I'm beginning a design which will have a front end working in the subnanoamp
region. Fortunately that part of the circuitry is a pretty small area of the
design. Is there an affordable conformal coating which I could apply to keep
leakage currents due to contamination to a minimum once I get it clean and
working?
The simple answer is "no; put it in a sealed box".
If this is a production unit, you need to study the conformal coating
issue before you layout the PCB. In my experience, a sealed box is less
costly in the long run.
Also:
Don't use "no-clean flux" and be careful in selecting water washed fluxes.
It looks like some lead free solder actually works so if CE will be an
issue you can try them. I think they replace the lead with an mixture of
plutonium and unobtainium.
Dry the board before coating or placing in the box. This is usually done
by placing them in the hot box over night or for a few days. In the long
run the coated board will have the same vapor pressure inside as out but
at least you get a clean start on it.
Never apply power to the board until the coating has aged to the point
where the strong smell is gone. Some coatings contain ions that you can
move around when they are wet. If it still smells wet, its still "wet".
Don't believe anything the maker of the coatings tells you. If they say
the sky is blue, check. This is from sad experience. The people that
apply the stuff are much more likely to know the right answers.
Mil-spec coatings are just ones that have made it past the approval
process. They worked well on tube based radios. If you aren't having to
meet some mil-spec. do your own picking based on performance.