You carefully wrap great-grandma’s wedding china in tissue paper, cover your car to shield it from salt in snowy weather and place your newborn baby in a snug car seat. Why? Because when you want something to stay safe for a long time, you protect it. That’s what parylene is, a protective coating for products you want to last.
Parylene coatings are hydrophobic, ultrathin, lightweight and highly conformal, wrapping around every edge available. The vapor phase coating process deposits the same thickness all around the objects they coat, resulting in pinhole-free coatings that are also free from defects. In addition, the parylene traps and immobilizes any particles that may be present on substrates.
Unlike many solvent-based protective coatings, parylene coatings are free of catalysts, plasticizers, and of course, solvents. Plus, parylenes don’t outgas, have leachable ingredients, and don’t require time to cure or cross-link.
Instead, the polymer chains in parylene pack tightly against one another, making it resistant to chemicals passing through the coating and reacting with the coating itself. Parylene coatings are impervious to moisture and insoluble in chemicals found in most end-user and industrial environments. The parylene coating also effectively blocks gases that could lead to corrosion of the coated object.
Parylene’s chemical properties form an excellent physical barrier that protects the underlying objects, especially electronics, from external contamination and electrical shorting. As a physical barrier, it protects from various types of problematic contamination, such as dust, foreign object debris (FOD), metal filings, and airborne salts.
Chemical Vapor Deposition Polymerization (CVDP)
The chemical vapor deposition (CVD) process doesn’t require the object being coated to be directly in front of the showerhead in the deposition chamber. Regardless of its position in the chamber, the object’s surfaces will be coated evenly. The most common type of CVD process for depositing parylenes is the “Gorham Process”, developed by William Franklin Gorham at Union Carbide in the late 1960s and is described in general below.
The Gorham Process
The dimer is placed in the vaporizer chamber, the system is placed under vacuum, and the vaporizer is heated to around 150 to 170 °C, until the dimer sublimes from a solid to a gas. The dimer gas travels through the pyrolyzer, which is heated at a much higher temperature, from 550 to 700 °C, where the dimer is “cracked” into two activated monomers. From there, the monomer travels into the room temperature deposition chamber and coats everything in the chamber with monomers that connects to other monomers and forms a polymer film. The thickness of the polymer film generally depends on the amount of dimer that you start with in the vaporizer. An overview of the chemical structure changes for this CVD process for parylene N is illustrated in Figure 1 below.
Figure 1. Typical parylene deposition process, illustrated with parylene N.
Parylene Chemical Structure-Property Relationships
The chemical structures for the starting materials for the five most common types of parylene are shown in Figure 2 below. The dimer types vary by the number and type of halogen, either chlorine (Cl) or fluorine (F), as added to the parylene N dimer. Parylene C has one chlorine replace an aryl hydrogen on each ring. Parylene D has two chlorines replace two aryl hydrogens on each ring. Parylene VT-4 has fluorines replace all of the aryl hydrogens. Parylene AF-4 has fluorines replace all of the aliphatic hydrogens.
Figure 2. Chemical structures of the common parylene starting materials, aka “dimers”.
Figure 3. Chemical structures of the common parylene polymers.
The chlorinated parylenes C and D are better moisture, chemical, and gas barriers than the N, VT-4, and AF-4 variants. Parylene D has slightly higher thermal resistance than C, but is much more difficult to deposit due to D’s very high sticking coefficient.
The N, C, and AF-4 variants have met FDA Class VI criteria and are suitable for biomedical applications. For electrical applications, the N and AF-4 parylenes have the best dielectric properties of the set.
The deposition processes of parylene types are affected by their chemical composition. N and VT-4 deposits much slower than C and D. D deposits faster than C, but it can be harder to control D’s coating uniformity and dispersion within the deposition chamber. AF-4 is particularly suited to high temperature and outdoor applications where the product may experience extended UV exposure, but it’s much more limited due to the cost of its starting material and limited availability. Additionally, the deposition is slow and may require the deposition chamber to be below room temperature to improve the coating process efficiency.
Adhesion Promoters for Parylene Coatings
Without adhesion promotion, parylene relies solely on its ability to physically wrap around features and on chemical Van der Waals forces (coordination) to keep the parylene on the substrate, which is a much weaker force than chemically bonding to the substrate with an adhesion promoter.
With adhesion promotion, the coating connects chemically to the product. The typical adhesion promoter used with parylene is Silane A-174, a.k.a. 3-(trimethoxysilyl)propyl methacrylate. Both solution and vapor deposition processes can be used to apply the A-174 adhesion promoter. Its chemical structure is shown in Figure 4 below.
Figure 4. Chemical structure of the Silane A-174, a.k.a. 3-(trimethoxysilyl)propyl methacrylate.
Cleaning and Surface Preparation for Parylene Coatings
Cleaning and surface preparation is critically important prior to coating performance and long-term reliability. A variety of cleaning methods using aqueous and non-aqueous media are available and include isopropyl alcohol (IPA), warm deionized water, aerosol defluxers, vapor degreaser, ultrasonic wash, and spray cleaning. Each method has their own advantages and disadvantages, including affordability, time to clean, and cleaning efficacy, but can be tailored to fit a multitude of needs.
Even with conventional cleaning, adhesion may still be an issue with some substrates, especially polyimide-based flex or rigid-flex circuit board assemblies. After conventional cleaning, treatment with an argon and oxygen plasma mixture may be necessary to have the argon plasma “roughen up” the surface by bombarding and physically ablating contaminants off the surface, and have oxygen plasma “burn off” any remaining organic contaminants and introduce chemical functional groups [hydroxylation (OH groups)] to which adhesion promoters can chemically bond. Keep in mind though that oxygen plasma may oxidize the surface and the oxidation may be undesirable for some materials (e.g., gold) and may affect surface properties.
Benefits of Parylene for Product Protection
The parylenes, especially parylene C, provides excellent physical and chemical protection from solid, liquid, and gaseous forms of contamination, including corrosive substances. Working with people experienced with many of the most common types of parylenes will go a long way in helping you select the best coating for your application, while also tailoring the parylene coating process to meet your goals and expectations.
Protection for your valuable products is attainable, affordable and predictable with parylene coatings. Protect your products as carefully as you protect great-grandma’s china, and your employees, clients and consumers will thank you.