Protecting printed circuit boards (PCBs) and similar electronics from the incursion of water is an essential responsibility of parylene (XY) conformal coating.  Suitable XY permeation barriers assure no form of liquid passes through to underlying components and that the water vapor transmission rate (WVTR) is minimal.  WVTR measures the level of water vapor migration through the applied barrier film, in terms of area and time.  Optimal WTVR ratings are represented by lower numerical values.  In comparison to liquid coatings, parylene typically provides lowest-level values, indicating better moisture barrier provision.  

Acrylic, epoxy, silicone and urethane coatings can be more quickly affected by water, its vapor, and other sources of moisture, such as: 

• acid rain,
• mists of other airborne pollutants,
• salt-air and
• chaotic weather.

          The polymer parylene (XY) provides exemplary, ultra-thin conformal coating for printed circuit boards (PCBs), solar cells, light emitting diodes (LEDs), medical implants, aeronautical/military equipment and numerous other products, with uniform, insulative protection in the nanometer (nm.) range. 

Application of parylene’s xylylene monomer employs a chemical vapor deposition (CVD) process implemented under a vacuum.  Unlike wet coating application methods – brushing, dipping, spraying, etc. – parylene CVD is not line-of-sight.  Because the vaporous monomer envelopes all sides of the assembly being coated, appropriate process control allows vacuum deposition of an entirely conformal coating, one that penetrates deep into any crevices, rivulets, or sharp edges and points that exist on the assembly’s surface.  The resultant parylene film is insulating, ultra-thin, and pinhole-free, exhibiting superior protective barrier qualities and very low moisture permeability.

Parylene Chemistry and Production Requirements