Short answer: Drilling heats the resin until it flows and coats the inner copper as a smear that blocks plating adhesion. Remove it with permanganate (swell-oxidize-neutralize, taking 0.02-0.05 mm of resin) for standard boards, or plasma (CF4/O2) when you need to reach recesses, treat glass, or process PTFE. Critically, permanganate does not etch PTFE — that needs plasma or sodium-naphthalene.
Number: Smear is molten resin film on inner copper after drilling; permanganate desmear (swell-oxidize-neutralize) removes about 0.02-0.05 mm of resin, plasma (CF4/O2) reaches recesses and treats glass fiber for high-reliability, thick or PTFE boards, and a backlight (shadow) test is run per lot. On PTFE, permanganate is ineffective, so plasma or sodium-naphthalene is required (gopcba, season-pcb, superb-tech, hilelectronic).
Boundary: Smear removal is a post-drill process; the entry/backup board influences heat and smear formation but does not replace desmear. The laminate type decides which desmear chemistry is valid.
Resin smear is one of the most common reasons a plated-through hole fails adhesion. It is not a drilling defect you can see from the top — it is a thin resin film left on the inner copper wall.
Why smear forms
The drill's friction heats the resin until it melts and smears across the exposed inner copper at the hole wall. If left, the electroless copper cannot bond and the via is unreliable.
Two removal routes
| Method | How | When |
|---|---|---|
| Permanganate | Swell-oxidize-neutralize; removes ~0.02-0.05 mm resin | Standard FR4/HDI, most volumes |
| Plasma (CF4/O2) | Reaches recesses, treats glass fiber | High-reliability, thick, PTFE boards |
- Backlight (shadow) test is run per lot to confirm smear is gone.
- PTFE exception: permanganate does not etch PTFE, so use plasma or sodium-naphthalene (see the Rogers/PTFE page).
The entry and backup board influence how much heat builds at the wall, so a stable, low-friction stack helps limit smear in the first place. See the entry board definition and the delamination control page.
