Bill of Materials Control for PCB Assembly

A bill of materials is a controlled document, and the discipline that keeps it correct is the same discipline that keeps the fabrication data correct. Where it is weak, the assembly line builds whatever the machine finds on the feeder, and the difference is discovered at test or in the field.

What Belongs on the Bill of Materials

Every item that is placed on the board belongs, including the parts that are not fitted but have a footprint. Each line should carry the manufacturer part number, the manufacturer, a description, the reference designators and the quantity per board.

The reference designators are what tie the document to the layout. A line without them cannot be verified against the board, and a line with a range that does not match the layout is a defect waiting to happen. Our design release checklist lists the fields to confirm.

Approved Equivalents and Substitution

Where a second source is allowed, the substitute should be listed as an approved equivalent with the criteria that make it acceptable, rather than left to the assembler’s judgement. The criteria usually cover the electrical rating, the package, the tolerance and the temperature range.

Substitution is where most bill-of-materials defects enter. A part that is electrically equivalent but mechanically taller can break a height restriction, and a part that is smaller can leave a pad pattern that no longer matches. Our component tolerance notes describe the parameters that matter.

Reference Designator Discipline

A reference designator identifies a component uniquely and permanently. Reusing one after a deletion, or leaving a gap that suggests a missing part, creates confusion that persists for the life of the product.

Where the layout is derived from a schematic, the designators should flow from that schematic rather than being renumbered to make the layout tidier. Renumbering is a common practice and it costs a re-verification of every document that references a component.

Value, Tolerance and Rating

The value alone is not a specification. Two resistors of the same value may differ in tolerance, in power rating, in temperature coefficient and in voltage rating, and each of those can matter for a particular position in the circuit.

Where a parameter is not critical, it should be marked as such rather than left blank, because a blank field invites a substitution that the designer did not intend.

Obsolescence and Lifecycle

A part that is available today may not be available in three years, and the risk is highest for the specialised parts: a particular processor, a specific connector, a custom magnetics component. The lifecycle status belongs on the bill of materials as a field rather than in someone’s memory.

Where a part is near the end of its life, the design should either provide a footprint that accepts a replacement or reserve space for a redesign. Both are cheaper than discovering the problem at the last purchase. Our industrial systems notes describe how the risk is managed on long-life products.

Packaging and Handling Requirements

Some parts require specific packaging: moisture-sensitive devices need dry packing, and parts that are sensitive to electrostatic discharge need shielding. Those requirements belong on the document rather than in a separate standard that no one reads at the machine.

The floor-life constraint that follows from a moisture sensitivity level is also a build instruction. A part that must be assembled within a defined period after the bag is opened imposes a limit on how much of the board can be built at once.

Verification Against the Layout

The bill of materials should be verified against the layout by a mechanical comparison rather than by reading. A tool that extracts the reference designators from the layout and compares them with the document finds the discrepancies that a human review misses.

The comparison should be run after every change, because a change to the layout can add or remove a designator without anyone updating the document.

Records and Change Control

The bill of materials is a revision-controlled document, and a change to it follows the same process as a change to the fabrication data. The two revisions must always be consistent, because a board built to one revision and assembled to another is difficult to detect and impossible to justify.

Where the customer supplies the document, the assembler should confirm the revision in use at the start of the build and report any discrepancy before the first board is placed.

Process Control and Verification

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Verification and Records

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Points to Confirm at First Article

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Documented Process Control

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Inspection and Traceability Notes

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

Can do-not-populate parts be omitted from the document? They should be listed with a zero quantity, because the footprint exists on the board and the assembler needs to know that the absence is intentional. An unlisted footprint is a part that may be fitted by mistake.

Who is responsible for an approved equivalent? The design authority, because the acceptance criteria are electrical and mechanical rather than commercial. Purchasing may propose a substitute, but the approval belongs to whoever owns the design.

What does gopcb check on a bill of materials? We check the reference designators against the layout, the package against the footprint, the quantity, the polarity marking where it matters and the moisture sensitivity level. Where a discrepancy is found we report it before the build rather than substituting a part ourselves.