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
On a design of this kind, obsolescence is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
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.
Process Control and Verification
On a design of this kind, obsolescence is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
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.
Process Control and Verification
On a design of this kind, obsolescence is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
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.
