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Parylene Versus Acrylic Conformal Coatings
April 24, 2013 |Estimated reading time: 3 minutes
Parylene and acrylic resins are both conformal coatings, but the similarities end there. The properties of each vary so much that they have their own unique uses and capabilities.
Application
Acrylic conformal coatings are a liquid conformal coating and can be applied four ways: Via spray, dip, brush, or robotics. Each method varies by a couple of factors, including quantity of products, complexity of masking requirements, and the skill level of operators. They are usually applied between 0.002” and 0.005”.
Parylene coating, meanwhile, is applied through a vapor deposition process onto the substrate or material being coated. Depending on the coating type and required thickness, typical parylene deposition rates are approximately 0.2/mils per hour; so machine runs can vary from as little as 1 hour to over 24 hours.
In parylene coating, the process begins with raw dimer in solid state (parylene C, parylene N, parylene D, parylene AF-4, or other variants) being placed into a loading boat, which is then inserted into the vaporizer. The raw dimer is heated to between 100 and 150°C. At this time, the vapor is pulled, under vacuum, into the furnace and heated to very high temperatures, which allows for sublimation and the splitting of the molecule into a monomer. The monomer gas continues to be drawn by vacuum one molecule at a time onto the desired substrate at ambient temperatures in the coating chamber. The final stage of the parylene deposition process is the cold trap. The cold trap is cooled to between -90 and -120°C, and is responsible for removing all residual parylene materials pulled through the coating chamber.
Based from the above, the application process of parylene is quite different than a traditional wet chemistry coating like acrylics. Parylene films are usually applied between 0.005” and 0.002”, but can be applied as little as 0.0001”.
Common Uses
A common use of acrylics is on PCBs for moisture protection. On the other hand, while parylene can be used on PCBs, it has a wide range of uses in medical products such as stents, catheters, and needles.
Disadvantages
In applications with exposure to solvents, acrylic conformal coating is not the best choice because it can be removed with a weaker solvent such as isopropyl alcohol or xylene. When it faces even stronger solvents, it will not offer the protection needed, especially if the product is a mission-critical device.
Also, for products that require a high-temperature application, acrylic coatings will fall short of expectations. For HumiSeal 1B31, arguably the most popular acrylic coating, the maximum continuous operating temperature is 125°C. Compare this to silicone conformal coating with an operating temperature that can exceed 200°C.
One disadvantage of parylene, however, is cost. The cost for parylene is typically higher than other conformal coatings due to many factors: The process itself, the raw materials involved, and the labor required to properly prepare a device for coating. While this is not necessarily true for all applications, typically for an item quoted in parylene and wet chemistry, the parylene pricing will be higher.
Another major issue that comes up often for some high-volume manufacturers is the limited throughput of parylene. Runs of the parylene machine can take anywhere from eight to over 24 hours. As a result of the limited chamber space, there is a fixed amount of product that can be processed during one coating cycle. This, coupled with the high capital cost of new equipment, can wreak havoc with a company and its customer’s internal and delivery schedules.Sean Horn is vice president and chief financial officer at Johnstown, Pennsylvania-based Diamond MT, a firm specializing in contract applications of conformal coatings for Department of Defense and commercial electronic systems. Contact Horn at shorn@diamond-mt.com.