Checking Copper Clearance Around the Actual NPTH Tool Envelope

A non-plated hole can approach copper more closely than the nominal CAD clearance suggests. The finished hole size, tool position and layer registration each contribute to the actual separation. The absence of intended plating does not prevent the machining operation from exposing a nearby conductor.

Start from the material removed by the tool

Eurocircuits defines inner-layer hole isolation from the production tool envelope. This is a useful distinction when evaluating clearance. A pad or drill symbol in an exported drawing may not represent the maximum material removed during manufacture.

Identify the hole’s maximum permitted size and position relative to the copper layers. Include layer-registration allowance for internal planes. For a routed non-plated slot, examine the complete swept tool path, including rounded ends and any corner cleanup. Do not reduce a slot to a single center-point clearance.

Review the electrical and mechanical purpose together

A mounting screw may intentionally contact an outer ground land while remaining isolated from internal planes. That requires a clear construction detail. An accidental inner-plane exposure is not equivalent to a designed plated chassis connection, and the surface land should not conceal the difference. Review the intended electrical state with the mechanical designer, including whether a conductive screw can touch the bore during insertion before it reaches its final centered position.

• Identify intentional copper contact on each layer.
• Apply the agreed hole-size and position tolerances.
• Check clearance to planes as well as visible traces.
• Include washer and screw movement where it affects electrical contact.

Inspect the location that consumes the allowance

For example, a hole near a dense power plane may have ample top-layer clearance but a small internal margin. A surface photograph cannot confirm that margin. Use the fabrication registration evidence or a representative section where the risk requires it, and preserve the hole’s position within the panel.

Coordinate the result with hole-position references and drill-to-innerlayer offset analysis. If the fabricator proposes changing a hole’s manufacturing sequence, verify that its copper isolation and finished dimensions still satisfy the drawing. The release should describe the intended non-plated feature and its protected copper envelope, so a visually clean hole does not stand in for evidence about hidden layer clearance.