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Solder mask expansion, also known as solder mask clearance or solder mask swell, is the distance between the edge of a pad and the edge of the solder mask opening. The expansion is essential to provide sufficient clearance between the pad and the solder mask to prevent solder bridging and ensure proper soldering.
Along with the KiCAD v6 release, the Solder mask expansion feature changed.
Since the release of KiCAD v6 the Solder mask expansion setting in Board setup can expand Solder mask of all footprint WITHOUT the expanded mask.
Again:
1️⃣ If your footprint has defined solder mask expansion, the manufacturing data will follow the footprint setup.
2️⃣ If your footprint does not have that expansion, the Board setup – Solder mask expansion setting overrides that footprint.