM environment, which is useful for sourcing validation before you submit.

EasyEDA: In the schematic editor, go to Fabrication → BOM. EasyEDA exports a CSV with reference, value, footprint, and LCSC part number (if you used LCSC library parts). NextPCB's system fully supports EasyEDA outputs and can automatically cross-reference LCSC part numbers to our global supply chain, making this output nearly ready to submit.

Regardless of tool, the exported file is a starting point. Plan to review and clean it before submission.

>> Recommend reading: Top 10 Free PCB Design Software (2026)

Step 7: Clean and Format the BOM

Raw CAD exports rarely come out submission-ready. Before you send the file, go through these cleanup steps:

Normalize units. A BOM where some resistors are "10k", some are "10K", and one is "10000Ω" will confuse anyone reviewing it manually. Pick one convention and apply it throughout. Standard practice: use engineering notation (10kΩ, 100nF, 4.7µH) rather than raw SI values.

Verify footprint consistency. Cross-check the footprint field against the actual PCB land patterns. An 0603 resistor listed in the BOM but placed on an 0402 pad is a sourcing error waiting to happen. Your CAD tool's netlist or BOM-to-layout cross-check feature can catch these mismatches.

Remove empty rows and duplicate entries. Empty rows from partial exports and duplicate component rows from library merge issues are common. Sort by reference designator and scan for anything that doesn't look right.

Check the reference designator list for gaps. If your board has R1 through R25, but R14 doesn't appear in the BOM, either R14 is intentionally DNP (mark it as such) or it was accidentally omitted. Either way, it needs to be addressed.

Use a consistent file format. Most manufacturers accept .xlsx or .csv. Avoid .ods or proprietary formats. If using Excel, don't put the BOM data inside a formatted table with merged header cells — plain rows and columns are easier to parse programmatically.

Step 8: Validate Before Submission

The final step before uploading is a deliberate review against the PCB layout and schematic. Specifically:

• Open your PCB layout and confirm that every reference designator in the BOM has a corresponding footprint on the board.
• Check that DNP components are visible in the layout but excluded from the CPL (pick and place) file.
• Confirm total component count makes sense relative to the complexity of your design.
• If you have a specific sourcing requirement (RoHS, AEC-Q, military screening), verify the MPNs you've listed meet those qualifications.

Many manufacturers, including NextPCB, offer an online BOM checker that cross-references your MPNs against distributor stock in real time. Running your BOM through one of these tools before submitting can surface availability issues before they become production holds.

For complete peace of mind, we highly recommend running your files through NextPCB's free HQDFM software. It automatically cross-references your Gerber, BOM, and CPL files in a 3D environment, highlighting footprint mismatches, missing MPNs, and rotation errors before you even upload them to the site.

4. BOM Format Example for PCB Assembly

Below is a representative example of a well-formatted BOM ready for PCBA submission. Reference designators are grouped, all rows have MPNs, DNP components are explicitly marked, and units are consistent throughout.                                                                                                                                                                         

A few things to note in this example: R6 and R7 are DNP jumpers present in the design for a variant option — they're included in the BOM with their MPN, but clearly marked DNP so they won't be placed or sourced. Note: Even if quantities are listed for DNP parts, our system will automatically exclude them from your final component cost. Every row has a complete MPN, and the unit formatting (kΩ, nF, µF, µH) is consistent throughout.

5. Download BOM Template for PCB Assembly

If you're starting from scratch or need a clean, manufacturer-ready format, the NextPCB BOM template below includes all required and recommended fields, pre-labeled column headers, sample data rows you can replace, and a built-in instructions sheet explaining what each field means and how to fill it.

The template is formatted as .xlsx — the format most PCBA manufacturers, including NextPCB, accept directly. Save your completed BOM in either .xlsx or .csv before uploading.

Download NextPCB BOM Template (.xlsx) Use NextPCB BOM Tool

If you already have a BOM but aren't sure whether your part numbers are current and in stock, NextPCB's BOM sourcing service can cross-reference your MPNs against live distributor inventory, suggest alternatives for out-of-stock components, and flag obsolete parts — before your order enters the queue.

6. Common BOM Mistakes That Delay PCB Assembly

Missing or Incomplete MPNs

This is the most frequent cause of BOM holds. A description like "N-Channel MOSFET, 30V, SOT-23" could match dozens of parts across multiple manufacturers. Without an MPN, procurement has to stop and ask — and your order waits.

Impact: 1–3 day delay minimum, plus risk of wrong component substitution if the factory makes a judgment call without checking.

Footprint Mismatch with Actual Package

The footprint field in your BOM might say "0402", but the MPN you listed is for an 0603 package. Or you listed a SOIC-8 part but the PCB has an MSOP-8 footprint. This happens when components are swapped late in the design process without updating all related fields.

Impact: Components arrive that physically don't fit the pads. Either the order is held for a corrected BOM, or — worse — the wrong part gets assembled and passes visual inspection but fails electrically.

Inconsistent Value Notation

When the same nominal value appears as "10k", "10K", "10,000", and "10kΩ" across different rows, it creates unnecessary review work and can cause grouping errors if the BOM is processed programmatically. Manufacturers who use BOM-matching software will treat these as different parts.

Impact: Duplicated sourcing effort, potential for ordering different parts at the same value due to automated mismatch.

DNP Components Not Marked

Leaving DNP components out of the BOM entirely is not the same as marking them DNP. If a reference designator appears in the CPL file but not in the BOM, the factory doesn't know whether it's intentionally absent or was accidentally omitted. They'll hold the order to ask.

Impact: Engineering hold, delayed production start.

Ungrouped Components

A BOM where each reference designator is its own row — even when the same MPN is used across 20 resistors — is harder to review and more likely to have quantity errors. It also gives the factory less information about how components are distributed across the board.

Impact: Slower review, higher chance of kitting errors on the factory floor.

Quantity Per Panel Instead of Per Board

If your PCB is panelized (multiple boards per panel), the BOM quantity should reflect one board — not the entire panel. The factory calculates panelization quantities on their end. Submitting panel-level quantities leads to either over-ordering or under-ordering of components.

Impact: Component shortages or excess inventory at the factory, potential production interruption.

7. BOM Checklist Before Sending to Manufacturer

Before you upload or email your BOM, run through this checklist. Each item corresponds to a real failure mode that causes assembly delays.           

If you're unsure about any of these points before submitting, NextPCB's engineering team reviews every order — but catching issues yourself ahead of time eliminates the back-and-forth that adds days to your lead time.

8. BOM vs CPL vs Gerber: The Complete PCBA File Set

A PCB assembly order requires three files, and each serves a distinct function in the manufacturing process. Understanding how they relate helps you spot inconsistencies before they reach the factory.

The Gerber files define the physical board: copper traces, drill holes, solder mask openings, and silkscreen. These go to the fabrication side — the bare board manufacturer uses them to produce the PCB substrate. They tell the manufacturer nothing about what components go on the board.

The BOM is the procurement document. It answers the question "what parts do we need to buy, and how many?" The factory's sourcing team uses it to order components, verify stock, and prepare kitting for the assembly line. It references the board only through reference designators.

The CPL (Component Placement List), also called a Pick & Place file, tells the SMT machine exactly where each component goes. It contains X/Y coordinates and rotation angle for every surface-mount component, referenced by designator. It's generated from the PCB layout, not the schematic.

All three files must be internally consistent. If U3 appears in the BOM but not in the CPL, the factory doesn't know whether it's a through-hole part (acceptable) or was forgotten (a problem). If R12 appears in the CPL but not the BOM, there's nothing to source for it. The BOM and CPL are cross-referenced during engineering review — mismatches are caught, and the order is held until resolved.

For a detailed walkthrough of all files in a PCBA submission package — including Gerbers, stackup notes, and assembly drawings — see our guide on How to Prepare PCB Files for Assembly.

9. How to Submit Your BOM on NextPCB

NextPCB offers two ordering paths depending on your timeline and order type. Both accept the same BOM format — the difference is how quickly the system processes it and how much manual review is involved.

Option 1: Rev0 — AI-Powered Instant Quote

Rev0 is NextPCB's AI-powered PCBA quoting platform. When you upload your BOM, it cross-references your MPNs against a database of 500M+ component SKUs in real time, flags any issues — missing MPNs, out-of-stock parts, obsolete components — and surfaces alternative matches automatically. The result is a production-ready quote in minutes rather than the 24–48 hour engineering review cycle of a standard order.

Rev0 is particularly suited to prototype runs where speed matters and you want to catch sourcing issues before they interrupt production. The upload flow is straightforward:

• Go to nextpcb.com/rev0-pcba and sign in or create an account.
• Upload your Gerber file first — this defines the board for fabrication.
• Upload your BOM (.xlsx or .csv) and CPL/Pick & Place file.
• The AI parser reads your BOM immediately. Any flagged issues appear on-screen with suggested fixes — missing MPNs show auto-matched alternatives, out-of-stock parts show available substitutes.
• Review the sourcing summary, confirm or adjust substitutions, and proceed to quote.
• No email exchanges. No waiting for an engineer to open your files. Production starts from the confirmed quote.

Try Rev0 — Instant PCBA Quote

Option 2: Standard PCBA Quote

For larger production runs, turnkey orders with complex sourcing requirements, or boards with through-hole components that require wave soldering, the standard PCBA quote path gives you direct access to NextPCB's engineering team for pre-production review.

• Go to nextpcb.com/pcb-assembly-quote.
• Upload your Gerber file via the Gerber Viewer.
• Specify board quantity, assembly options, and surface finish.
• Upload your BOM and CPL files. Include any assembly drawings or special instructions in the notes field.
• An engineer reviews the complete package and returns a detailed quote, typically within 24 hours.

Get Standard PCBA Quote

Need Help Sourcing Components First?

If your BOM has gaps — parts marked TBD, obsolete MPNs you haven't replaced yet, or components you want to cost-optimize — NextPCB's BOM sourcing service handles this before the order is placed. The team checks availability across authorized distributors, identifies drop-in alternatives, and returns a sourced BOM ready for submission. This step is optional but useful for production-scale orders where a single out-of-stock line item can delay an entire run.

10. Conclusion

A BOM that clears engineering review without a hold isn't complicated to produce — it just requires discipline at a few specific points: assigning MPNs in the schematic, grouping components correctly on export, verifying footprint-to-package consistency, and marking DNP parts explicitly. The eight steps in this guide cover all of them.

Most BOM-related delays aren't caused by design complexity. They're caused by omissions that are easy to miss when you're moving quickly. The checklist above catches the common ones before they become email threads.

When you're ready to order, Rev0 validates your BOM automatically on upload — flagging stock issues and missing MPNs in real time, so you get a confirmed quote in minutes rather than days. For larger or more complex runs, the standard PCBA quote path connects you with NextPCB's engineering team for a full pre-production review.