Does anyone know how to create an area on PCB that will not be covered with solder mask?
You can create a shape using an obtsacle on solderamask layer, but it still fills the solder mask inside of it.
In my experience, the soldermask layer is like a negative layer. It shows where soldermask won't be. In other words, it shows where the openings in the soldermask layer are.
Add->Rectangle, then in the Options Tab select an appropriate class/subclass; i.e. Board Geometry/Soldermask_Top, and draw the rectangle. You'll also need to make sure this class/subclass is part of the soldermask film in your Artwork Control Form.
First clarify what do you mean by "solder mask". The OBTACLE you draw or the PAINT on the board. It is a clasical use of word confusion.
I am going to name SOLDER MASK the obtacle you draw in Orcad.
Suppose you DO NOT want paint on the whole bottom layer (RF board a must), you want only finished metal exposed.
In this case you have to copy the board outline on the soldermask layer.
In conclusion, you have to have an obtacle on the SM layer over the area where you do not want paint (or you want only metal exposed).