Parylene as conformal coating for PCBs
There are many choices with tradeoffs that depend on your requirements for dielectric thickness (pF), protection from the environmental stress, reworkability, cost.


AR - Acrylic - easiest to use, but prone to chemical solvent
ER - Epoxy - highest protection, thermal or UV cure, hardest to repair
UR - Urethane - polyurethane 1 or 2 part, UV cure excellent protection
SR - Silicone - best for a high temp to 200'C
PR - Parylene - highest cost, excellent protection.


can be slightly flakey if underprocessed ( anecdotal inside HDA's )



Ref info: Diamond-MT

Acrylic Resin (Type AR):

 
Acrylic conformal coatings are easily applied. They dry to the touch at room temperature in minutes, have desirable electrical and physical properties and are fungus resistant. They have long pot life and low or no exothermic during cure, which prevents damage to heat sensitive components. Also, they do not shrink. The main disadvantage is solvent sensitivity, but this also makes them easier to repair.

Epoxy (Type ER):

 
Epoxy systems are usually available as “two-component” compounds. These rugged conformal coatings provide good humidity resistance and high abrasion and chemical resistance. They are, however, virtually impossible to remove chemically for rework because any stripper that will attack the coating also dissolves epoxy-coated or epoxy-potted components and the epoxy-glass printed circuit board itself. The only effective way to repair a board or replace a component is to burn through the epoxy coating with a knife or soldering iron.

Polyurethane (Type UR):

 
Polyurethane conformal coatings are available as single component, two components, UV curable, and water borne systems. As a group, all provide excellent humidity and chemical resistance plus outstanding dielectric properties for extended periods.

Silicone Type (SR):

 Silicone conformal coatings are particularly useful for high temperature service, up to about 200° C. They provide high humidity and corrosion resistance along with good thermal endurance, making them desirable for PWA’s that contain high heat dissipating components such as power resistors. Silicone coatings are susceptible to abrasion (low cohesive strength) and have high coefficients of thermal expansion.

 Performed per IPC-7711 and IPC-7721 Specifications.

Parylene Type:

Parylene is a transparent polymer conformal coating that is deposited from a gas phase in a vacuum. These polymers are polycrystalline and linear in nature, possess superior barrier properties, have extreme chemical inertness, and because of the deposition process can be applied uniformly to virtually any surface and shape 

Surface cleanliness can affect conductivity

Plating chemistries,    
Flux activators,    
Perspiration ,   
Ionic surfactants.