Parylene Coating Thickness – How Much is Enough?
Sean Clancy, Ph.D. | August 18, 2020
From ensuring medical devices operate as expected in the rain to offering reliable protection throughout thermal cycling in an automobile, Parylene’s exceptional chemical and physical properties deliver potent benefits in a relatively thin coating. You may be aware that of all the conformal coatings, Parylene is applied in the thinnest layer possible, still offering effective barrier protection that, in many ways, goes unparalleled.
If you are considering a Parylene coating for your printed circuit board assembly, you may be wondering what thickness you should use. You may have surmised that the more coating, the better, but that is not always the case. The thicker the coating, the longer it takes to coat – you will face diminishing returns at some point. Additionally, there is not a universal thickness ideal for all operating environments and applications.
To gain a better understanding, let’s look into how Parylene coating works, and how to determine what Parylene thickness is most suitable for your requirements.
What is the Parylene Coating Process?
The Parylene deposition process begins with dimer, which is a powdered chemical. The more dimer that is in the first chamber, the thicker the coating will be. The dimer is heated into vapor, which is pulled by a vacuum into a connected chamber, and heated to an even higher temperature, further pyrolyzing the dimer. It is pulled into a final deposition chamber, where it penetrates crevices, coating exposed surfaces, and becomes a polymer.
What Parylene Thickness is Best?
Every project has priorities, what are yours?
- Are there cost considerations to meet, or do you need to meet the highest level of protection, no matter the price?
- Are there time constraints?
- Do you need to meet industry-standard requirements?
When Time is an Issue
Do you have a project that needs to be completed yesterday? A thinner coating may be your best bet. However, when you do go thin, say, 0.1 to 5 μm (microns), you run the risk of breaches or pinholes in the coating. If your product has simple geometry or relatively flat structures, your coating will probably be pinhole-free.
But for products with more substantial differences in structural or component heights or components that have more complex geometry, this level of thinness may not offer enough coating material to form a coherent film, leaving voids or pinholes. And any breaches in the film are potential pathways for ingress of corrosive compounds.
On the other hand, a thinner coating may provide a sufficient level of protection for products not exposed to corrosives, such as products requiring only a quality barrier against minimal exposure to contamination. Additionally, when you go thin, you decrease the time to coat and the cost.
For Product Reliability
Is product reliability the largest of your concerns? The IPC-CC-830 and MIL-I-46058 coating standards might be good references. According to these standards, thickness ranges of 12.5 to 18 and 12.5 to 25 μm, respectively, are thick enough for protection against very harsh environments. Coatings this thick are pinhole-free for a myriad of products and are time-tested, with decades of usage in demanding industries such as defense, aerospace, medical device, and telecommunications.
For products in the most demanding of environments, the NASA-STD 8739.1 standard calls for a range of thicknesses from 13 to 51 μm. This thickness is more than sufficient to provide a level of protection for incredibly harsh environments. Bear in mind, however, that at thicknesses around 50 μm, a single coating run may take well over a day, depending on the Parylene type you have selected.
The Big Picture
There is no optimal Parylene coating thickness across all applications and requirements. Instead, you should select a thickness based upon several factors, such as the operating environment that your product will be exposed to, and how your customers expect to use the product. Additionally, budget and timeframe are essential factors. Although there is no perfect thickness of coating suitable for every application, with enough planning, you can find the perfect thickness suitable for yours.
Although there is no perfect thickness of coating suitable for every application, with enough planning, you can find the perfect thickness suitable for yours.
Sean Clancy, Ph.D.
Dr. Clancy, Director of Coating Technology at HZO and Anti-Corrosion Expert, has authored 19 publications with four co-authored publications, two patent applications, and has delivered over 550 individual projects and $1M+ revenue in root cause failure analysis.
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