A plated finished hole diameter describes the opening remaining after processing. A production drill diameter describes the tool used before plating. They are related, but they are not interchangeable dimensions. The safest fabrication drawing specifies the required finished geometry and tolerance while allowing the fabricator to select a qualified drill and plating process.
Define which diameter your requirement controls
For a component lead or mechanical pin, the opening after fabrication is usually the relevant interface dimension. For process analysis, the drilled hole and plating thickness are also important. A CAD hole field may be interpreted differently by different output and ordering workflows, so confirm whether the fabrication package describes finished diameter or tool diameter.
For a non-plated hole, do not simply copy the plated-hole compensation rule. Its process and dimensional requirements are different. Identify plated versus non-plated features explicitly, especially when mounting holes, connector pins and electrical vias appear in the same drill package.
Work a geometric example without prescribing a production process
Consider an ideal circular drilled hole of 0.300 mm and an assumed uniform conductive buildup of 0.025 mm on each wall. The remaining diameter is 0.300 − 2 × 0.025 = 0.250 mm. The factor of two represents the opposing walls; subtracting the coating only once gives the wrong opening.
| Illustrative quantity | Value | Meaning |
|---|---|---|
| Production drill diameter | 0.300 mm | Starting geometric hole |
| Assumed buildup per wall | 0.025 mm | Uniform radial thickness for this example |
| Calculated remaining opening | 0.250 mm | Idealized finished diameter |
| Requested finished tolerance | To be agreed | Not inferred from the calculation |
Actual production includes drill tolerances, wall preparation, plating distribution and finish effects. The simple subtraction is an explanatory model, not an instruction to use a particular tool or a claim that every point in the barrel has equal thickness. Ask the fabricator to confirm the compensation it uses.
Understand the consequence for annular ring
For concentric circular features, the nominal radial difference between a pad and a production hole is half their diameter difference. A 0.600 mm pad around a 0.300 mm production hole gives (0.600 − 0.300) / 2 = 0.150 mm before positional and dimensional variation. Substituting the 0.250 mm finished opening would yield 0.175 mm and overstate the pre-plating geometric ring in this example.
This nominal ring is not the minimum remaining ring on a manufactured board. Registration, drill positioning and feature size variation can reduce it. The acceptance definition and calculation basis must be agreed with the fabricator; do not turn a nominal arithmetic result into an unsupported capability limit.
Relate hole geometry to the complete construction
On an eight-layer board, the hole passes through the full construction unless its layer span is explicitly limited. Board thickness, drill diameter, copper requirements and layer registration interact. A supplier may discuss aspect ratio using its own defined thickness and hole-size basis. Record that basis rather than comparing ratios calculated with different diameters.
For example, 1.600 / 0.300 = 5.33 and 1.600 / 0.250 = 6.40. Both divisions are mathematically correct, but they answer different definitions. Neither result establishes manufacturability without the process owner’s qualification and acceptance requirements.
Issue a hole table that can be checked
- Give the required finished diameter and tolerance for each critical hole family.
- Mark plating status and specify finished slot dimensions separately where relevant.
- Identify press-fit or mechanically critical features instead of treating them as ordinary vias.
- State fill, cap, tenting or backdrill requirements independently of diameter.
- Request the proposed process interpretation and record any approved exceptions.
At inspection, compare the measurement method and location against the agreed requirement. A top-surface measurement does not necessarily describe the narrowest barrel opening. If a connector does not fit, retain the drawing, measurement evidence and affected lot identification before attributing the problem to drill compensation.
References and next step
Eurocircuits: production hole diameter · Eurocircuits: drill-class calculation basis
Use the fabrication requirements guide to document the result. For a project-specific review, include the controlled drawing revision and relevant assumptions in your RFQ.