Parylene is a transparent polymer offering uniform and pinhole-free conformal coatings. Different varieties of parylene (Parylene N, C, D, AF4, and F) formed by a modification in their molecular structure. Each modification results in a set of material properties that are applicable in different service conditions. The basic type of parylene derivatives is the Parylene N (poly-para-xylylene) monomer.

The polymer parylene (XY) is a reliable protective conformal film that safeguards the visual clarity and color of printed circuit boards (PCBs), similar electronic assemblies and other products.  XY optical clarity seldom diminishes to the extent either the coating or the underlying substrate becomes visually indistinct, although over-exposure to ultraviolet (UV) light may eventually interfere with optical perception.  However, in the majority of cases, colorless parylene generates advantageous optical properties for a wide range of uses -- including artwork/museum artifacts, cameras/sensors, computer touchscreens, healthcare/medical devices, light-emitting diode systems (LEDs), and optoelectronic components maintaining consistent aerospace, scientific, and telecommunication operations.

Parylene (XY -- poly(para-xylylene)) organic polymers are highly regarded through a wide range of industries – aerospace/defense, automotive, commercial, industrial, medical – for their utility as conformal coatings.  Chemically inert, colorless, linear/polycrystalline and optically clear, XY coatings provide exceptional barrier protection, dielectric reliability, and insulation for printed circuit boards (PCBs) and similar electronic assemblies whose components must maintain performance through all operating conditions.  Parylene conformal films safeguard function in the presence of biogases, biofluids, chemicals, moisture/mist, salt compounds, and temperature fluctuations. 

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A primary function of all conformal coatings is maintaining sufficient insulation and avoiding dielectric breakdown while protecting printed circuit boards (PCBs) and related electronic assemblies. Providing a completely homogeneous coating surface, parylene (XY) conformal coatings are exceptionally corrosion-resistant, dense and pinhole-free. Among other performance advantages, ultra-thin XY protective films offer superior dielectric properties. Dielectric substances maintain electrical insulation, simultaneously transmitting electricity without conduction. They have the potential to store energy because they support electrostatic fields that release only low levels of thermal energy.

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Conformal coatings primary purpose is protecting the performance of highly sophisticated electronics such as printed circuit boards (PCBs), sustaining their functionality through often unfriendly operating conditions.  Among the most important coating-requirement is safeguarding PCBs from the negative impact of moisture incursion.  Sources are many.  Liquidized obstacles to appropriate assembly function can result from unwanted contact with acid rain, aggressive solvents, atmosphere pollutants, chemicals, fog, high humidity, intermittent immersion, persistent rain, snow, salt water/mist and wet sprays of any kind. 

Chemically inert parylene (Poly-para-xylylene/XY) conformal film is often selected because its micron-thin protective films generate precise coating uniformity, regardless of substrate topography.  To this extent, XY far exceeds the capacities of liquid materials – resins of acrylic, epoxy, silicone or urethane – for a wide range of coating assignments.  It is true that pre-synthesized liquid coatings are easier to apply.  However, their conformal films are dimensionally thicker, making them difficult to position in constricted operating spaces.  Liquids are also generally less resistant to contaminant incursion and other problems that interfere with reliable performance of printed circuit boards (PCBs), and most other contemporary electronics, including biomedical implants.

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A specialized chemical vapor deposition (CVD) process attaches conformal coatings composed parylene (XY) to substrates.  CVD uniformly encapsulates all exposed substrate surfaces as a gaseous monomer; completely eliminating wet coatings’ liquid phase and need for post-deposition curing.  Synthesizing in-process, CVD polymerization requires careful monitoring of temperature levels throughout. 

Beneficial thermal properties of XY protective coatings include reliable performance through an exceptional range of temperatures.  Parylene is available in variety of material formats, prominently Types C, N, F, D and AH-4.  Each has a particular range of properties that determine its optimal uses.  Types C and N exhibit faster deposition rates than other parylenes, making them useful for a wider range of coating functions.  However, operating temperature is a significant determinant of use:  Much depends on chemical composition. 

• Used more frequently than other XY varietals, Parylene C is a poly-monochoro para-xylene.  It is a carbon-hydrogen combination material, with one chlorine group per repeat-unit on its main-chain phenyl ring.  In oxygen-dominated atmospheres, C conformal films regularly provide reliable assembly security at temperatures of 100° C (212° F/water’s boiling point) for 100,000 hours (approximately 10 years).  C is suggested for use in operating environments reflecting these temperature conditions.  Chemical, corrosive gas, moisture, and vapor permeability remain consistently low.  C generates exceptional vacuum stability, registering only 0.12% total weight-loss (TWL) at 49.4° C/10^-6 torr (1 torr = 1/760 SAP (standard atmospheric pressure, 1 mm Hg).   C can also be effective at temperatures below zero, to -165º C.
• With a completely linear chemical format, Parylene N is the most naturally-occurring of the parylene series.  Used less regularly than Type C, N is highly crystalline; each molecule consists of a carbon-hydrogen combination.  N’s melting point of 420° C is greater than most other XY types.  Vacuum stability is high, registering TWL-levels of 0.30% at 49.4° C, and 10^-6 torr.  These properties encourage higher temperature applications.  Compared to other XY varietals, N’s low dielectric constant/dissipation values also recommend uses with assemblies and parts subjected to higher levels of unit vibration during operation.  N’s electrical/physical properties are not noticeably impacted by cycling from -270º C to room temperature, adding to its versatility.  
•  Parylene F has fluorine atoms on its aromatic ring.  Possessing aliphatic -CH₂- chemistry, F’s superior thermal stability is attributed to this aliphatic C-F bond, compared to Type C’s C-C bond.   Better thermal stability, and reduced electrical charge/dielectric constant expand its use for ILD (inner layer dielectric) applications, such as those for ULSI (ultra large-scale integration), where a single chip can incorporate a million or more circuit elements.   F is a good choice for many microelectromechanical systems (MEMS)/nanotech (NT) solutions. 

• Originating from the same monomer as Type C, Parylene D’s chemical composition contains two atoms of chlorine in place of two hydrogen atoms.  Like Type C, D conformal films can perform at 134° C (273° F), dependably securing assembly performance in oxygen-dominated environs for 10 years, at a constant 100° C.  Parylene F resists higher operating temperatures and UV light better than C or N.  
• Parylene AF-4’s melting point is greater than 500° C.  It survives at higher temperatures/UV-exposure better than other parylenes for long durations because it possesses CF₂ units, situated between its polymer-chain rings.