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Definitive Guide to Conformal Coating Application Methods

Posted on June 3rd, 2021 by

Conformal coating materials, including acrylics, epoxies, urethanes, silicones, and Parylene, introduce different benefits, drawbacks, and challenges to any project. Because there is no one-size-fits-all solution to unique electronic protection requirements, you must carefully consider these variables and how they may apply to your needs.

Consideration must also be taken for the conformal coating application method itself, which includes brushing, spraying, dipping, and chemical vapor deposition. Your choice of application will hinge upon the complexity of the substrate to be coated, required performance, and throughput requirements. Curing time, capital investment, and rework may also factor into the overall selection of the coating application method you choose.

This blog post will briefly discuss a selection of popular conformal coating techniques and touch upon each method’s benefits, drawbacks, and potential challenges.

Download Primer Paper on Coating Methodologies

Brushing

Brush coating is a well-known application technique used to coat a limited quantity of substrates. In this process, the operator formulates coating material, dips a brush into the coating container, and manually applies it to the substrate. With proper application, this method can yield excellent results on even surfaces.

Brush coating is the cheapest and easiest application method, requiring minimal maintenance, repairs, or upgrades. Equipment costs are low, and due to the controlled nature of the process, there is often a reduced need for masking. Although the process is simple, with little capital outlay, the quality and results vary as they are operator-dependent. The process itself is labor-intensive, and the operator needs proper protection from the coating environment. Obtaining a uniform build over an entire assembly is complicated, and contamination issues are a concern.

Manual brushing is a viable choice for low volumes, such as prototype runs. Brush coating can also be suitable for touchup after repairs or rework. This method also works well for high topography PCBAs due to the operator’s measure of control.

Advantage Disadvantage
Straightforward and low startup costs Difficult to control the material thickness
Suitable for low-volume, high-mix production Easy to create voids and bubbles
Suitable for rework or touchup application The brush can be the cause of residual FOD (bristles)
Can protect against airborne FOD Operator experience dependent
Very good for small parts of for where masking needs are challenging Part to part variability

Spraying

Spraying, either manual or automated, is another popular method of application. This technique is faster than brush coating, and when the solvent dilution, pattern, and nozzle pressure are appropriately combined, consistent, reliable results are possible. With this method, masking is required to shield sections or components of a PCBA before an operator uses a handheld spray gun or automated equipment to apply a thin film of conformal coating to the boards.

It may be necessary for complex boards and components for an operator to spray coating on several planes to ensure complete coverage underneath. The type of feed system, the temperature of the material, line speed, and atomization pressure affect coating thickness, and viscosity control is critical. If the coating is too thick, the PCBA gets cob webbing. If too thin, wicking and running can occur. Typically, thinning the coating with solvent is necessary for spray application. 

Although spraying costs are low to moderate, more masking time is required with spraying than brushing, possibly offsetting the savings. Although spraying may be less expensive than other methods, the temperature and humidity of the coating environment must be conditioned and factored into the startup costs. Additionally, operators should have adequate respiratory equipment.

Advantage Disadvantage
System not complicated Need to contain excess over-spray or any harmful vapors
Reasonable implementation cost Material wastage/loss in the process
Aerosol is suitable for rework Usually much higher in VOCs as dilution is needed for spray
Angled spray may provide a better coating on high topography assemblies Thin material may require multiple coat/cure cycles to get desired thickness

Dip Coating

Dip coating, which can be manual or automated, is one of the oldest coating application methods, effective at applying conformal coatings to assemblies that are not too irregular or bulky in shape. With this technique, units are masked, immersed into a tank of coating material, and subsequently withdrawn. Excess material is allowed to drip off, followed by demasking and curing. Coating thickness is contingent upon immersion and withdrawal speeds. 

Dip coating completely encapsulates the PCBA or component and is a low-cost, efficient process for high-volume applications. Correctly done, dipping yields uniform coverage and repeatable, predictable film thickness. Additionally, both sides of the board coat simultaneously, improving efficiency. That said, the open atmosphere around the dip tank can cause contamination. Consequently, the coating material in the tank must be periodically purged and replaced, which potentially increases costs for high-volume production. Variability of viscosity variations and inconsistent coating thickness can make the dipping process a crude one with little control. 

Dip coating is a popular choice for high volume production, as carriers may allow for the coating of as many as 40 PCBAs each cycle, equating to around 500 units per hour.

Advantage Disadvantage
System not complicated/easy design Open to environmental impacts – temperature/humidity
Relatively not expensive Material viscosity must be monitored
Reused material/process savings Coating reservoir can become contaminated

Chemical Vapor Deposition (CVD)

Chemical vapor deposition is used exclusively with Parylene coatings. The CVD process is performed under vacuum, with specialized equipment that includes a coating chamber.

Parylene dimer is placed in the vaporizer chamber, and the system is placed under vacuum. 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, 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 connect to other monomers, forming a polymer film.

An overview of the chemical structure changes for this process for Parylene N is illustrated below.

Figure 1 Typical deposition process, as illustrated with Parylene N

Since it deposits as a vapor that lands on and begins building up a thin film that wraps around components and substrates with little to no change in thickness, Parylene is a truly conformal coating that ensures protection from corrosive environments even at tight corners.

All surfaces can be coated evenly regardless of chamber position, and the coating deposits the same thickness all around the objects being coated. This vapor phase coating process also leads to pinhole-free coatings, absent of defects. Masking and demasking are typically required, but no curing process is necessary.

However, CVD material and equipment can cost more than other methods, and particular expertise and tools are required for rework. CVD is a batch process, and most CVD equipment yields limited volume.

Advantage Disadvantage
Uniform coverage on all surfaces Batch-mode
Excellent material properties Material/equipment can be expensive
No harmful vapors during the process Requires specific processes for rework

CVD Process with HZO

After reviewing the options above and you have decided that chemical vapor deposition with Parylene is an ideal application process for your project, consider HZO. Proprietary CVD equipment with the largest coating chambers in the industry and optimized square chambers addresses many of the limitations of the CVD process. Our team of dedicated engineers looks forward to helping guide you through the coating process from beginning to end, so send a message today.

Watch the webinar regarding the four critical things to know about conformal coating

Mallory McGuinnessMallory McGuinness

Mallory is an electronics protection evangelist who writes content for HZO. In her free time she is reading non-fiction, and hanging out with her beta fish, King Awesome.

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