Parylene is a family of plastic coatings applied as an ultra-thin, protective film through a specialized vapor deposition process. Unlike paints or liquid coatings that are brushed or sprayed on, parylene is deposited one molecule at a time in a vacuum chamber, allowing it to form a perfectly uniform layer over even the most complex shapes. It’s widely used to protect electronics, medical devices, aerospace components, and anything else that needs a reliable barrier against moisture, chemicals, and electrical interference.
How Parylene Coatings Are Applied
Parylene starts as a solid powder called a dimer. The coating process, known as the Gorham process, takes place inside a sealed vacuum system with three connected chambers. First, the dimer is loaded into a sublimation chamber and heated until it turns into a vapor. That vapor then passes into a much hotter pyrolysis chamber, where the heat breaks each dimer molecule into two smaller monomer molecules.
These monomer molecules flow into a room-temperature deposition chamber where the parts to be coated are waiting. When the gas contacts any exposed surface, it polymerizes on the spot, building up a thin, continuous film. Because the coating arrives as a gas rather than a liquid, it seeps into tight crevices, under components, and around sharp edges with perfectly even coverage. No curing step, no heat stress on the parts, and no solvents involved. The thickness of the final coating scales linearly with how much dimer powder you start with, giving manufacturers precise control.
The Main Types of Parylene
There are several parylene variants, each with a slightly different chemical structure that shifts its performance profile. The three most established types are N, C, and D.
- Parylene N is the base form and the strongest electrical insulator of the group, with a dielectric strength of 7,000 volts per mil (a mil is one-thousandth of an inch). Its electrical properties stay remarkably stable across different frequencies, making it ideal for high-performance electronics. It does, however, let more moisture through than the other types, with a water vapor transmission rate of 3.0 (g·mm)/(m²·day).
- Parylene C is the most commonly used variant. Its chemical structure includes a single chlorine atom substituted onto the molecular ring, which dramatically improves its moisture barrier. Its water vapor transmission rate is just 0.4, roughly seven times lower than Parylene N. It also resists corrosive gases well. Its dielectric strength is 5,600 volts per mil, slightly lower than N but still excellent.
- Parylene D has two chlorine atoms on its ring instead of one. It’s similar to C in most respects but can handle somewhat higher operating temperatures. Its moisture permeability sits between N and C at 1.8, and its dielectric strength is 5,500 volts per mil.
High-Temperature Variants
Newer fluorinated parylenes (sometimes sold under names like AF-4 or VT-4) push the thermal limits much further. Standard parylene N and C begin to decompose around 489°C and 503°C respectively when tested in an inert atmosphere. Fluorinated parylene AF-4, by contrast, holds up to 568°C before decomposing, and it can survive short exposures to 300°C even in the presence of oxygen, such as during electronics soldering. These variants are particularly valuable for protecting circuit boards that go through high-temperature manufacturing steps.
Where Parylene Is Used
The combination of thin, uniform coverage, strong moisture resistance, and excellent electrical insulation makes parylene a go-to coating in several industries.
In electronics, parylene protects circuit boards, sensors, and connectors from humidity, salt spray, and chemical exposure. Because the coating is applied at room temperature and conforms perfectly to complex board geometries (including the undersides of tiny surface-mount components), it can protect assemblies that liquid coatings would struggle to cover evenly. The strong dielectric properties mean it also serves as an electrical insulator between closely spaced conductors.
In the medical field, parylene is valued for its biocompatibility. It meets the requirements of ISO 10993, the international standard for evaluating medical device materials in contact with the body. Coatings are used on implantable devices, surgical instruments, and diagnostic equipment where the surface must be chemically inert and non-reactive with tissue or bodily fluids. The coating’s pinhole-free nature at very thin layers is critical here, since even microscopic gaps could allow corrosion or biological contamination.
Aerospace and military applications rely on parylene to protect sensitive components from extreme temperature swings, altitude-related condensation, and chemical exposure. The coating adds almost no weight or bulk, which matters when every gram counts.
How Parylene Compares to Other Coatings
Traditional conformal coatings like acrylics, silicones, and epoxies are applied as liquids by brushing, spraying, or dipping. They’re cheaper in raw material cost and simpler to apply, but they come with trade-offs.
Acrylic coatings dissolve in solvent and are left behind as the solvent evaporates. They can take up to 24 hours to fully cure. While they don’t shrink during curing, they can’t match parylene’s ability to penetrate tight spaces uniformly. Epoxy coatings, meanwhile, tend to shrink as they cure, which can stress delicate components underneath. They’re also nearly impossible to remove if you need to rework the board later.
Parylene’s vapor deposition process eliminates these issues. There’s no liquid pooling in corners, no uneven thickness, no curing shrinkage, and no solvent residue. The trade-off is cost: the vacuum equipment and dimer material are more expensive than a can of spray coating, and the process takes longer. For high-reliability applications where failure isn’t an option, that cost is generally justified.
Removing and Reworking Parylene
One practical challenge with parylene is that its durability cuts both ways. When you need to access a coated component for repair or rework, the coating doesn’t simply dissolve or peel off like acrylic.
The most common removal method is mechanical: physically scraping, peeling, or picking the film off the surface. It’s straightforward but tedious, especially over large areas. For small, targeted spots, a soldering iron or localized heat can soften and remove the coating. Neither approach is as clean or easy as dissolving an acrylic coating with solvent, which is worth considering if frequent rework is expected.
Typical Coating Thickness
Parylene coatings are remarkably thin, often measured in microns (thousandths of a millimeter). Typical applications use films between 0.5 and 25 microns thick, though thicker coatings are possible. Even at these minimal thicknesses, the coating provides a continuous, pinhole-free barrier. This is possible because the molecule-by-molecule deposition process builds the film uniformly rather than relying on a liquid to flow and settle. For context, a single layer of parylene C at just a few microns thick already provides better moisture protection than many liquid coatings applied at ten times the thickness.