PCB Assembly BOM: from parts list to build instruction

What is a PCB assembly BOM?

A PCB assembly BOM, or bill of materials, identifies the components required for a particular board revision and assembly variant. It connects each fitted reference designator to a quantity, a purchasing identity and the information needed to distinguish that part from a similar-looking substitute. A useful BOM tells purchasing what may be bought and tells assembly what belongs on the board.

The BOM does not describe copper geometry or provide machine placement coordinates. Those tasks belong to fabrication data and the pick-and-place file. Release them together, with the assembly drawing and any variant notes, so that an approved component list is not paired with yesterday’s layout. For a checklist covering the complete package, use required files for PCB assembly.

An early quotation may start from an incomplete list if its gaps are identified. Purchasing approval needs a more definite instruction. Label uncertain components, proposed alternatives and unresolved quantities explicitly; do not let a convenient spreadsheet become accidental authorization to buy or substitute a component.

BOM fields that prevent avoidable questions

Use one unambiguous purchasing identity per fitted line. Group references only when they use the same approved component and assembly status. “10 kΩ resistor” is a description, not a complete purchase instruction: size, tolerance, power, voltage, temperature behavior and qualification requirements may still distinguish candidates. Conversely, a distributor stock code should not erase the manufacturer’s identity.

Keep board identity, revision, variant, issue date and approval status with the BOM. These can live in a controlled cover sheet or release manifest rather than being repeated in every row. The downloadable CSV below demonstrates the checker’s narrow input format; it is not a universal industry template or an IPC-mandated column order.

Check structure before sending the BOM

Paste CSV text or read a local CSV file. The checker looks for duplicate references, mismatched reference counts, missing purchasing fields and ambiguous fit status. Enter the number of boards to see fitted demand before attrition, packaging and minimum-order effects. It does not connect to a parts database, verify stock, interpret an XLSX workbook or decide that a replacement is equivalent.

For this checker, Quantity means the number of references represented by a row, including a DNP row. DNP references contribute zero installed demand. If your company instead uses a zero purchase quantity for DNP items, map that field to a separate column before using the example format. Avoid silently converting an unfamiliar spreadsheet convention.

A clean structural result is a useful starting point, not a release certificate. Footprint dimensions, pin mapping, voltage ratings, lifetime requirements and component condition require engineering and sourcing review. To compare the fitted reference set against placement data, continue to the existing DFA file-consistency tool.

Installed quantity and purchased quantity are different

Suppose R1 and R2 use the same resistor and the build requires 100 boards. The design installs 200 resistors. If the agreed setup and handling allowance is five pieces, the kit requirement becomes 205. An order multiple of 100 would then produce a purchase quantity of 300, leaving 95 pieces to allocate, return or retain. Those numbers are an example, not a recommended allowance.

An expensive processor supplied on a tray does not necessarily need the same overage rule as a small passive on cut tape. Ask how the assembler handles feeder loading, leaders, losses, replacements and shortage reporting for the actual package. Record the agreed extra quantity separately so the BOM’s per-board design quantity remains intelligible.

Do not multiply a total order quantity by the build quantity a second time. Common spreadsheet errors arise when one column means “per board” in some rows and “per purchase order” in others. Name the basis in the header. Keep planned build quantity, allowed overage, MOQ, order multiple and purchased quantity distinguishable even if the purchasing system eventually combines them.

Control DNPs, variants and substitutions

An optional connector may be fitted in Variant A and absent in Variant B. Export or approve the intended variant, rather than asking the production team to infer the choice from crossed-out notes. The placement list and assembly drawing must express the same decision. Reference designators should remain traceable through the change even when an option is not loaded.

Altium’s variant documentation explains that variants can affect manufacturing outputs. Use the documentation for your installed EDA version and review the generated files. A variant selected for the BOM is not evidence that every other output used the same selection.

Separate “approved alternative” from “suggested candidate.” An alternative needs review against the circuit, package, process and product requirements, not just a matching value. Record who may approve it and whether approval applies to one purchase or to the released design. The BOM alternative approval guide covers that decision trail in more detail.

Do not put several mutually exclusive manufacturers into an unexplained free-text MPN cell. Use a primary approved line and an alternative table with exact identities and conditions. This lets purchasing distinguish a permitted option from a list that still needs an engineering decision. Keep distributor identifiers as supporting information and confirm their mapping to the manufacturer part number.

A BOM also defines a sourcing handover

For turnkey assembly, state which parts the assembler may source and what requires approval before purchase. For consigned assembly, add supplied quantities, reel or tray identity, package condition and any relevant handling history. A partial-turnkey build needs an owner per line; “we supply the difficult parts” is too vague to reconcile a kit.

Manufacturer packaging information matters when material is supplied outside its original packing. For example, TI packaging resources provide package and moisture-sensitivity information for TI devices. Use the actual component’s documentation; a photograph of a reel or a generic packaging label does not establish the handling limits of every part.

Define how shortages are reported, how long material waits for clarification, who owns excess stock and whether a repeat order may consume retained inventory. These decisions belong alongside the turnkey versus consigned comparison. The BOM can identify ownership, but the commercial agreement must settle storage, loss and disposition terms.

A practical release check for engineering and purchasing

Start with identity: board number, revision, variant and build quantity. Then compare the reference set and fit status against the approved drawing. Resolve duplicate references before interpreting totals. Check the manufacturer and exact MPN for every fitted purchase line, paying particular attention to package and ordering suffixes. Finally, identify the approval status of proposed alternatives and any customer-supplied material.

Use a simple exception log with the reference, question, decision owner and current disposition. “U3 package suffix to confirm” is actionable. A general comment such as “BOM may change” is not. Keep changes in the release history rather than replacing the file under the same ambiguous name after quoting or purchasing has begun.

Altium’s BOM report documentation is a useful reference for configuring exported fields. Regardless of the software, inspect the output as a manufacturing document: grouped rows, hidden columns, custom field names and alternate choices can change how another team interprets it.

When requesting a review, send the available BOM and explain the decision you need: quotation, sourcing feasibility, variant reconciliation or release readiness. RAYPCB can discuss an incomplete package when the gaps are clearly identified. Files and requirements, rather than a generic list of component names, make the conversation useful.

Already have an approved BOM and a revised export? Use the BOM comparison tool to list added, removed and changed references before reviewing the next assembly release. A difference report supports the review; it does not approve substitutions.

PCB assembly BOM questions