How to tell plated vs non‑plated holes in Excellon .drl files?

Short answer

You usually cannot determine plating from a bare Excellon .drl alone. Use split PTH/NPTH drill files, explicit attributes/comments, a drill drawing, or cross‑check with copper Gerbers.

What in the .drl header should I look for?

Classic Excellon has no standard code for plating. Some CAD/CAM tools add nonstandard comments or attributes. Search the header and tool table for strings like these (case varies):

• ;TYPE=PLATED or ;TYPE=NON_PLATED

• ;PTH / ;NPTH in tool notes or file comments

• ATTR,Plated,Yes|No or %TF.HolePlated,P|NP%

If you find nothing, the file itself is ambiguous.

Do EDA tools separate PTH and NPTH files?

Often yes. Many exports produce two drill files, commonly named like Board-PTH.drl and Board-NPTH.drl (or .txt). If you see two drill files and the headers or filenames clearly say PTH vs NPTH, that’s the intended split. If there’s only one .drl, designers usually expect all holes to be plated unless otherwise noted in a drill chart or fabrication notes.

Cross‑check with Gerbers to classify holes

When the .drl is ambiguous, correlate drill coordinates with copper layers:

• Likely PTH: hole locations coincide with copper pads (top and/or bottom); typical via sizes are 0.20–0.35 mm finished, with annular ring ≥0.10 mm.

• Likely NPTH: hole locations have copper clearance (no pad) on all copper layers; common for tooling, fiducials, and mechanical holes (e.g., 2.5–3.3 mm for M2.5–M3).

Slots follow the same rule: a plated slot will have copper pads or connected copper; a non‑plated slot will be cleared in copper.

These are heuristics—confirm with a drill chart when available.

What if I only have one drill file?

State your intent on the fab drawing: identify which tool codes (T01, T02, …) are NPTH vs PTH, or provide a note such as “All holes plated unless marked NPTH.” Without this, CAM will often treat holes tied to copper as plated and copper‑cleared holes as non‑plated.

Preferred deliverables for unambiguous plating

• Two Excellon files: one PTH, one NPTH, with a drill table/chart that maps T‑codes to sizes and plating.

• Or a drill file with explicit attributes/comments per tool indicating plated/non‑plated.

• Or a modern, attribute‑rich package (e.g., Gerber X2/XNC with hole‑plating attributes, or IPC‑2581), plus a fabrication drawing.

Tolerances and CAM implications

NPTH are typically drilled after plating and do not receive barrel copper; common finished hole tolerances are about ±0.05 mm for NPTH and ±0.076 mm (±3 mil) for PTH. Mixing plated and non‑plated tools in one file without clear flags risks wrong drill stack usage and tolerance misapplication.

Quick checks that catch most cases

• Two drill files clearly named PTH/NPTH: trust the split.

• Header/comments explicitly say plated or non‑plated: follow them.

• No comments and one file: assume holes with copper pads are PTH; holes with copper clearance are NPTH; then confirm on the drill chart with the fabricator.