Conformal coatings are used to protect a PCB and its components from many environmental factors. Things like humidity, corrosive substances, oxidizing substances, acidic/basic solutions, and dust can all attack a PCB and lead to reliability challenges. Conformal coatings are formulated to be easily applied so that a barrier layer can be applied across the entire surface of a PCB.
There are different classes of materials used to formulate conformal coating products. In some instances, a conformal coating will need to be removed for purposes of testing, rework, debug, or repair. If a conformal coating needs to be re-applied, is it possible to mix conformal coatings without creating a new reliability problem? We’ll look at compatibility in this guide.
Conformal Coating Compatibility
In general, there are five different base materials used for conformal coating formulations. These are listed in the table below.
Best for humidity and solvent protection
Best for temperature, solvent, and humidity protection
Polyurethane, silicone, acrylic, and epoxy conformal coatings are not generally compatible with each other. Each type of conformal coating material has different chemical properties, curing mechanisms, and adhesion characteristics that can affect their compatibility with other materials. For example, it is generally not recommended to apply a silicone-based conformal coating over an acrylic-based conformal coating, or vice versa. If a silicone conformal coating is being removed from a PCBA, then it should be patched with another silicone-based conformal coating.
This is because the two materials may not adhere well to each other. Weakly adhering thin films can experience slow delamination over time, where the coating material begins to flake away from the applied surface. The other reason to not mix coatings is the differing levels of protection they can provide. In general, it is best to select a single conformal coating material and only use that material if the coating ever needs to be re-applied.
Parylene-Based Materials Are More Compatible
There is one exception to this compatibility issue: parylene-based coatings. Parylene-based coating materials can be applied over most other conformal coatings in the above list. Parylene has excellent adhesion to a wide range of substrates, including metals, plastics, ceramics, glasses, and thin films. It forms a conformal coating that is highly resistant to moisture, chemicals, and extreme temperatures.
The problem with parylene-based materials is that they are applied using a vapor phase deposition system. This is more complex than an automated spray coating system. The coating must be brought to a vapor at the right temperature and pressure, and then it is allowed to deposit on the assembly. These more complex deposition systems carry more cost and they are not used by many assembly houses.
Re-application of Conformal Coatings
On an assembly line, especially at high-volume, conformal coatings will be applied with an automated spray coating system using pressurized air (does not apply to parylene). These systems have high throughput and will generally be used to apply coatings on multiple boards in parallel. They also allow coating through a mask if a coating needs to be applied selectively.
When performing repair or rework, an automated system should not be needed, and indeed this would be impractical in most situations. Instead, most conformal coatings are available in spray cans so that a new coating can be applied by hand. A thin, relatively uniform layer can be applied, and the extent of the coating can be verified visually by looking at the sheen of the coated surface.
If you’re running into frequent repair issues on conformally coated boards, you might consider investing in a vapor phase deposition system. These systems carry some high costs, so this should be balanced against running a full quality control investigation to find the root cause of failure that requires repairs. Designers should focus more on the main applications and reliability of your conformal coating needs, and let the repair technicians focus on selecting an appropriate replacement.
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