Automotive Conformal Coatings: Silicone vs Parylene

Conformal coatings have long played a crucial role in protecting printed circuit boards (PCBs) and vital automotive electronics from challenging conditions. Their significance in automotive design and manufacturing has only grown over time. Delicate electronic components and their connections necessitate shielding to ensure consistent performance. By adapting to the topographies of PCBs, these coatings insulate assembly components, preserving the operational integrity of specialized electronics in even the harshest conditions.

Automotive PCBs can house up to 100 million lines of code and boast sophisticated circuitry with numerous microprocessors. The demand for precise coating application has heightened due to PCB miniaturization and increased component density. Presently, liquid silicone and vapor-applied Parylene stand out as the two most indispensable conformal film materials for these applications.

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, dipping (immersion) or spraying. Unlike other conformal coatings, silicone is applied in a relatively thick coat — between 0.003 in to 0.008 in — 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 deposition (CVD) allows the substance to penetrate deep within substrate surfaces, from all angles, covering crevices, edges, corners and underneath components if necessary.  Parylene coatings are extremely thin (0.0005 in).

Silicone and Parylene Conformal Coatings for Automotive Electronics

Roughly equivalent to a very soft rubber, silicone resin is used frequently for automotive electronics. Applied in a sufficiently thick coating layer, it absorbs impact and shock to the coated assembly. Parylene forms a thin, resilient coating with less abrasion resistance. However, Parylene coatings are chemically inert and extremely tough, withstanding exposure to brake fluid, battery cooling fluids, antifreeze, salt air and automotive chemicals with solvent properties, like gasoline. As global initiatives continue to drive toward the elimination of halogens in electronics, materials like halogen-free ParyFree^® are being utilized more to protect automotive electronics.

Thermal resistance is important for automotive applications, as many applications experience high-temperature environments. Silicone resin works effectively at higher operating temperatures (>200ºC).

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, and Parylene HT^® can function continuously at 350ºC (450ºC for a short-term service). For overall performance consistency, Parylene remains stable at a constant temperature of 80ºC for 10 years. For automotive purposes where intense heat is a factor, a higher temperature variant of Parylene or silicone will be the best solution.

Silicone has remarkable water-resistant qualities; thickly-applied SR repels moisture where other coatings fail. Silicone conformal films also exhibit the following properties:

• Flexible and soft
• Adhere well to PCB surfaces not requiring thinner film covering to ensure operation
• Corrosion/UV resistance superior to most competing conformal coatings
• Provide a smooth/quick curing coat (about one hour at room temperature)
• Easy to apply/rework

These factors minimize production cost and time, especially for assemblies requiring further attention after coating. However, silicone’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 – while also maintaining dielectric/thermal protection through fluctuations of electrical current; its micro-thin films effectively coat MEMS/nanotechnology that manage vehicle communication and signal-processing functions, frequently situated in areas of high-performance activity and stress. Parylene 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, Parylene provides excellent dielectric and moisture barrier properties, generating bubble- and pinhole-free conformal coating layers as thin as .0005 in. Parylene’s other benefits include:

• High optical clarity
• Tin whisker growth mitigation
• Flexible conformability for adaptation to all surfaces
• Film penetration of extremely small spaces and crevices

Summary

Parylene and silicone are both used to conformally coat automotive electronics. Silicone cures rapidly and is a reliable dielectric barrier, 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, ultra-thin coating provides excellent value even with a very thin layer. Their respective material properties and film application methods are critical to determining which is best applied for a specific automotive purpose.