The BOM checklist

What the BOM does in manufacturing

In design, the bill of materials is a list of what the circuit needs. In manufacturing, it is three things at once: a purchasing document (what to buy, how many, from whom), an assembly document (which part goes at which reference designator), and a traceability record (which approved alternates were used in which build). A BOM that works for design review frequently fails at least one of these jobs, and the failure shows up as a question during the file review, a shortage during kitting, or a wrong part on a board.

Everything in this guide follows from those three jobs. If a column helps purchasing, assembly, or traceability, it belongs in the BOM. If it does not, it is optional.

Required columns

Useful but optional columns include distributor part numbers (in addition to the MPN, never instead of it), lifecycle status if you track it, RoHS or other compliance flags, and unit cost from your own quotes for comparison.

Alternates and the substitution policy

Component availability changes weekly. A BOM with no approved alternates guarantees a substitution conversation at the first shortage; a BOM with alternates pre-approved lets sourcing proceed without one. Approve alternates where you can — passives are usually easy, with tolerance, voltage, dielectric, and package matched — and mark the lines where you cannot.

For lines marked no substitute, be specific about why if it is not obvious: a qualified part in a safety path, a component with a characterized performance, a customer-mandated source. The reason lets the reviewer propose a sensible path if the part becomes unavailable, rather than simply stopping.

The policy Atherton Forge applies is simple: substitutions are proposed with reasons and approved by you in writing before purchase. The approved alternate is recorded against the BOM line so that the next build repeats the decision. Your BOM can shorten that loop by carrying the approvals from the start.

Do-not-populate lines and variants

Do-not-populate (DNP) parts are legitimate: debug headers, optional filters, footprints reserved for a future revision. The problem is inconsistency. If a part is in the BOM without a DNP flag, it will be quoted and bought. If it is in the pick-and-place file, it will be placed. If it is DNP in the BOM but shown populated on the assembly drawing, inspection will flag a missing part. Mark DNP in the BOM, remove or mark those designators in the CPL export, and note them on the assembly drawing — all three, from the same revision.

Product variants — a board built two ways from one layout — are best handled with one BOM and a column per variant showing the quantity for each, or with a clearly named BOM per variant that references the same fabrication revision. What does not work is a single BOM with prose notes explaining which parts belong to which variant.

Quantities, attrition, and minimums

The BOM states quantity per board. The purchased quantity is that number multiplied by the build quantity, plus attrition: small parts are lost to feeder setup, rework, and the tape leader that cannot be picked. Attrition allowances vary by part type and are agreed during the review rather than assumed. Minimum order quantities and reel-versus-cut-tape pricing also mean the buy quantity will exceed the placed quantity, and the surplus is either held for the next build or returned to you, by agreement.

State the build quantity and the expected annual volume with your request. Both change the sourcing decision: whether to buy a reel now, whether a last-time buy is warranted for a lifecycle-flagged part, and whether long-lead lines should be ordered ahead of the board schedule.

Consigned kits

If you supply components, the BOM becomes the packing list. For each consigned line, supply the part in manufacturer packaging where possible, labeled with the MPN and the BOM line number. Cut tape should include a leader; loose parts in bags are placeable by hand but not by machine. Moisture-sensitive devices should arrive in sealed bags with indicators, or be noted so they can be baked before reflow.

Include the attrition quantity agreed at review. A kit that arrives with exactly the placed quantity will run short, and a shortage on one line stops the build for all of them. A receiving check against the BOM is planned per build; discrepancies are reported to you before assembly starts.

Lifecycle and obsolescence

Every component has a lifecycle: introduction, active production, not recommended for new designs (NRND), last-time buy, and end of life. A BOM built during prototyping can carry parts that have quietly moved to NRND by the time production starts, and a part that was merely expensive at prototype can become unobtainable at volume.

Check lifecycle status at BOM review and again before each production run. Manufacturers publish product change notifications (PCNs) and end-of-life notices; distributors surface status in their catalogs. Flag anything not marked active, and decide the response deliberately: qualify an alternate now while both parts are available, make a last-time buy sized to the product’s remaining life, or plan a design change at the next revision. Record the decision on the BOM line.

Pay particular attention to single-source parts — microcontrollers, sensors, specialty power devices, connectors with unique footprints. These cannot be substituted without a layout change, so their lifecycle risk is design risk. A column noting “single source” on such lines lets the reviewer weigh them appropriately and lets you see at a glance how exposed the design is.

Consigned parts are not exempt. If you supply a part that has gone end of life, the shortage still stops the build. Include your consigned lines in the lifecycle check, and tell the review how much stock you hold.

The errors that cause rework

• Description-only lines with no manufacturer part number.
• Distributor part numbers in the MPN column.
• Designator count that does not equal the quantity on the line.
• Designators present in the CPL file but missing from the BOM, or the reverse.
• Merged cells, multi-row headers, or hidden columns that break import.
• MPNs with the packaging suffix omitted, so the ordered part arrives in the wrong packaging for the machine.
• Package column disagreeing with the footprint on the board (0805 in the BOM, 0603 on the layout).
• Polarized parts with no orientation guidance anywhere in the package.
• DNP handled by deleting the line, so the designator on the drawing has no explanation.
• End-of-life parts with no alternate and no flag.
• A BOM revision that does not match the fabrication data revision.
• Prose notes in place of columns (“use the Samsung one if Murata is out”).

Format and revision control

Export the BOM as XLSX or CSV, one sheet, one header row, one line per part. Name the file with the product, the design revision, and the BOM revision, and keep the BOM revision in step with the fabrication data: a BOM change that alters a footprint is a new board revision, not a new BOM revision. State who owns the BOM on your side, because substitution approvals will be sent to that person.

The checklist

1. Every line has a manufacturer and an exact manufacturer part number.
2. Every line has a quantity per board, and the designator count matches it.
3. Every designator in the pick-and-place file appears in the BOM, and vice versa.
4. DNP lines are flagged in the BOM, handled consistently in the CPL, and noted on the assembly drawing.
5. Approved alternates are listed where you can accept them; no-substitute lines are marked with a reason.
6. Package and mounting type are filled in and agree with the layout.
7. Consigned lines are marked, with kit quantities including attrition.
8. Lifecycle-flagged or long-lead parts are identified.
9. One sheet, one header row, no merged cells; file named with product and revision.
10. The BOM revision matches the fabrication data revision you are submitting.