In a practical sense, solder mask alone may not give you the protection you
need if your PWB assembly ends up in high temperature/humidity and/or
corrosive environment. Enen though the solder mask will protect all your
circuits in a harsh enviroment, you will still have metallic solder joints
exposed. Therefore, you would run the risk of shorting, the metallic circuits
reacting with the corrosive materials, etrc.

Conformal coatings were developed to completely seal the printed wiring board
assembly after all the components were soldered or joined in place. At this
point, the protective conformal coating covers everything and there are no
conductors exposed to the environment.

Hope this helps

Larry Fisher
Dexter Electronic Materials
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