PCB design glossary
Drill File
AKA: excellon, drill data
A file (usually Excellon format) that tells the fab where to drill holes — for vias and through-hole pads.
Drill files (Excellon: .drl, .exc, .xln) describe every hole that needs to be drilled into the PCB: position, diameter, plated or non-plated. They ship alongside the Gerber files in the fab package.
Drill data and Gerber artwork together describe the entire board. Drill is its own file (not part of a Gerber layer) because the drilling step happens with different machinery from the photolithography that defines copper.
What the drill file describes
- Every hole's position and diameter.
- Plated holes (PTH): vias and through-hole pads, copper-lined to connect layers.
- Non-plated holes (NPTH): mounting holes and alignment holes with no copper.
- Slots, which some tools write into the drill file and others onto a mechanical layer.
Finished size vs drill size
Plating adds copper inside the hole, so a plated hole ends up smaller than the drill that made it. Most fabs let you specify the finished hole size and choose the drill themselves. For component leads, a finished hole a few tenths of a millimetre larger than the lead is typical.
Common drill file problems
- Drill file missing from the fab package, so the board comes back with no holes.
- Units or zero format mismatched between drill and Gerber files, so holes land in the wrong place.
- Plated and non-plated holes merged into one file, so mounting holes get plated.
- Holes smaller than the fab can drill for the board thickness.
Work it out with a free tool
Check the drill hits against the copper
The Gerber viewer reads Excellon drill files with the layers, so you can see whether each hole lands on its pad before the fab drills it.
View drill and copper together