I like your statement "My general opinion about cleaning is that easiest way
to clean something is if that "something" doesn't exist. On the printed
board in this case,oxidation, contamination in the laminate, residues et
cetera."

We all know some type contamination will exist no matter the manufacturing
or cleaning process used. How it affects a "steril" high vacuum environment
is another matter. You have acceptance criteria during operation?

Using Steve Z's logic, I would start with the least "porous" material
possible then laminate it in a vacuum press, for dimensional, outgassing,
and other purposes. The choice of ENIG seems right. Solder mask or not, if
it is LPI it is epoxy approximating the laminate material and finished board
so, I think, go for whatever is needed.

Clean as required to meet specified requirements. Follow this with a good
high vacuum "soak" simulating the operational environment. Then, go for it.

MoonMan

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