Engineering resources
Finished hole size versus drill tool size
A PCB hole drawing describes the feature the finished board must provide. A drill tool describes one manufacturing input. Confusing them can lead to a board that passes one interpretation of the drawing but does not accept the component lead or mounting hardware.
State what the dimension means
For every hole class, identify the nominal finished dimension, tolerance, plating status and function. Separate electrical vias, component holes, press-fit locations and mechanical mounting holes when they have different requirements. A press-fit hole must be reviewed against the connector supplier’s specified finished-hole system, not treated as an ordinary clearance hole.
Eurocircuits explicitly explains how its CAM process interprets incoming drill sizes as finished hole sizes. That is useful evidence of why the handoff convention matters; it is not proof that every supplier uses the same convention. Ask your chosen fabricator to acknowledge how the drill file, hole table and notes will be interpreted.
A simplified plating calculation
Hypothetical geometry only; these numbers are not process settings or acceptance limits. If a cylindrical opening were exactly 0.350 mm before plating and received a uniform 0.025 mm deposit on each wall, the remaining diameter would be 0.350 − 2 × 0.025 = 0.300 mm. Counting the deposit once would incorrectly predict 0.325 mm.
Real hole preparation, plating distribution and measurement introduce additional considerations. Consequently, this subtraction must not be used to assign a production drill tool without the fabricator’s process review. Its purpose is to show why a finished dimension and a tool size are different quantities.
| Hole-table column | Useful entry | Ambiguity to avoid |
|---|---|---|
| Function | Via, component, press-fit or mounting | One tolerance for every hole |
| Size basis | Finished diameter, with units | A bare number labeled “drill” |
| Plating | Plated or nonplated | Inferring it only from a pad shape |
| Acceptance | Approved tolerance and method | Relying on an unreferenced default |
Reconcile the files before release
Compare the hole count and locations in the fabrication package with the CAD output. Confirm units, coordinate origin and the intended handling of plated slots. Check that mechanical holes are present in the same revision of the outline drawing. If separate drill files are supplied, make their purpose evident in the manifest.
Review pads and registration alongside the proposed drilling process; specifying a finished hole does not by itself resolve annular-ring requirements. Keep any approved tool compensation with the fabrication records rather than silently modifying the customer drawing. The manufacturing review should close these questions before release, including for specialty laminate constructions.
Discuss your fabrication package
Submit project requirements and fabrication files for review. Material suitability, availability and commercial terms require project-specific confirmation.
Sources and scope
Manufacturer references checked 9 October 2026. Examples and review worksheets are original editorial guidance, not approved process instructions. Historical reference documents are used for fundamentals, not current product availability.