Long used to safeguard printed circuit boards (PCBs) and other essential automotive electronics from harsh operating environments, conformal coatings’ importance in auto-design/manufacture has never been greater. Fragile electronic components and the paths between them require protection for PCBs to perform reliably. Conforming to PCBs’ topographies, coatings insulate assembly components, safeguarding specialized electronics’ functional integrity through extreme operating conditions.
Automotive PCBs contain as many as 100 million lines of code and have advanced circuitry with scores of microprocessors. PCB miniaturization and expanded component density requires precise coating application. Liquid silicone and vapor-applied parylene are currently the two most valuable conformal film materials for these purposes.
Silicone and Parylene: A Basic Comparison
Technically polymers, silicone and parylene differ fundamentally. A combination of silicon and oxygen atoms, silicone is chemically unique among conformal coatings. While its chemistry is distinctive, silicone’s liquid deposition resembles other wet coatings – acrylic, epoxy and urethane; it is applied to substrates via brushing, dip-immersion or spraying. Unlike other conformal coatings, silicone is applied in a relatively thick coat -- between .003"-.008" -- to assure effectiveness per IPC Standards.
In contrast, parylene dimer is a hydrocarbon molecule, chemically related to virtually every other available plastic. However, its application method is unique. Deposited as a gas in a vacuum, parylene’s chemical vapor-based deposition (CVD) allows the substance to penetrate deep within substrate surfaces, from all angles, covering crevices, edges, corners and underneath components if necessary. Unlike silicone, parylene coatings are extremely thin (.0005”).
Silicone and Parylene Conformal Coatings for Automotive Electronics
Roughly equivalent to a very soft rubber, silicone resin (SR) is used frequently for automotive electronics. Applied in a sufficiently thick coating-layer, it absorbs impact and shock to the coated assembly. Parylene (XY) forms a thin, resilient coating with less abrasion resistance. However, parylene coatings are chemically inert and extremely tough, able to withstand exposure to brake fluid, antifreeze, salt air and automotive chemicals with solvent properties, like gasoline.
Thermal resistance is one area of performance where silicone conformal films exceed parylene. SR works effectively at higher operating temperatures (>200ºC), a functional advantage compared to parylene, which tops out at 80ºC. Because many automotive applications have high temperature requirements, parylenes C or N are not viable options. Some SR-variations provide reliable conformal coating at 600ºC, making them a superior alternative to parylene, at far lower cost. While certain fluorinated parylenes possess higher temperature-performance capabilities, excessive cost makes large-volume production economically infeasible.
Nevertheless, parylene supports a broad range of temperatures, and performs more effectively than silicone in exceptional cold, withstanding temperatures as frigid as -165ºC, without physical damage. For overall performance-consistency, XY remains stable at a constant temperature of 80ºC for 10 years. Nevertheless, for automotive purposes where intense heat is a factor -- temperatures in the engine compartment can reach 175ºC -- silicone offers a wider range of uses.
Table 1: Thermal Properties of Selected Parylenes, in Comparison with Silicone Conformal Coating
Melting point, °C
T5 point (where modulus = Taken from secant modulus temperature curve)
T4 point (where modulus = Taken from secant modulus temperature curve)
Thermal conductivity, 25°C
3.5 - 7.5
Specific heat, 25°C
Silicone has remarkable water-resisting qualities; thickly-applied SR repels moisture where other coatings fail. SR conformal films also:
· are flexible and soft,
· adhere well to PCB surfaces not requiring thinner film covering to ensure operation,
· offer corrosion/UV resistance superior to most competing conformal coatings,
· provide a smooth/quick-curing coat (about one hour at room temperature), and
· are easy to apply/re-work.
Combined with easy film-application, these factors minimize production costs and time, especially for assemblies requiring further attention after coating. However, SR’s inability to resist solvents limits its use for automotive electronics in contact with solvents during operation.
Parylene resists both moisture and chemicals -- water, corrosive materials, acids, bases and solvents – maintaining dielectric/thermal protection through fluctuations of electrical current; its micro-thin films effectively coat the MEMS/nano-technology managing vehicles’ communication/signal-processing functions, frequently situated in areas of high performance activity and stress. XY protects micro-machined circuits from the potentially deleterious effects of aggressive automotive environments.
Of all conformal coatings, parylene adheres to the widest selection of substrate materials and surface geometries. Chemically and biologically inert, XY provides excellent dielectric and moisture barrier properties, generating bubble- and pinhole-free conformal coatings layers as thin as .0005”. Parylene’s other benefits include:
· high optical clarity,
· mitigated tin whisker growth, and
· flexible conformability for adaptation to all surfaces,
· enabling film-penetration of extremely small spaces and crevices.
Table 2: Mechanical/Physical Properties of Selected Parylenes, in Comparison with Silicone Conformal Coating
Tensile strength, psi
6,000 – 11,000
800 – 1,000
Yield strength, psi
Elongation to break %
20 – 25%
40 – 45 (Shore A)
Coefficient of friction - static
Coefficient of friction - dynamic
< 0.01%/24 hours
< 0.01%/24 hours
1.10 – 1.12
1.05 – 1.23
Refractive index nD23
SR rivals XY for automotive electronics. Silicone cures rapidly, is reliably dielectric, displaying exceptional stability across a wide temperature range. Roughly equivalent to very soft rubber, silicone can lack sufficient utility for coating high-profile, consistently active electronic components. Parylene’s resilient, but ultra-thin coating sometimes lacks strong abrasion resistance. Their respective material properties and film application methods are critical to determining which is best-applied for a specific automotive purpose.
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