Parylene’s (XY) reputation as the most versatile and reliable of major conformal coating materials is well-earned.  However, unlike liquid coatings –resins of acrylic, epoxy, silicone and urethane – parylene cannot be applied via relatively economical brush, dip or spray methods.  XY can be the most expensive of the major conformal coatings to use, a factor influenced both by:

Parylene (XY) polymer conformal films are recognized for their exceptional range of desirable functional properties for coating printed circuit boards (PCBs) and similar electronics.  Beneficial properties include biocompatibility, chemical/solvent resistance, dielectric/insulative reliability, and ultra-thin pinhole-free film thicknesses between 1-50 μm.  They also generate complete surface conformability, regardless of substrate configuration, exceeding the coating capabilities of liquid conformal materials, such as acrylic, epoxy, silicone and urethane.   

For conformal coatings, elongation is a measure of material ductility -- a specific coating's ability to undergo significant plastic deformation before rupture.  A coating’s yield elongation is the maximum stress the material will sustain before fracture.  Thus, computed parylene (XY) elongation measurements represent the total quantity of strain the conformal film can withstand before failure.  While elongation is equal to a material’s operating failure strain, it has no exclusive units of measurements.  Typically,

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.

          As the electrical components used to power printed circuit boards (PCBs) grow smaller, conventional conformal films become less effective for coating them.  Ongoing development of microelectricalmechanical systems (MEMS) and nano technology (NT), has little room for the thicker conformal films provided by liquid materials, such as acrylic, epoxy, silicone and urethane.   Nanocoats (NCs) are increasing in prominence, frequently surpassing micro-thin parylene (XY) for many MEMS/NT purposes.  

Hydrophobic Basics and Hydrophilicity

Parylene (XY) conformal coatings are applied to substrate materials through a specialized chemical vapor deposition (CVD) process that completely eliminates the liquid phase of wet coatings.  No initiators or catalysts are involved in CVD polymerization, which synthesizes truly conformal protective film in-process.  This is in stark contrast to wet coating materials such as acrylic, epoxy, silicone and urethane, which are synthesized prior to application via, brush, dip or spray methods.  Wet during application, liquid-coated substrates requiring further drying and curing.

With reliable moisture barrier properties, parylene (XY) conformal coatings generally have a hydrophobic surface when deposited onto substrates, causing liquids to form separate droplets on film surfaces.  While this outcome is useful for many XY applications, greater hydrophilic response, wherein XY molecules form ionic or hydrogen bonds with water molecules, can also be desired.  This can be achieved by applying glue or epoxy on top the deposited parylene; surfaces acquire enhanced hydrophilic properties, becoming more wettable. 

Parylene:  Properties and Processes

Taber tests are designed to measure a material’s capacity to withstand abrasion and its effects during operation.  Conformal coatings – both liquid and parylene (XY) – are