I understand the purpose of solder mask on PCB copper features, it helps prevent oxidation, reduces the risk of solder bridging, and protects areas that shouldn’t be soldered.
However, I’m not sure about non-plated holes (NPTHs). Since these holes don’t contain copper, is there any reason to apply solder mask around or inside them?
Are NPTHs typically left free of solder mask, or are there cases where masking them is recommended? What are the practical reasons for choosing one approach over the other?
From a fabrication standpoint, it’s common to provide a small solder mask clearance around an NPTH rather than running the mask right up to the hole edge. A clearance of around 5-10 mil is typical, assuming there are no nearby copper features.
If the hole is used for mechanical mounting, it’s also worth ensuring enough clearance around the hole so washers or screw heads don’t interfere with adjacent solder mask openings or exposed copper.
NPTHs are subject to drill and registration tolerances, so if the solder mask opening is too tight, slight misalignment can leave mask partially overlapping the hole edge, creating ragged edges or loose mask flakes after drilling. A modest clearance around the hole accommodates those tolerances and results in a cleaner finished board.
When an NPTH serves as a tooling or alignment hole, mask clearance becomes an assembly requirement, not just a fabrication preference. Alignment pins need clean, accurately sized hole edges to seat correctly, so mask encroachment here affects registration regardless of what the electrical design requires.