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In Protel 2004 SP2 by default only pads are given a solder mask expansion, which is defined in the Design Rules or on a pad by pad bases within the pad properties.  Protel uses negative layers for the Solder Masks which means any object you see in that layer is a gap in the Solder Mask.

Therefore if we wish to define an area of our board to have no solder mask we need to place an object on the Solder Mask layer of the correct shape.

Within Protel 2004 the Solder Mask layers are defined as Top Solder and Bottom Solder.

To place a shape on the Solder Mask layers first select the Solder Mask layer you wish to use.  If the shape you wish to use is a square or rectangle I would suggest using the Place » Fill command and for more complex shapes you could use Place » Copper Regions or even Arcs.

The same rules and techniques can be applied to the Paste Mask Layers also.