In addition to what Graham has said about salts under conformal coating,
consider also that when a salt crystal encounters water, which WILL come
through conformal coatings, it will expand up to 200 times its original
volume, creating a localized blister.  This effect is often referred to as
"mealing".  Once the blister is formed, it is a place for water vapor to
collect and condense.  Liquid water together with the original salt material
forms an electrolytic solution, which is generally detrimental to circuit
reliability.

Doug Pauls
CSL

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