You protect your car with a car cover. You protect your skin with sunscreen. You protect your phone screen with a plastic film to prevent scratches. When something matters, you protect it. The same is true for electronics and the best protection for your electronic devices is parylene. But what makes parylene a superior coating material?
A History You Can Trust
Parylene has been in commercial use since 1965, so it’s predictable and can be modeled by product designers to work out any type of issues in Design for Manufacturing (DfM) and other pre-production steps.
This predictability is very important if a coating is used on active RF (radio frequency) devices, circuits, and assemblies, since each layer above the active areas will affect the RF signals. By being able to predict a coating’s properties ahead of time, its effects can be compensated for by incorporating signal information into the modeling phase.
Parylene has been used for decades in high-reliability markets, such as aerospace, defense, and all throughout the tech industry. It’s even been used in highly sensitive medical devices, such as pacemakers and other implantable electronic devices. If our government, space program, and doctors trust parylene to be reliable and predictable, you too can feel confident knowing your products are safe.
Conformal Coating Means Even Coverage Everywhere
Parylene doesn’t conduct electricity, which is very important for a film that coats and separates conductive areas on electronics. Parylene makes a fantastic electrical insulator, a.k.a. dielectric, coating since it coats every surface on a product with uniform thickness. Conformal coatings aren’t supposed to be used as the primary means of electrical insulation, but they can supplement other forms of insulation and isolate electrical ground from active traces and pins. The lack of pinholes and other point defects helps parylene prevent arcing. An example of how parylene deposits is shown in Figure 1.
A coating with variable thicknesses on an electronics assembly that is running at high voltage may have a greater risk of failure if the device is operating near the dielectric breakdown voltage of the coating. At the breakdown voltage, the coating essentially undergoes a complete failure and any insulative properties are left negligible or lost completely.
Parylene Thickness-Electrical Properties Relationship
Though defined by its fundamental composition, the insulating properties of a parylene coating increase with thickness. This means by selecting a specific parylene thickness, you can fine-tune the electricity-blocking properties. Since each parylene type has different dielectric properties, there’s a suitable parylene for virtually every device.
The fundamental properties of the parylene is determined by both its chemical composition and its thickness. The parylene with the best general electrical properties is the original formulation, parylene N. With the addition of chlorine (Cl) in parylenes C and D and fluorine (F) in parylenes F and AF-4, the electrical properties shift, as shown in Table 1 below.
Table 1. Parylene electrical properties of common types based on industry literature.
Barrier To Conductive Contamination
As mentioned previously, parylene coatings are great barriers to chemicals and corrosive compounds. As a barrier, the parylene is also blocking materials that can lead to electrical shorts, such as water and other conductive liquids, as well as conductive solids, such as dust.
In environments where small particle debris travels or is generated, making its way through housings or chassis, and onto the electronics assemblies, a conformal coating can drastically improve the reliability of those electronics. This small particle or foreign object debris (FOD) can be composed of conductive and non-conductive materials.
Many types of dust are actually salts, especially near marine locations where salt spray can travel long distances. Salt, along with moisture and electrical bias can lead to electrical chemical migration (ECM) or dendrites. Dendrites are tiny metallic structures that can cause electrical shorts and possibly lead to critical failures. Parylene drastically reduces the risk of dendrites and failures due to dendrites.
The parylenes provide excellent physical and chemical barriers to conductive contamination. Moisture has a difficult time passing through parylene films, as well. So, only electrical bias is left while your assembly is operating under expected conditions.
Tin Whisker Mitigation
Numerous studies by U.S. Defense Contractors, such as Lockheed Martin and others, have shown parylene to be a great barrier to help mitigate issues caused by tin and other metal whiskers. In a device where multiple components are coated in parylene, even if the parylene coating on one element of the device isn’t able to prevent a tin whisker from poking through as the whisker grows, it’s extremely unlikely that the whisker would be able to pierce through yet another layer of parylene on the element next to it to reach an electrically conductive surface and cause an electrical short.
Tin whiskers have led to critical failures of aerospace systems, including satellites, many of which are listed on a NASA website focused on issues relating to tin and other metal whiskers.
Of all the coating types evaluated, only parylene and polyurethanes performed well in mitigating the risk of tin whiskers.
Wrapping It Up
The electrical properties of a conformal coating are a critical parameter that many product designers rely upon to build devices that are meant to last. The parylenes are without a doubt the coating of choice when one needs chemical, physical, and electrically insulative barriers.