Parylene for Automotive

High-tech electronic systems increasingly regulate automotive management functions for emissions’ controls, fuel systems, fluid monitoring, lighting, and powertrain mechanics, frequently comprised of miniaturized, multi-layer MEMS/Nano packages. Systems’ survival in hostile vehicular environments typified by condensation, corrosive fluids and vapors, excessive temperatures, humidity and prolonged UV exposure is partially assured by protective conformal coating.

All types of conformal coating exhibit some degree of physical flexibility, and protection against mechanical/thermal shock, while securing the component from exposure to corrosive elements. Understanding the precise end use of a vehicular printed circuit board (PCB) system is basic to deciding among conformal coatings for automotive application.

Material cost-effectiveness can actually be enhanced with a more expensive initial investment in solutions that continue working without replacement or repair in the longer-term. Wet coatings like acrylic, epoxy, silicone and urethane offer simple application and relatively easy removal, repair and replacement. In contrast, parylene’s specialized chemical vapor deposition (CVD) process is slower-to-finish and considerably costlier to enact. However, because CVD causes parylene to penetrate far deeper into the substrate surface, it generates the highest levels of protection available for many automotive purposes. Parylene is a superior substrate covering for automotive PCBs, providing durable, ultrathin protection for strategically-situated moving parts and related electro-mechanical assemblies.

Expanded Automotive Use of Parylene Conformal Coatings
Electrically insulating, parylene provides long-term surface insulation resistance (SIR) which stimulates optimal function of such automotive system as:

• analytical performance measurement of automotive systems' functioning, 
• anti-lock brakes control,
• emissions' management,
• engine control unit (ECU),
• instrument pods' circuitry (IPC), 
• power controls/modules for doors, mirrors, seats, sunroof, and windows,
• power train/chassis operations,
• passenger comfort/convenience, and 
• tire-pressure gaskets/seals.  

With parylene, PCBs and related operational equipment benefit from enhanced functional integrity, despite ongoing exposure to operational ecosystems characterized by the presence of acidic substances, chemical contaminants, fluctuations of electrical current, humidity, moisture, temperature changes or a mixture of these conditions

Providing chemical, dielectric, moisture and thermal protection that far surpass competitive coatings for automotive purposes, parylene is the conformal coating of choice for such applications as:

• PCBs in contemporary automobiles that contain approximately 100 million lines of code and have advanced circuitry with scores of microprocessors. Parylene coatings’ excellent dielectric properties provides insulation that permits PCB traces to be situated closer together, reducing the potential for nearby electronics interference with performance, while diminishing component size. 
• Microelectricalmechanical Systems (MEMS) that manage signal-processing and communication functions. Such electronic vehicular systems as manifold air pressure (MAP), manifold air temperature (MAT), and power train/chassis control are regulated by MEMS’ sensors.  Frequently situated in areas of high performance activity and stress, MEMS’ sensor maintenance is essential to assure safe, efficient automotive performance.   Of conformal coatings, parylene best safeguards micro-machined circuits from the potentially deleterious effects of aggressive automotive environments.       
• Ruggedization of automotive components and systems stabilizes performance through exposure to harsh engine fluids, high levels of vibration, and temperature extremes. Parylene is well-adapted to ruggedization, providing superior resistance to heavy vibrations and shock/internal disruption within the engine, for such automotive systems as detection/door switches, environmental protection monitors, mechanical/physical performance gauges, motion controllers, and components regulating safety, rocker and tactile functions.  Parylene is applicable for specialized, highly refined purposes, where sensors and components need to be appropriately insulated.  
• LED illumination is increasingly essential to safe driving. Parylene conformal coatings extend the functional life of the LED light itself, improving performance.  
• Elastomer/polymer seals normally confronted by harsh operating conditions benefit from parylene films, which stimulate enhanced operational efficiencies and consequent functional upgrades.   This can be especially true in the case of older, high-mileage vehicles, where systems have already experienced considerable use. Such automotive communicative functions as GPS, vehicle telemetry, and voice/data systems (WiFi/Bluetooth) also benefit from application of parylene film

Conclusion
Increasing computerization of automotive systems is applied to virtually all a vehicle's operational technology. Often situated in demanding end-use environments, PCBs, electronic sensors and similar assemblies benefit from application of parylene conformal coatings.

Parylene CVD processes generate both:

• deep penetration of the component substrate and
• the thinnest possible effective layers of pinhole-free conformal coating for automotive assemblies,
• supporting their function in even the most spatially-confined segments of any automotive engine,
• in the presence of various combinations of aggressive chemicals, harsh liquids and vapors.  

Parylene conformal coatings significantly lessen the likelihood of performance degradation in automotive environments characterized by persistent exposure to corrosive engine liquids and byproducts, winter road-salts, mechanical vibration, noxious exhaust gases, and fluctuating thermal conditions.

Parylene coatings also provide dependable component protection for automotive computers.

Rapidly evolving products for automotive information technology (IT), electrical systems, engine components and sensors increasingly depend on parylene coatings to assure ongoing performance through all conditions. Parylene protects assemblies and systems within the engine, while adding to the functional performance, better assuring the vehicle operates as intended.

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.

Thermal Properties of Selected Parylenes, in Comparison with Silicone Conformal Coating

Melting Point
Parylene C: 290°C
Parylene D: 380°C
Parylene N: 420°C
Silicone: Cured

T5 point (where modulus = Taken from secant modulus temperature curve)
Parylene C: 125
Parylene D: 125
Parylene N: 160
Silicone: 125

T4 point (where modulus = Taken from secant modulus temperature curve)
Parylene C: 240
Parylene D: 240
Parylene N: 30
Silicone: -80

Thermal conductivity, 25°C
Parylene C: 2.0
Parylene D: --
Parylene N: 3.0
Silicone: 3.5 - 7.5

Specific heat, 25°C
Parylene C: 0.17
Parylene D: --
Parylene N: 0.20
Silicone: --

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.

Mechanical/Physical Properties of Selected Parylenes, in Comparison with Silicone Conformal Coating

Tensile Modulus
Parylene C: 3.2
Parylene D: 3.0
Parylene N: 2.8
Silicone: .007

Tensile Strength, psi
Parylene C: 10,000
Parylene D: 11,000
Parylene N: 6,000 – 11,000
Silicone: 800 – 1,000

Yield Strength, psi
Parylene C: 8,000
Parylene D: 9,000
Parylene N: 6,100
Silicone: --

Yield Elongation
Parylene C: 2.9
Parylene D: 3.0
Parylene N: 2.5
Silicone: --

Elongation to Break %
Parylene C: 10 -39%
Parylene D: 10%
Parylene N: 20 – 25%
Silicone: 10%

Rockwell Hardness
Parylene C: R85
Parylene D: R80
Parylene N: R85
Silicone: 40 – 45 (Shore A)

Coefficient of Friction - Static
Parylene C: 0.29
Parylene D: 0.33
Parylene N: 0.25
Silicone: --

Coefficient of Friction - Dynamic
Parylene C: 0.29
Parylene D: 0.31
Parylene N: 0.25
Silicone: --

Water Absorption
Parylene C: 0.06%/24 hours
Parylene D: < 0.01%/24 hours
Parylene N: < 0.01%/24 hours
Silicone: 0.12%/7 days

Density g/cm2
Parylene C: 1.289
Parylene D: 1.418
Parylene N: 1.10 – 1.12
Silicone: 1.05 – 1.23

Refractive Index nD23
Parylene C: 1.639
Parylene D: 1.669
Parylene N: 1.661
Silicone: 1.43

Summary
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.