Parylene and acrylic resins are both conformal coatings. Most of the similarities stop there. Because their properties vary so much, they have their own unique uses and capabilities.
Acrylic conformal coatings are a liquid conformal coating and can be applied four ways: spray, dip, brush, or robotics. Which method selected will vary by a couple of factors including: Quantity of products, complexity of masking requirements, and skill level of operators. They are usually applied between .002” and .005”.
Parylene coating is applied through a vapor deposition process onto the substrate or material that is being coated. Depending on the coating type and required thickness, typical parylene deposition rates are about .2/mils per hour, so machine runs can vary from as little as 1 hour to over 24 hours. The process begins with raw dimer in solid state (these are: 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 between 100-150º 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 -90º and -120º C and is responsible for removing all residual parylene materials pulled through the coating chamber. As you can see, the application process of parylene is quite different than a traditional wet chemistry coating like acrylics.
Parylene films are usually applied between .0005” and .002”, but can be applied as little as .0001”.
A common use of acrylics is on printed circuit boards for moisture protection.
While parylene can be used on printed circuit boards, it has a wide range of uses in medical products such as stents, catheters, and needles.
In applications that have an exposure to solvents, acrylic conformal coating is not the best choice. Acrylic conformal coating can be removed with a weaker solvent such as isopropyl alcohol or xylene. Whenever it faces even stronger solvents, it will not offer the protection that is needed, especially if your product is a mission critical device.
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 max continuous operating temperature is 125ºC. Compare this to silicone conformal coating, whose operating temperature can exceed 200ºC.
One disadvantage of parylene is cost. The cost for parylene is typically higher than other conformal coatings. This is because of many factors, such as 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 several of our high volume manufacturers is the limited throughput of parylene. Runs of the parylene machine can take anywhere from eight to over twenty-four 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 our internal and our customer’s delivery schedules.