This PCB will be used in a high vacuum cryogenic instrument.
Normally I have a surface plane on top and bottom. These are done with Gold flash or similar and No Solder Mask.
This metal layer keeps the total exposed FR4 surface area to a minimum, which reduces the effects of outgassing for the FR4. The Gold finish is also easy to clean from finger prints and oils.
However for this design I have some small pitch SMT parts and the assembler really wants Solder Mask to keep the solder on the pads from wicking between lands. ( I can certainly sympathize with his perspective. )
So I want to put a small area of Solder Mask only where it is needed for manufacturing.
I.E. What I am looking to achieve is to maximize the total area of bare metal and minimize the total area of Solder Mask exposed to the vacuum.
I do not seem to be able to produce Keepouts on the Solder Mask layer like we can with copper pours.
Any hints would be greatly appreciated.