What is Parylene Coating?
Parylene conformal coating is a thin film coating technology used to improve the capabilities of leading-edge technologies. Applied as vapor, the coating layer perfectly conforms to complex shapes and provides complete and even coverage.
Product designers use parylene to waterproof electronics, add dry lubricity or enhance adhesion to other coatings. Parylene coatings are a popular choice in applications where reliability and performance matter most.
Parylene has become the protective coating of choice for industrial and consumer electronics, aerospace and medical applications.
What makes parylene unique:
- Ultra-thin coating, starts at 0.5 microns thick
- Exceptional moisture and chemical protection
- Use temperatures range from -270ºC through +250ºC
- Drug and hydrophilic coating adhesion tie-layer
- FDA approved for human implantable devices
- Dry lubricity comparable to PTFE
- Truly conformal barrier layer
What is Parylene Coating Used For?
Parylene has several benefits engineers use to enhance products. Below is a list of the most common benefits. Most people looking at parylene find that the easiest way to evaluate an application is to schedule a call with an expert.
Parylene is a superior barrier layer that provides protection from moisture, corrosion, salt spray, solvents and airborne contaminants. It is chemically inert, ultra-thin, pinhole-free and conforms to components evenly and consistently. Combined with its unique molecular-level deposition, this high level of protection is achieved with 10% of the mass than spray or dip coatings.
Parylene is a powerful insulator with very high dielectric strength compared to other materials. The ability to apply a very thin dielectric layer that allows high voltages to flow in close proximity, without discharge concerns, is helpful for many designs. As a result, it’s often used to control the electrical path or eliminate arcing when space is limited.
When a sub-micron layer of parylene is applied to a part, it acts as a lubricating layer. Parylene has a low coefficient of friction that is comparable to PTFE (Teflon). Parylene improves lubricity, without the risk of shedding particles and reduces the resistance force of pushing devices through restricted anatomy. It’s also used on common elastomers to reduce tackiness and improve cleanliness.
Adhesion Tie Layer
Parylene acts as an adhesion layer when it is applied between a device and a top coating. Its inert and biocompatible properties make it a unique solution for bonding drug and hydrophilic coatings to drug-eluting devices or catheters. Parylene has been shown to reduce particle counts when used with hydrophilic coatings. Additionally, the vapor deposition process allows complex shapes to be consistently coated with a high level of control.
Parylene’s completely-conformal properties are used to physically reinforce and add strength and rigidity to delicate connections on printed circuit boards (PCBs). Think of this as parylene welding. The combination of parylene’s crevice-penetrating ability and its consistent coating thickness provide a protective “jacket” that greatly reduces failures caused by solder fatigue from thermal cycling and vibration, without board redesign.
Often a first consideration in selecting high-value coatings for medical industries, parylene is an FDA approved material that meets USP Class VI and ISO 10993 biocompatibility requirements for use on human implantable devices. Standing the test of time, the use of parylene is well-documented in an increasingly wide range of medical coating applications over the past 40 years.
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What you’ll learn:
- Parylene benefits and applications
- How the vapor deposition process works
- Detailed material properties of parylene
- Compare parylene to other coatings
- Design guidelines
What is parylene coating used for?
Waterproof barriers help make sure critical electronics perform in a wide range of environments. Parylene offers unmatched protection from airborne contaminants, corrosion, chemicals, gases and moisture. It achieves this level of protection with 10% of the mass of spray or dip coatings.
Parylene has a very high dielectric strength compared to other materials. The ability to apply a very thin dielectric layer is helpful for many designs. As a result, parylene is often used to control the electrical path or eliminate arcing when space is limited.
Parylene acts as an adhesion layer when it is applied between a metal or polymer part and a top coating. Parylene’s biostability and chemical inertness make it a unique solution for bonding drug and hydrophilic coatings to drug-eluting devices or catheters.
A very thin layer of parylene applied to a part acts as a lubricating layer. Its low coefficient of friction is comparable to PTFE (Teflon). Compared to PTFE, PVP and silicone parylene can be a better choice if flaking and particulates are a concern. Dry film lubricity is important in the design of medical devices that interact with a patient vessels. Parylene is also used on common elastomers to reduce tackiness and improve cleanliness.
How is Parylene Deposited?
Parylene films are “grown” as vapor deposits molecule by molecule on parts in a room-temperature vacuum chamber. Understanding how the deposition process works is helpful to understand how parylene can provide unique solutions.
Chemical Vapor Deposition
Parylene is applied in a specialized vacuum system using a three-stage, vapor-deposition process. During this process parylene deposits molecule by molecule onto parts placed in a vacuum chamber. This creates an extremely conformal coating that evenly covers grooves, crevices, gaps, and even sharp points. Because the coating is applied molecule by molecule, the thickness can be controlled from hundreds of angstroms to a hundred microns.
The process stages are:
Stage 1: Parts are fixtured into a vacuum coating chamber. The solid parylene dimer – in powder form – is placed inside the vaporizer, where it is heated and turns from a solid to a gas.
Stage 2: The dimer vapor then flows into the pyrolysis furnace, which adds additional heat to the dimer gas and turns it into a monomer vapor.
Stage 3: Finally, the monomer vapor enters a room-temperature deposition chamber in a highly excited state. All of the parts are fixtures in this chamber. Here the individual monomers find other monomers and polymerize onto everything in the chamber. The deposition process creates a thin and highly conformal coating.
Truly Conformal Coating
Vapor-deposited parylene coatings are truly conformal and reach surfaces that are unreachable by liquid coatings. Parylene’s vapor deposition process produces thin films that ‘grow’ uniformly on a surface one molecule at a time.
Liquid conformal coatings, applied by spraying or dipping, tend to collect and pool in low crevices while pulling away from raised edges and sharp points. Bubbling, cracking, pinholes and orange peel are typical in liquid coatings.
Though many factors go into determining the final cost of adding parylene in your production process, there are three main factors to consider: coating thickness, masking complexity and component size
Impacts: Machine Hours
Optimal coating thickness is determined by your specific application and benefits desired. As coating thickness increases, additional time inside the deposition chamber is required increasing total machine hours.
Impacts: Labor Hours
Masking complexity is determined by your product’s design and operating requirements. As complexity increases, operators must take more time to process each individual part increasing total operator hours.
Size of a Component
Impacts: Batch Size
Parylene coating is applied inside a vacuum deposition chamber of fixed, physical size. As individual component size increases, total quantity of product’s coated decrease reducing batch size.
The Parylene Coating Process
The parylene coating process consists of four steps that make sure the coating properly adheres to the substrate and sections remain uncoated as needed.
The benefits of parylene depend on effective adhesion between parylene and the underlying substrate. Most parts go through a parylene adhesion promotion step to encourage the parylene layer to adhere to the substrate. VSi offers a variety of solutions custom to the product and application.
Liquid Adhesion Promotion
With liquid promotion, parts are submerged in a series of liquid baths that include a silane adhesion promoter. Silane coupling agents have the ability to form a durable bond between organic and inorganic materials. A-174 Silane is commonly used to improve adhesion to a wide range of materials including elastomers, glass, metal, plastic or quartz.
Plasma treatment activates the surface energy of many substrates and promotes the adhesion to parylene. Plasma treatment, a weak version of plasma etching, can also be applied to parylene after parts have been coated to promote adhesion to silicone overmolding or epoxy on top of the parylene.
Mechanical abrasion can be used to create a rough surface that provides a topography that parylene can better attach to. This is an effective pre-treatment method commonly used in the production of coated wire mandrels.
Unique material combinations can often present adhesion challenges. VSi Parylene offers custom adhesion promotion development targeted to your specific application.
After adhesion promotion, parts move on to masking. Masking applies a physical barrier to selectively keep areas free of parylene coating. This keeps any areas that need to remain free of parylene sealed so that parylene cannot deposit on the masked area.
After masking, parts are placed onto a fixture which is then loaded into the vacuum deposition chamber.
After the parts are prepared and placed into the coating chamber, coating can begin. Parylene coatings are applied at room temperature using specialized vacuum deposition equipment. Parylene application occurs through a process known as Chemical Vapor Deposition (CVD). In this process parylene raw material is vaporized (~150°), pyrolyzed (~650°C ) and then vapor-deposited (~22°C) directly onto parts. Parylene films are “grown” as vapor deposits molecule by molecule in a room-temperature vacuum chamber.
After the chemical vapor deposition process is complete, there are a few important final steps remaining.
Coating Thickness Measurement
Parylene films can be measured using spectral reflectance directly on the parts, or by witness coupons that were coated along with the parts.
After coating thickness has been measured and verified, parts are carefully removed from the coating fixture. If the application requires that some areas do not have coating, the sealed masking is removed.
Final Inspection and packaging
Finally, parts are inspected to ensure they meet customer specification requirements. Quality attributes for parylene typically specify the coating thickness, area of coverage and adhesion-testing requirements. It is common to inspect for these attributes under a microscope. Once the inspection is complete, parts are labeled per customer specifications and prepared for shipping.
Parylene Type C
Parylene C is the most popular type because it provides a combination of barrier and dielectric properties while also having cost and processing advantages.
Parylene C has a chlorine atom on one of the aromatic hydrogens. This gives parylene C very low permeability for better protection from moisture, chemicals and corrosive gases.
Parylene C deposits much faster than other types which allow a thicker layer to be applied with less machine time.
Parylene C is the best choice for:
- Implantable medical devices.
- Pinhole-free barrier layers to electronics or materials from harsh environments.
- Encapsulating electronics to provide dielectric protection.
- Meeting IPC-CC-830 or MIL-I-46058C standards.
Parylene Type N
Parylene N is the base structure of the parylene group. Type N has excellent dielectric properties. It has a very low dissipation factor, high dielectric strength, and a low dielectric constant that does not change with frequency.
Parylene N is more molecularly active than parylene C during the deposition process. An advantage of the higher activity is increased crevice penetration, which allows parylene N to get farther into tubes and small openings. A disadvantage of higher activity is slower deposition rates which increase the machine time and cost for thicker layers.
Parylene N is the best choice for:
- Dry lubricity.
- High frequency/RF applications because of its low dissipation factor at high frequencies.
- Applications that require high penetration.
Parylene Type F
Parylene F fills a niche because it is capable of higher operating temperatures and is more resistant to UV than type C or type N. Parylene F also has very good dielectric properties and good crevice penetration.
The chemical structure of Parylene F has four fluorine atoms on the aromatic carbons. Parylene F has a slower deposition time and the raw material is more expensive.
VSi offers parylene F for applications that require the increased temperature and UV resistance.
Parylene F is the best choice for:
- Applications with higher temperature requirements.
- Applications that require UV resistance.