Parylene offers the best protection against solvents of any conformal coating. It is also brings to the table excellent moisture and gas protection, very high dielectric strength, and is bio-compatible. Even with all of these benefits, there are still some disadvantages to using parylene versus other conformal coatings.
One of these factors 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.
The parylene process is a batch process. This means that there is only a finite amount of space available in the chamber for every coating machine run. The goal is to maximize the amount of items to be coated in the chamber. If there is a suboptimal amount of items to be coated available, the difference in price per piece could escalate drastically.
The raw material, parylene dimer, is rather expensive ranging from $200-$10,000+ per pound. Because parylene is applied through a vapor deposition process, everything, including items that do not need to be coated like inner diameter of the chamber, gets coated. This makes parylene an inherently inefficient process and wasteful with materials, which escalates the end cost to the customer.
Masking and otherwise prepping an article for parylene coating can be a labor intensive affair. Because parylene is applied as a vapor, it literally gets everywhere that air can. Our operators and quality inspectors have to take this into account prior to coating to ensure that every one of the customer’s coating free areas are just that.
One major issue that often comes up 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.
One final disadvantage of parylene to consider is the poor adhesion to many metals. Parylene has always had poor adhesion to gold, silver, stainless steel and other metals. Many printed circuit board manufacturers use gold in their products because of its conductivity. While there are some adhesion promotion methods that will greatly improve adhesion to these metals, they are either material or labor heavy and can increase costs significantly.
So whenever people tout all of the benefits of parylene, don’t hesitate to say “Yeah, but..” and bring these points up for discussion. Only by addressing and working through these issues will you be able to determine the correct coating for you.