I'm not quite sure what you mean by 'panel board.'
If you mean the steel boxes that make the cases of panels and switchgear, that is defined by the appropriate UL standards, and the size of the box is a major consideration in the sheet metal thickness.
If you mean the wood pieces folks iften use when mounting electrical equipment to masonry walls, I am not aware of any standard defining either the material or the thickness. It's very likely that there will be a local or PoCo standard, though - so check there!
My own experience suggests that anything less than 3/4" will not be thick enough for the mounting screws to hold much; If you have access to the back to place nuts and washers, you might be able to use 1/2". These are design issues, though- not code rules.
FWIW, the last 'board' I made was made from 2x6 Tongue & Groove, covered with two coats of spar varnish.