Assembly Drawing and BOM Accuracy

An assembly runs on two documents: the bill of materials, which says what to place, and the assembly drawing, which says where and in what orientation. When those documents disagree with the board layout, the line stops, and when they disagree with each other but nobody notices, the boards are built wrong. Most assembly errors that are blamed on the line originate in the documentation.

What the Bill of Materials Has to Contain

A usable bill of materials lists the reference designator, the quantity, the manufacturer part number, the approved alternatives and the package for every item on the board. It also carries a revision that matches the layout, because a bill of materials without a revision is a document that cannot be checked against anything.

The most common defect in a bill of materials is a part number that identifies a family rather than a specific part. A resistor value with no tolerance or package, or a connector family with several variants, forces the assembler to guess, and a guess is a defect waiting to happen. Approved alternatives should be listed explicitly rather than described as equivalent.

Reference Designators and Their Order

Reference designators should be unique, stable and logically ordered. Renumbering them between revisions breaks every inspection programme, every test record and every repair note that refers to them, and the cost of that disruption is far higher than the tidiness it delivers.

Designators should also appear legibly on the silkscreen where they are needed for assembly and repair. A designator hidden under a component or removed by a mask opening provides no reference and will be worked around rather than used. The legibility rules are set out in our article on silkscreen design rules.

The Assembly Drawing

The drawing shows the board with component outlines and the information that a bill of materials cannot carry: polarity, orientation, the location of parts with more than one acceptable position and any note about how a specific part should be fitted. It is the document the operator actually works from.

Polarity must be marked with the convention the operator sees on the part, and the marking should be visible after assembly. Where a part can be fitted in more than one orientation without electrical consequence, the drawing should say which one is preferred so that inspection has a reference.

Revision Control

Every document needs a revision, and the revision has to be tied to the others. A layout revision without a matching bill of materials revision is the classic way to build a batch with the wrong part, and the error is invisible until inspection or test.

Changes should also be reviewed for their effect on other documents. A component change may alter the manufacturer part number, the feeder setup, the inspection programme and the test limits, and the release process should require each of those to be updated rather than left to whoever notices.

Checks Before Release

A release check compares the bill of materials against the layout netlist to confirm that every placed part appears and that no part appears twice. It then compares quantities, packages and part numbers, and finally checks that the assembly drawing matches the layout for polarity and orientation.

The check is mechanical, and that is the point. Automated comparison catches the errors that a careful reader misses, and it takes minutes. Where the comparison tool is not available, a structured manual check against a checklist is far better than an unstructured review.

Communication with the Assembler

Documentation should answer the questions that arise on the line before they are asked. Which alternative part may be substituted, what to do if a reel is short, which side of the board is the primary side, and how a specific part is to be oriented are all typical questions that a good drawing resolves.

Open questions should be resolved in writing rather than by verbal agreement, because a verbal agreement at setup becomes a defect on the third shift. The engineering change process that carries those decisions is described in our article on design release.

Documentation for Test and Repair

The same documents serve test and repair. A test programme refers to reference designators, and a repair technician works from the assembly drawing, so both depend on the accuracy of the same source. Where a repair uses an outdated drawing, the replacement part may be wrong even though the work is done correctly.

Records should also identify which revision was built. Where a product has been through several revisions, the serial number and the revision together tell a technician what is actually on the board, which is information no photograph can provide.

Common Failure Patterns

Three patterns account for most documentation defects. The first is a part number that is ambiguous, forcing a choice on the line. The second is a drawing that lags the layout, so the operator places what the drawing shows rather than what the board needs. The third is an undocumented substitution made during a shortage and never recorded.

All three are prevented the same way: a single source of truth, a release check against it, and a rule that any change goes through the same process regardless of how small it appears. The quality of the finished assembly depends on the quality of the paperwork as much as on the machine settings, and the same evidence driven approach applies as in PCB quality assessment.

Process Control and Verification

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

FAQ

Can the bill of materials live in the CAD tool only? It can, provided the assembler can access the correct revision. A printed or exported copy that matches a revision number is easier to control and easier to audit.

Why not renumber designators when the layout changes? Because every downstream document and record refers to them. Stability is worth more than tidiness.

Who should check the documents? Someone who is not the author, using a comparison against the layout rather than a reading of the document alone.