The Assembly Drawing: What It Must Contain
What the Drawing Is For
The assembly drawing is the document that tells a contract manufacturer what to build and how to check it. It is not the schematic, and it is not the layout file. Its job is to convey everything that is not expressed in the data: which side carries which parts, which orientation marks matter, which areas must not be coated or must be coated, which vias are filled, and how the board is separated from the panel. Where the drawing is thin, the manufacturer makes an assumption, and an assumption that is not written down is the origin of most assembly defects.
Reference Designators and Orientation
The drawing should show the reference designator and the orientation of every part, in a legend that is legible at the size it will be printed. For polarised parts, the mark has to be unambiguous and consistent: a diode with a band on one drawing and a dot on another is a part that will be fitted backwards somewhere in production. Where the reference designators on the silkscreen differ from those in the bill of materials, the drawing should say which governs. In practice the designator in the placement file governs, and the drawing should confirm it rather than introduce a second convention.
The Bill of Materials Link
The assembly drawing should identify the bill of materials by revision and by part number, and it should state that the two are used together. Where a part has an alternative or a substitution is permitted, the drawing is the natural place to record it. Where a component has a specific requirement that the bill of materials cannot express, such as an adhesive under a part or a specific flux, that belongs on the drawing. The rule is that anything a machine cannot read from the data has to be written down.
Notes That Prevent Assumptions
The notes section is where the drawing earns its keep. Typical entries include the soldering method for through-hole parts, the requirement for a no-clean or a water-soluble flux, the cleaning requirement and the acceptance criteria, the requirement that a board be baked before reflow, and any restriction on the reflow profile. Where the assembly has a step sequence, such as a selective soldering operation after the surface mount reflow, the sequence should be stated, because a manufacturer working from the data alone will choose an order that suits their line rather than the design.
Via in Pad and Filled Vias
Filled and capped vias look like ordinary pads to a placement machine, and a shadow under a pad in an X-ray image looks like a void to an inspector. Marking the filled via locations on the drawing removes both problems: the manufacturer knows which pads have a reduced paste volume, and the inspector knows which shadows are expected. The same applies to any via that must remain open for probing, and to any test point that must not be coated.
Coating and Masking Keep-Outs
Where the board will be conformally coated, the drawing should define the keep-out areas as polygons with dimensions rather than describing them in words. Connectors, test points, press-fit areas, anything that will be soldered later and any region likely to be reworked should be shown. The coating material and the thickness specification belong on the drawing as well, since the manufacturer will otherwise apply their house material. Where a coating must not enter a mechanical feature such as a counterbore, that should be explicit.
Test and Inspection Notes
If the assembly will be tested in circuit, the drawing should identify the test points and any fixture constraints, such as components that must be loaded after test. If an inspection step such as X-ray or automated optical inspection is required, and what the acceptance criteria are for voids or for fillet shape, that should be stated. Where the design has a component that cannot be inspected optically, the drawing should say so, because otherwise a report of “no faults found” may be reporting nothing at all.
Panel and Depanel Instructions
The drawing should identify the panel, the rail, the tab locations and the depanelling method, and it should state which edge must not be stressed. Where the board will be broken out by hand, the direction of the break matters, and where a tab is near a plated feature, that should be flagged. The depanelling step is where the mechanical damage happens, and it is the step most often omitted from the drawing.
Keeping the Drawing Current
A drawing that was correct at the first prototype and never updated is worse than no drawing, because it is trusted. The most common drift is a change made to the layout without a corresponding change to the drawing: a component moved, a polarity mark rotated, a via filled, a connector substituted. The assembly drawing should be regenerated from the layout as part of the release process rather than edited by hand, so that the positions and orientations it shows are the ones in the data the manufacturer will use. Notes, on the other hand, are written by hand and should be kept short and stable, since every note is a potential source of a discrepancy between two revisions. Where a change is unavoidable, the revision and the date should be visible on the sheet, and the manufacturer should be asked to confirm which revision they are building from, because a build made from an obsolete drawing is only discovered when the units arrive.
FAQ
Is the assembly drawing the same as the fabrication drawing? No. Fabrication covers the bare board; assembly covers the placement, soldering and any post-processing of the assembled unit.
Does the drawing need every component? Yes, with a designator and an orientation where the part is polarised.
Why mark filled vias? So the manufacturer knows the paste volume is reduced and the inspector does not read the fill shadow as a void.
Should coating keep-outs be polygons or a note? Polygons with dimensions, ideally on their own layer, because a note cannot be interpreted precisely.
Who owns the assembly drawing? The designer, and it should be under revision control together with the bill of materials and the placement data.
Conclusion
The assembly drawing exists to remove assumptions, so anything a machine cannot read from the data has to be written on it: orientations, keep-outs, via treatment, process sequence and depanelling. Keep it in step with the bill of materials and under revision control, and it will prevent most of the errors that are normally attributed to the manufacturer. Assembly services are described under PCB assembly, the soldering processes in SMT PCB assembly, and the verification that follows in PCBA testing. A complete drawing set is the normal starting point for prototype PCB assembly in 2026.
