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,

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. 

The phrase “in-line parylene processing" is deceptive  because it does not accurately describe the method in which parylene (XY) is applied as a conformal coating.  It is true that some aspects of the traditional production line are relevant, but primarily in a fractional way. without the traditional station-to-station regimentation of standard in-line manufacturing processes.

Parylene conformal coating (XY) provides insulative protection for complex electronic circuit assemblies expected to function through rigorous operating conditions -- potential chemical, electrical, moisture and vapor incursion during performance.   Applied through chemical vapor deposition (CVD), parylene penetrates deep within substrate surfaces, generating a level of assembly security surpassing that offered by liquid coatings such as acrylic, epoxy, silicone and urethane.  Yet, although XY is applied in a vacuum, it’s capacity to provide these extraordinary qualities does not exist in one.  Parylene’s durable protective value depends on film adhesion, a quality subject to persistent, thorough inspection throughout the production process.

Accidentally discovered in 1947, by chemist Michael Szwarc, the polymer parylene originally bore his name, and was known for a brief period known as Szwarcite. Working to thermally decompose the solvent p-xylene at temperatures exceeding 1000 °C, Szwarc identified the monomer para-xylylene di-iodide as the only product resulting when para-xylylene was reacted with iodine.

The Need for Cleanliness Testing                                          

For various reasons, even people familiar with the variety of existing conformal coatings, their strengths, weaknesses and respective use often assume that the chemical vapor deposition (CVD) process used for parylene films incorporates a solvent, as an integral component of the procedure.  This is false, for the reasons detailed below.