Exit burr, the torn-copper collar at a hole bottom, is the most common PCB drilling defect and breaks continuity at electrical test. Four levers cut it by up to 40%: backup board choice, entry sheet, drill sharpness and drilling parameters. A harder, flat backup with the right entry stops fibres tearing on exit.
Exit burr is the small collar of torn copper left at the bottom of a drilled hole. Left unchecked, it breaks continuity at electrical test or, worse, opens a joint in the field. The good news: burr is controllable, and most of the control lives in the consumable stack.
Burr forms when the drill exits into a soft or unsupported surface and the last fibres of copper tear instead of shearing. The usual suspects:
A denser, smoother-faced backup backs the hole so the final copper fibres break clean. Our UV Melamine 880 and Phenolic PHE are specified for ≤0.05 mm exit burr on typical stacks. Moisture is the silent killer — store backup flat at ≤25°C and <50% RH.
A clean, flat entry sets the bit straight so the exit is centred. Aluminum entry 1100 H18 is the standard; a coated entry adds location for fine vias. See how to choose a backup board.
A dull bit smears. Track hits per resharpen and match point geometry to the stack — a sharper point for small holes, a stronger web for thick copper.
Lower the feed near exit, raise spindle speed for small holes, and avoid stacking too many panels. Parameters interact with the material — tune them together.
| Lever | Action | Typical gain |
|---|---|---|
| Backup | Denser, dry, smooth face | Largest single gain |
| Entry | Flat, correct alloy | Better location |
| Bit | Resharpen on schedule | Stops smear |
| Feed | Ease near exit | Fewer tears |
For HDI and fine microvias, pair the backup with a coated entry and confirm the set on a HDI sample kit. For automotive thick copper, start from the automotive material set.
Need a side-by-side on your own panels? Request samples and we return a burr comparison.