PCB conformal coating is a layer applied to circuit boards to protect them from environmental elements like temperature fluctuations, dust, moisture, and chemicals. The conformal coating can highly to extend the PCB lifespan and minimize the risk of component failures caused by environmental influences. This blog will cover the top 5 PCB conformal coating types and key considerations for choosing the ideal coating. Besides, you can learn how to apply, cure, or remove them.
Top 5 PCB Conformal Coating Types
Each kind of PCB conformal coating has unique properties, and we will discuss the five major types below.
| Aspect | Acrylic Resin | Silicone Resin | Urethane Resin | Epoxy Resin | Parylene |
| Properties | Moisture resistance Fair abrasion resistance UV resistance High dielectric strength | Excellent moisture & corrosion resistance Good salt spray & chemical resistance Vibration stress resistance | Excellent moisture & chemical resistance High abrasion & solvent resistance | High humidity & chemical resistance Excellent abrasion resistance Moderate solvent resistance | Superior moisture & solvent resistance Extreme temperature resistance High dielectric strength |
| Removability | Easy (non-corrosive solvents like isopropyl alcohol) | Difficult (specific solvents, prolonged soaking, ultrasonic bath/brush) | Difficult (specific solvents, prolonged soaking, ultrasonic bath/brush) | Extremely difficult (chemical methods ineffective) | Challenging (requires abrasion techniques) |
| Pros | Cost-effective Easy application and removal Fast curing time(as little as 30 minutes) | Excellent temperature range Vibration resistance Ideal for outdoor sign applications | Excellent chemical and abrasion resistance Suitable for harsh environments | Tough and durable Fast cure time | Ultra-thin, uniform coating No curing required |
| Cons | Poor solvent resistance Limited temperature range (up to 125°C) | Requires specific solvents for repair | Requires specific solvents for repair | Can become brittle after curing, limited flexibility Require specialized equipment for mixing materials | Requires specialized CVD equipment |
Acrylic Resin (AR)
Acrylic resin is an affordable conformal coating that offers a moderate degree of protection. This coating has the advantages of high dielectric strength, fair abrasion resistance and moisture resistance, and UV resistance. There are several ways to apply it to the circuit board, including brushing, dipping, and spraying. Acrylic-based conformal coating can be also easily removed using non-corrosive solvents such as isopropyl alcohol, so repairs and rework are easy to make. However, its main limitation is poor solvent resistance and unsuitability for use in environments above 125°C.
Silicone Resin (SR)
Silicone resins offer excellent protection over a wide temperature range (up to 200°C), with some silicone coatings even reaching 600°C. This PCB conformal coating has excellent moisture and corrosion resistance as well as good salt spray and chemical resistance. Silicone-based conformal coating is not resistant to abrasion because of its rubbery nature, but this property helps the coating withstand vibration stress. Special formulations allow LED lights to be coated without reduction of intensity or color shift, making SR an ideal option for outdoor sign applications.
Removing silicone resin can be difficult, involving the use of specific solvents, prolonged soaking times, and agitation with an ultrasonic bath or a brush. Furthermore, the high thermal resistance of this conformal coating can hinder heat dissipation.
Urethane Resin (UR)
Urethane resin provides excellent moisture and chemical resistance. It is frequently utilized in aerospace applications where fuel vapor exposure is a frequent issue. Because this PCB conformal coating is very resistant to abrasion and solvents, it is hard to remove. Similar to silicone conformal coating, complete removal typically calls for specific solvents, a lengthy soak period, and brush or ultrasonic bath agitation. Most urethanes take longer to cure than acrylics, which can cure in as little as 30 minutes. This is also an important factor to consider when working on high-volume production or time-sensitive projects.
Epoxy Resin (ER)
Epoxy resins are available as two-component compounds that form a tough, smooth, and non-chippable surface. This PCB conformal coating has good humidity resistance as well as high chemical resistance (especially to solvents) and abrasion resistance. However, the coating material is complex to prepare and apply, requires specialized equipment for mixing, and has a fast cure time (usually measured in minutes). Once cured, it is extremely difficult to remove by chemical methods, making subsequent repair work difficult. The tough coating lacks flexibility after curing, which may cause shear damage to components or circuits when the circuit board bends due to thermal expansion and contraction or external forces.
Parylene
Chemical vapor deposition (CVD) is used to apply parylene, a special type of conformal coating material. This coating can penetrate into the gaps and under components of the circuit board in gas form, forming an ultra-thin protective layer without curing. Parylene conformal coating has superior moisture resistance, solvent resistance, and extreme temperature resistance. This kind of PCB conformal coating is suitable for special applications due to its low expansion and high dielectric strength. Once applied, removal for rework is challenging, necessitating the use of abrasion techniques, and reapplication is impossible without vapor deposition equipment.
Key Considerations for Selecting PCB Coating Material
After learning about the five PCB conformal coating types, here are three key considerations to keep in mind when making a choice.
Product Intended Use
Different coatings have both drawbacks and positives, selecting the coating needs to be according to the final use of the product. If the PCB will be used in a product exposed to water for an extended period of time, then it must have first-rate moisture resistance. Because of its superior moisture resistance, parylene can become the best of the various conformal coatings.
Temperature Tolerance
Second, it is important to consider the operating temperature range the coating can support, especially for demanding applications like aerospace. For example, PCBs used in aircraft need to withstand temperatures between -65°C and over 100°C.
Silicone conformal coatings operate normally and remain stable within this temperature range. In addition, how the coating affects the heat dissipation of the PCB must also be considered. The high-insulation coating causes heat to accumulate on the circuit board, while the good thermal conductivity coating allows for easier heat dissipation.
Environmental Conditions
The operating conditions of the PCB can affect its conformal coating selection to a certain extent, since humidity, corrosive gases, and chemicals can affect circuit board performance. If the PCB is in a partially sealed environment, the main concern is humidity and condensation, irrespective of outside corrosive factors.
The physical properties of the conformal coating will affect circuit board performance in vibration and stress conditions. Rigid coating, which fails to deform upon bending or vibration of the PCB, can cause mechanical stress to elements such as component pins, causing cracks in solder joints.
5 Common Methods for Applying Conformal Coating
Protective coatings are applied using various techniques, and the most appropriate method depends on the board size, production volume, and project budget, among other things. Let’s look at five popular methods for applying PCB conformal coating.
Manual Spraying
Manual spraying involves hand application of the coating to the board using a spray gun or an aerosol can. It is extremely time-consuming and is typically utilized for low-volume production runs. The outcomes may differ from board to board, and the quality and consistency of the PCB conformal coating depend on the operator.
Selective Coating
Selective coating is an automated process, using programmable spray nozzles to apply PCB conformal coating to specific areas on a circuit board. This process does not require tedious masking procedures and is very popular in mass production.
Automated Spraying
Automatic spraying is a programmed system that utilizes programmed reciprocating spray heads to precisely apply consistent coatings to PCBs moving on a conveyor, greatly reducing the need for masking. Automated systems require a high initial investment, but the consistency, accuracy, and efficiency of the coating they apply make them indispensable in large-scale production.
Dipping
Dipping is a traditional coating-applied method, in which multiple PCBs are simultaneously immersed in a conformal coating solution and withdrawn and dried. Immersion rate, withdrawal rate, dipping time, and coating material viscosity control the resulting coating effect. The process supports high processing speed and is used in mass production and online processing. But there has to be a great deal of masking off of areas that must not be coated, and this will only happen when both sides of the circuit board have to be coated.
Brushing
This simple brushing technique is primarily used for repair and rework, using a brush to apply the coating to specific areas of the board. Brush type, coating viscosity, operator experience, and others determine the coating quality and consistency. Brushing is labor-intensive, but it is less expensive and suitable for low-production volume.
How to Cure PCB Conformal Coating?
There are many types of techniques to cure circuit boards and some of the most widely used methods are mentioned below.
Evaporative Curing Mechanism
The core principle of evaporation curing is that only the protective coating resin remains after the liquid carrier evaporates. This process is intuitive in theory, but it is necessary to dip the circuit board at least twice to guarantee that the edges of its components have a sufficient coating.
Moisture Curing
Moisture curing is commonly used for silicone and some urethane systems, which react with moisture in the ambient to form a cured polymer coating. Moisture cure is often combined with evaporative cure, where the carrier solvents are first evaporated, and then the moisture reacts with the resin to complete the final cure.
UV Curing
UV curing is a technology that uses ultraviolet energy to cure PCB conformal coating rapidly, within seconds. With no carrier solvents, they are entirely solid systems. Only the areas directly exposed to the UV light will cure, so a secondary cure mechanism is required under components and in shadowed areas.
Heat Curing
As a secondary cure process to evaporation, moisture, or UV cure, heat cure mechanisms can be applied in single or multicomponent systems. Heat can accelerate the curing rate and reduce the production time considerably. The heat sensitivity of components and circuit boards requires special precautions when curing at elevated temperatures.
How to Remove the Coating from a PCB?
PCB conformal coating needs to be removed from the circuit board in the course of repairing circuit boards or damaged PCB components. The following are some of the popular methods to remove the coating.
Solvent Removal
Most conformal coatings are easily removed by solvents, but it is necessary to confirm whether the solvent will damage the circuit board. Acrylic acid is easily removed, while epoxy resin, urethane, and silicone have the lowest sensitivity to solvents.
Peeling
This is a method of directly peeling off PCB conformal coating from circuit boards, typically used for silicone and some flexible coatings.
Burn-Through
This is a common coating removal technique that is suitable for most circuit boards. When reworking the circuit board, use a soldering iron to burn through the coating.
Microblasting
Micro sandblasting is the use of a concentrated mixture of soft abrasives and compressed air to abrade coatings, which can remove small areas of coatings. This method is commonly used to remove parylene and epoxy resin coatings.
Grinding/Scraping
Grinding or scraping the circuit board to remove the PCB conformal coating is effective for hard coatings such as parylene, epoxy resin, and urethane. This may cause damage to the circuit board, so it is only used as a last resort.
Final Words
When choosing the most appropriate PCB conformal coating, it is essential to consider very carefully the coating properties, such as moisture resistance, abrasion resistance, and chemical resistance. Equally, it is important to take into account how the coating performance at extreme temperature conditions, severe environmental exposures, and mechanical stresses. All of these must be weighed to determine the most suitable conformal coating for reliable protection.
