Circuit Board Conformal Coating 101

Next to my front door I have a rack that holds a wide assortment of coats, and one hat. The coats are for both me and my wife, and they are all different from each other in styling and materials depending on the needs of the day. There are light coats for brisk mornings, raincoats for the downpours, and even a full winter coat for the occasional freezing temperatures that come through. I have no idea about the hat though, I’m not even sure that it’s mine come to think of it.

Just as our coats protect us from unpleasant environments, our circuit boards also have coats available to protect them. This protective cloak for circuit boards is known as conformal coating, and it can vary in its composition as well depending on the environment that the PCB will operate in. We’re going to take a closer look at what circuit board conformal coating is, how it is applied, and what that means to you as a PCB designer. 

What is Circuit Board Conformal Coating?

Conformal coating is a thin film of protective insulating material applied to a fully assembled circuit board. It is made of a non-conductive material, and can cover the entire board depending on the needs of the PCB and what environment it will be operating in. It will cover and protect exposed areas of metal such as traces that aren’t already covered with solder mask, component leads, and solder joints. Conformal coatings will even increase the dielectric strength of a board allowing for reduced spacings between circuits in more compact and dense PCB layouts.

The non-conductive dielectric materials used in conformal coatings can be acrylic, silicone, or urethane resins. Here are some of the contaminants that conformal coating will protect a PCB from, but there are many others than what is listed here:

• Dust and dirt

• Salt-spray

• Fungus

• Condensing moisture

In addition, conformal coating is resistant to various chemicals and solvents. It will also help to prevent damage from thermal and mechanical stress as well as abuse during handling, all of which will help extend the circuit board’s operational life. And just as conformal coating will help to protect the PCB from these contaminants, it will also help seal delicate components preventing leakage and corrosion.

For printed circuit boards that are going to be operating in harsh environments, the protection that conformal coating provides is extremely important. Because of this, conformal coating circuit boards are commonly used in aerospace, marine, automotive, industrial, power, and military applications. And even though the price of conformal coating used to restrict it to more mission critical applications, with decreasing prices conformal coating is becoming more common. As conformal coating becomes even more widespread, and the technology needed to work with it continues to improve, the resulting drop in prices will see it mainstreamed into more consumer level products.

Conformal coating being applied through selective spraying

How is Conformal Coating On Circuit Boards Applied?

When you have your circuit board coated, there are many different ways that the conformal coating can be applied:

• Dipping: For boards that are in high volume production runs, dipping is the usual method of applying a conformal coat. Dipping will coat both sides of the board, so PCBs that have specific areas that either need or don’t need coating will have to use another method.

• Automated spraying: This method is also used for production runs. The circuit boards are run through the process on a conveyor belt, and reciprocating spray heads will apply a consistent coating on the board.

• Selective coating: This is a variation of the automated spraying, but programmable spray heads are used instead of reciprocating. This allows for boards with unique requirements to have specific areas programmed for applying the conformal coating.

• Manual spraying: For technicians working on a board, manually spraying conformal coating with a spray gun or an aerosol can is the fastest approach. This is a labor intensive operation used in low production runs, although it lends itself well for boards that have unique requirements. Those areas can be masked off to prevent the conformal coating from applied there.

• Brushed: This is the slowest method of applying conformal coating, and is usually done when a board is being reworked or repaired and the coating has been removed in specific areas. Brushing the coating on however could result in variations of the application as it is not as consistent as spraying the coating.

Applying the coating will depend on how many circuit boards that you are having produced, and if there are any unique requirements on how it is to be applied. The key to all of this is how you communicate the conformal coating needs to your contract manufacturer.

Conformal coating can protect the components, solder joints, and exposed traces on your PCB

What Do I Need to Know About Conformal Coating When Designing a PCB?

When setting up your printed circuit board for conformal coating, you will need to specify to your contract manufacturer what it is that you want. It is always best to work out the details on how to communicate this with your CM first, but they will usually be looking for this information in the manufacturing drawings. You need to be clear and concise as to where the conformal coating is required on the board, as well as where it is not.

Generally it is assumed that the conformal coating will cover the entire board, but your drawings should make this perfectly clear. For boards with areas that are not to be coated, your drawings should indicate the zones that should be masked in order to keep coating out. Areas of the board that are not normally coated would include EMI or RF shields, heat sinks, batteries. Adjustable devices like potentiometers, and connectors.

One thing that will help you with the design of a printed circuit board that requires conformal coating, is to use an advanced CAD tool withthat all of the features that you will need. An example of a PCB design system that has the superior circuit board design and drawing creation capabilities that you will need is available from Cadence. OrCAD PCB Designer has the tools and functionality to take your design from concept through design, and then create the manufacturing files that you will need to build the board.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.