KB: How to Create Custom Soldermask Openings

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Most objects have a Soldermask Expansion property setting that you can manually assign a value, allowing you to create an opening around specific objects. To do this, select the object and then open the Properties Panel. Once open, there would be an option for Solder Mask Expansion that is set to Manual and disabled by default. To set a Value, select the Checkbox next to the entry and change the value from 0mil to any value you want for the Soldermask opening around that specific object.

Since the Soldermask layer functions as a negative layer, any object placed on it will remove soldermask and create an opening. To make custom Soldermask Openings, you can place a Soldermask object, usually a Polygon, Fill, or Region, in the desired area.

Switch to either the Top or Bottom Solder Mask Layer, where you want the opening to be on and then Place ► Polygon pour \ Fill \ region ► Place it on the copper area wherever you want to expose the PCB copper plane areas ► Create the shape.

If we need some voids in the mask opening shape, we can use a polygon pour for the solder mask opening shape and a polygon pour cutout for the voids in it. This will help us achieve the custom solder mask opening with voids. 

With the custom Solder Mask object placed on the board, the generated Gerbers would show the object and correctly interpret it as a negative layer, resulting in the desired soldermask openings.