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. Parylene is deposited with a unique vapor deposition process, an example of how parylene deposits is shown in Figure 1.
Figure 1. Courtesy of CALCE, University of Maryland.
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.
What you’ll learn:
- What is parylene?
- The parylene deposition process.
- Coating benefits.
- Detailed properties of parylene.
- Design recommendations.
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.
|Electrical Properties||Parylene C||Parylene N||Parylene F|
Dielectric strength defines the maximum voltage required to produce a dielectric breakdown of the material. The higher the dielectric strength of a material the better its quality as an insulator.
|220 V/micron at 25.4 microns
5600 V/mil at 0.001”
|276 V/micron at 25.4 microns
7000 V/mil at 0.001”
|276 V/micron at 25.4 microns
7000 V/mil at 0.001”
Volume resistivity is the electrical resistance through a cube of insulating material. The higher the volume resistivity, the lower the leakage current and the less conductive the material is.
at 23°C, 50% RH
at 23°C, 50% RH
at 23°C, 50% RH
Surface resistivity is the electrical resistance of the surface of an insulator material. The higher the surface resistivity, the lower the leakage current and the less conductive the material is.
at 23°C, 50% Relative Humidity
|1×1013 ohm at 23°C, 50% Relative Humidity||4.7×1017 ohm at 23°C,
50% Relative Humidity
Dielectric Constant (k)
A ratio measuring the ability of a substance to store electrical energy in an electric field.
|60 Hz 3.15
1 KHz 3.10
6 GHz 3.06 ‐ 3.10
|60 Hz 2.65
1 KHz 2.65
6 GHz 2.46 ‐ 2.54
|60 Hz 2.20
1 KHz 2.25
Dissipation Factor (tan δ)
A measure of a dielectric material’s tendency to absorb some of the AC energy from an electromagnetic (EM) field passing through the material.
|60 Hz 0.020
1 KHz 0.019
1MHz 0.0136 GHz 0.0002 ‐ 0.0010
|60 Hz 0.0002
1 KHz 0.0002
1MHz 0.00066 GHz 0.0021 ‐ 0.0028
|60 Hz 0.0002
1 KHz 0.00021MHz 0.008
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. VSi Parylene is here to help with parylene prototype, development and production services.
What are the temperature limits of parylene coatings?
Parylene coatings have specific temperature limits that once reached, can shorten the usefulness of the coating. Near the end of the life of the parylene coating, it loses flexibility and becomes more brittle, can yellow, and forms cracks which lead to coating failure. This is known as a glass transition, when the parylene transitions from being flexible and salient, to brittle and opaque. Parylene glass transition temperatures depend on the type of parylene.
What are the melting temperatures of parylene coatings?
The melting points for all of the parylenes are very high and are typically above that of solder and other materials on circuit boards. Parylene N, C, D, and F are susceptible to oxidation, while AF-4 is more resistant to temperature extremes due to its CF2 units bridging between the rings in the polymer chain.
What happens when parylene is exposed to high temperatures?
Exposure to high temperatures causes microscopic breaks in most parylene films until it’s severe enough to cause cracks visible to the naked eye. In both cases of high temperature and oxygen or UV light and oxygen, most of the parylenes are oxidized. Carbon to oxygen bonds are added to the methylene (CH2) units in the polymer backbone, and when enough of the polymer chains have broken in such a manner, then a visible crack will form.
How does temperature affect outgassing in parylene coatings?
Any adhesive, masking, staking, and related materials should be fully cured prior to coating with parylene. Including a mild bake out or bake out under vacuum step prior to coating will vastly decrease the risk of outgassing during the coating process, in addition to mitigating the risk of delamination of the parylene. Outgassing can increase the length of time it takes to pump down the deposition chamber prior to coating, therefore increasing time-to-coat.