Finished Hole vs Drill Diameter

Specify the hole your assembled product needs on the finished PCB, including its tolerance and plating status. The production drill diameter is a manufacturing input used to achieve that result. For a plated through hole, the two dimensions normally differ because material is deposited on the hole wall. Before releasing files, confirm whether the fabricator interprets the sizes in your drill data as finished requirements or already compensated production sizes.

This distinction matters when a connector will not insert, a mounting pin is loose, or a supplier proposes changing a small hole. This guide covers mechanically drilled through holes and their release documentation. Blind vias, laser microvias, backdrilling and filled vias need additional construction-specific instructions. For the broader sequence, see the PCB manufacturing process guide.

Give each diameter a clear meaning

A finished hole dimension describes the opening to be accepted on the delivered bare board, at the agreed inspection stage. A production drill dimension describes the tool selected to manufacture it. The word “drill” in a filename or CAD dialog does not, by itself, establish which meaning the receiving fabricator applies.

Eurocircuits’ production hole diameter explanation distinguishes its production tool size from finished hole size and identifies copper plating and applied finish as contributors to the difference for plated holes. Its numerical compensation is a supplier rule. It should not be copied into every board’s drill data.

Assign a stable feature group such as H1 to holes with the same requirements. A group identifier lets a drawing, drill report and supplier query refer to the same features without relying on diameter alone. Two equally sized holes may have different plating, fit or inspection requirements.

Confirm the supplier’s drill-file convention

Ask a direct question before handoff: “Will you read these tool-table values as required finished diameters?” Record the answer with the board revision. If the package contains process-compensated sizes, label that exception and obtain explicit agreement. Otherwise the supplier may apply its own allowance to an already enlarged value.

The current Eurocircuits drilled-hole guidelines, for example, say its CAM input treats tool-list values as finished sizes. The same page explains separate PTH/NPTH identification and supplier-specific handling of small holes as vias. These are useful questions for a handoff review, not universal defaults to assume at another fabricator.

Mark small component and locating holes by function. If their opening must remain controlled for insertion or positioning, say so. A rule intended to permit via-size adjustments should not silently change a connector hole. Require approval for any proposed change outside the released acceptance limits.

Keep the drilling instructions in the same revision-controlled package as the approved stack-up drawing. Board construction and hole requirements should not arrive in different emails with conflicting revision labels.

Understand the geometry without inventing a drill allowance

In an ideal circular cross-section, a uniform inward deposit of thickness t on the wall reduces the diameter by 2t. That is a geometric relationship, not a complete manufacturing compensation model. A specified minimum copper thickness is not automatically the actual total wall deposit at every point, and a tool’s nominal diameter does not by itself establish the processed opening.

Illustrative calculation only: imagine an opening of 1.00 mm immediately before a uniform 0.025 mm deposit is added to its entire wall. The ideal resulting diameter is 1.00 − 2 × 0.025 = 0.95 mm. The inputs are invented to explain radial versus diametral change. This is not a drill recommendation, a plating specification or a promised finished-hole tolerance.

For purchasing, ask the supplier to confirm the finished limits it will meet and identify any exception. Request its proposed tool information when the design or component specification makes that information relevant. Do not replace a finished acceptance requirement with an informal instruction to “add the usual amount.”

Review the hole against its actual function

A soldered lead, a compliant press-fit contact and a locating dowel impose different requirements. Start with the component or mating-part drawing and the intended assembly method. Check the entire tolerance range, not only the nominal values. For non-round leads, use the specified insertion envelope rather than treating one side dimension as the diameter of a round pin.

A concrete component example is Samtec’s HDTF-4 footprint drawing, revision C. It distinguishes a PTH dimension, drilled-hole dimension and plating detail, and directs users to obtain the connector’s press-fit instructions and fixtures. That illustrates why a footprint must be read as a complete requirement set. Its dimensions are not recommendations for unrelated connectors.

Fit example only: suppose a round pin can be as large as 0.70 mm and the required finished hole is 0.80 ± 0.05 mm. At those limits, the minimum diametral clearance is 0.75 − 0.70 = 0.05 mm. This arithmetic does not prove assembly suitability. Position, shape, alignment and the assembly process still need review. The example is not a press-fit calculation or a suggested soldering clearance.

Check copper geometry and exported files together

Ask which diameter the fabricator uses when checking hole-to-copper spacing. Eurocircuits’ classification guidance measures its inner-layer isolation from the production tool size and distinguishes that size from the finished opening. No numerical clearance from that service is adopted here. The practical check is whether the actual proposed manufacturing geometry has been evaluated against the applicable rules.

Do not approve a larger production hole just because the finished diameter stays unchanged. Ask whether pads, unconnected copper, adjacent holes and relevant fabrication checks have been reviewed. If a supplier suggests changing surrounding copper, obtain the design owner’s approval and keep the accepted change with the release record.

Export the drill files, drill map and fabrication drawing from the same released design. The KiCad 10.0 drill-file documentation describes Excellon and Gerber X2 outputs, units and origin settings, and separate PTH/NPTH Excellon files by default. Confirm the receiving supplier’s requested format and inspect the actual export.

Overlay the exported drill data with copper and outline data in a viewer. Check a known distance, a few identified hole groups and their locations. A PDF with plausible-looking drill symbols is not enough to establish that the machine-readable file uses the correct units, origin or plating classification. Resolve discrepancies before sending the package described in our PCB quotation file checklist.

Use this hole-release checklist

1. Identify the board revision and the exact drawing, drill and copper files that belong together.
2. Label each relevant hole group as PTH or NPTH and identify component, mechanical and via functions.
3. State finished diameters and acceptance limits in consistent units; identify exceptions to general tolerances.
4. Confirm how the supplier interprets drill-table values and who applies process compensation.
5. Include applicable component drawings, plating requirements and restrictions on hole-size changes.
6. Resolve manufacturing queries affecting copper geometry, mating fit or the agreed construction.
7. Agree the inspection evidence needed for critical holes and retain the supplier’s acknowledgement.

For the last item, specify what the report should identify and which limits it must demonstrate. A hole-diameter result and a plating-thickness result answer different questions. Use the quality evidence guide to keep the requested records tied to the delivered board revision.

Frequently asked questions

Should I add plating allowance to every CAD hole?

First confirm the fabricator’s input convention. If it expects finished-hole values and applies its own compensation, adding an allowance beforehand can create an incorrect requirement.

Does an NPTH need a finished-size tolerance?

Yes, when the delivered opening matters to fit or function. NPTH identifies the plating condition; it does not define the acceptable size range or position.

Can a standard hole tolerance be used for every press-fit connector?

Use the applicable connector and application drawing for the exact part and revision. Confirm the hole, plating, board and assembly requirements together with the fabricator and assembler.

Does the drill-file tool list prove the finished holes are correct?

No. The tool list is input data whose interpretation must be agreed. Verify delivered holes against the finished dimensions, tolerances and inspection requirements in the released package.

Primary sources checked October 9, 2026. Supplier rules and the connector drawing are cited as identified examples. Numerical examples are hypothetical and do not establish manufacturing capability or component suitability.