PCB Conformal Coating: A Comprehensive Guide for Beginners

In today’s world, electronic devices play a vital role in our everyday lives, offering convenience and enhancing our overall experiences. Yet, they’re quite susceptible to external conditions; factors like moisture, temperature, and dust can disrupt their functionality. This is where PCB conformal coating steps in. It serves as a defensive barrier, preventing the board from any adverse factors. In this article, we will introduce the types of conformal coating, how to apply conformal coating, and how to choose the right one for your projects.

What Is PCB Conformal Coating

PCB conformal coating is a non-conductive chemical coating or polymer film, usually with a thickness of 10-25 microns. After the components are assembled, circuit board conformal coatings are applied to the surface of the board to protect the electronic circuit from various harmful factors to improve the lifespan and reliability of electronic equipment.

Types of Conformal Coatings

Circuit board conformal coatings are distinguished by their chemical composition, each offering unique properties and advantages. Here, we’ll explore five of the most commonly utilized types of them:

1. Acrylic Resin (AR)

Acrylic coatings are cost-effective, easy to use and repair. They have high dielectric strength along with moderate moisture and abrasion resistance. However, their suitability falls short in environments requiring chemical resistance, as they can be easily stripped away by certain solutions.

2. Silicone Resin (SR)

Silicone conformal coating has excellent resistance to chemical corrosion and can protect PCBs under high temperature conditions. It is flexible, elastic and vibration-resistant, so it is suitable for some outdoor applications. However, this coating is not easily removable, requiring specialized techniques or solutions.

3. UrethaneResin(UR)

The most outstanding performance of Urethane conformal coating is moisture resistance and chemical corrosion resistance, so it is often used in aerospace applications. But like silicone resin, it is difficult to remove.

4. Parylene (XY)

Parylene conformal coating is a type of coating that is applied through vapor deposition. It is featured with outstanding dielectric strength and moisture resistance. In addition, it is very thin and light, protecting the circuit board without adding significant bulk or weight to the circuit board.

5. EpoxyResin(ER)

Epoxy resin coatings are celebrated for their outstanding moisture resistance. Characterized by their hardness, these coatings offer high resistance to chemicals and abrasion. Moreover, this kind of PCB conformal coating can withstand temperatures up to 150°C. Epoxy coatings are often used in potting compounds that completely cover the electronics without penetration.

Types	Pros	Cons	Applications
Acrylic resin	cost-effective, easy to use and repair	poor chemical resistance,	general-purpose applications where chemical resistance is not critical
Silicone resin	excellent chemical resistance, flexible and vibration-resistant	difficult to remove, no abrasion resistance	applications requiring chemical resistance, high-temperature environments
Urethane resin	outstanding moisture resistance, excellent chemical corrosion resistance	difficult to remove. long time curing process.	aerospace applications
Parylene	outstanding dielectric strength, thin and lightweight	high cost, hard to rework	environment with extreme temperatures;
Epoxy resin	high abrasion resistance, great moisture resistance	difficult to remove, cannot withstand high temperatures	applications requiring potting compounds

How to Apply Conformal Coating

There are 4 primary methods to apply conformal coating on printed circuit board:

• Brushing

Brushing refers to using a brush to apply the coating to the corresponding area on the circuit board. The tools used are very simple. You only need a brush and a container for coating. Maintenance and rework are also very convenient. However, this method is only suitable for small batch production, and requires the operator to be skilled in operation and able to control the amount of coating well to ensure the consistency of the coating thickness.

• Dipping

Dipping is a traditional circuit board conformal coating method. The whole process is simple to operate. You only need to completely dip the PCB into the coating and wait for it to solidify. Note that only lower viscosity coatings can be used. Its disadvantage is that it requires a lot of masking and sealing preparation upfront, and the entire process is also very time-consuming.

• Spray application

Spray coating atomizes the conformal coating and applies it to the circuit board using air or gas. This includes aerosol spraying, batch spraying, and selective spraying. Aerosols use solvent-based coatings and a spray gun within a booth. Batch spraying employs compressed air spray guns for high-quality finishes. Selective spraying precisely coats specific areas, avoiding masked components. While capital investment is high for spray setups, this technique supports all coating types with proper programming and monitoring.

• Vapour deposition

Vapor deposition is among the more costly coating techniques due to its need for specialized machinery. This method involves evaporating the coating into a mist and then depositing it on the circuit board. It offers enhanced moisture protection and superior electrical performance. Primarily used for applying parylene and PTFE coatings, and well-suited for medium to high production volumes.

Process of Applying PCB Conformal Coating

Step 1: Cleaning

The PCB is thoroughly cleaned to remove any residues, contaminants, or impurities using techniques like aqueous chemistries, solvent cleaning, or plasma cleaning.

Step 2: Priming (optional)

Depending on the type of conformal coating resin, a primer may be applied to enhance adhesion and improve the binding of the coating to the PCB surface.

Step 3: Masking

In this step, we need to mask the area of the PCB that should not be coated. To perform this, we can use different tools such as masking tapes, liquid latex, or masking boots.

Step 4: Applying coatings

Now, it’s time to apply the conform coating to the circuit board. Many methods are available, as mentioned above, including brushing, dipping, or vapor deposition, among others.

Step 5: De-masking and finishing

After the coating application, the masking materials are removed, and any necessary finishing processes are performed to ensure the quality of the coating, prevent masking leakage, and address any defects.

Step 6: Drying and curing

The coated PCB goes through drying and curing stages to ensure that the coating is properly set and achieves its desired properties. The curing mechanism may involve heat, moisture, UV radiation, or catalytic processes, depending on the coating type.

Step 7: Inspection

The final step involves a thorough inspection of the coated PCB, either manually, using UV lamps for visibility, or through automated optical inspection (AOI) systems, to ensure the quality and integrity of the conformal coating.

How To Remove Conformal Coating

When the circuit board needs repair or component replacement, we need to remove the circuit board conformal coating. Below we list commonly used methods of removing conformal coatings:

Solvent Removal – Use specific solvents to dissolve the coating, but make sure the solvent you choose is appropriate and will not damage electronic components. Generally, Acrylic coatings are the easiest to dissolve, while silicone and urethane coatings are more difficult to remove.

Peeling – For some elastic coatings such as silicone conformal coating, we can remove them by peeling them off the circuit board with a knife. But this process requires special care and precise control by the operator, otherwise it might damage the components.

Thermal/Burn-through – During the repair process, you can use a soldering iron to simply burn through the coating, but you need to operate it carefully. This method is suitable for almost all types of conformal coatings.

Microblasting – The process involves using a micro-sandblaster, which employs a concentrated mix of soft abrasive and compressed air, to remove the coating effectively. This method is suitable for removing parylene and epoxy coatings.

Grinding/Scraping – You need to use a drill to grind off the unnecessary coating, which is suitable for some harder coatings such as epoxy resin and polyurethane. However, if the operator is not careful, there is a possibility to damage the circuit board, so this method is not a priority.

Choosing the Right Conformal Coating

Choosing the most suitable conformal coating needs to consider several factors, including:

• Operating temperature range of the coated components;
• Production volume and throughput requirements;
• Design specifications of the printed circuit board;
• Environmental conditions such as temperature and humidity;
• Equipment and machinery available for the coating process;
• Relevant industry standards, customer requirements, or national regulations.

By comprehensively considering these factors, I believe you can find a conformal coating suitable for your PCB project. If you are still not sure about how to choose the right coating, you can consult a professional PCB manufacturer such as MOKO Technology. Our professional engineers will provide the best solution according to the specific needs of your project. Contact us now to know details about our conformal coating service!