How do organic coatings protect electronic components

We report electrical conductivity measurements on surface electrodes as a function of relative humidity (RH). Experiments were conducted on both (1) bare surfaces and (2) surfaces with organic coatings. Electrically-insulating, hydrophobic liquids (e.g., toluene, hexane, silicone oil) were used as coatings. Under dry conditions (0% RH), both bare- and coated-electrodes show extremely low surface conductivity. Going from 0% to 100% RH with bare surface electrodes produced a large (expected) conductivity increase. This is attributed to sorption of a thin, electrically-conducting water layer. A less obvious result was that going from 0% to 100% RH also produced a large conductivity increase for surface electrodes coated with organic liquids. This suggests that water affinity for hydrophilic surfaces negates geometric surface coverage by the coatings - at least from a conductivity standpoint. Corresponding results were obtained with coated and uncoated printed wiring boards. The impact of this finding on the reliability of coated electronic assemblies will be discussed.