You may have multiple mechanical PCB layers, but you will need at least one to fabricate your board. The most basic mechanical layer (sometimes referred to as Mechanical 1) outlines the physical dimensions of your board. This is depicted by the pink outline in the image above. The fabricator will use this layer to cut out your circuit board from their stock material.
This outline can be a simple rectangle or a more complicated shape with rounded corners and / or cut outs. While less common, additional mechanical layers are sometimes included detailing tooling specifications and other miscellaneous mechanical information. For most PCBs these additional mechanical layers are not needed though, so we won’t dive deeper into this topic.
Where the mechanical layer defines the physical boundaries of your board, the keep out layer specifies constraints on the working area of your design. For example, you may want all components drawn back 1/2 inch from the perimeter of your board, the keep out layer will constrain your design in this manner. This is not a layer that is provided to the fabricator, but rather works with your design software to provide feedback to the designer as they develop their design to indicate whether or not they are in violation of the predetermined boundaries.
Keep outs aren’t restricted to the permitter of your board either, they can be used to identify segments on the interior that need to stay clear for mechanical or other reasons. In the image above the keep out restricts our working area from the outer edges of the PCB and near the two mounting holes on the left side of the board.
