When storing or operating in a harsh environment, electronic devices can be highly impacted by dust, moisture, mold, and other pollutants, causing poor performance and a shorter lifespan. In this case, PCB coating is a very effective way to protect them from those adverse factors, thereby improving the reliability of electronic products. In this ultimate guide, we’ll walk through everything you need to know about PCB coatings. With the right coating strategy, you can significantly extend the working life of PCBs in your products. Let’s get started!
What Are PCB Coating and Conformal Coating?
PCB coating, also known as conformal coating, is a layer made of synthetic resin or polymer that coats the surface of PCB and its components. After curing, the coating forms a transparent insulating protective film, which can precisely conform to the shape of the coated object. This layer can effectively isolate electronic components and circuit boards from the working environment to avoid corroding and extend their service life.
5 Different Types of PCB Coating
In terms of the materials used, PCB coatings can be divided into five types:
1. Acrylic PCB Coating
Acrylic is easy to apply and this coating material can remain in a good state for a long time after formulation. The curing time is short, and the coating does not release heat during curing, avoiding damage to heat-sensitive components. Also, there is no shrinkage after curing. However, it is not resistant to chemical reagents and high temperatures, thereby making it easy to rework or repair.
2. Polyurethane PCB Coating
Polyurethane is available in one-component or two-component materials. They both have good long-term dielectric properties. Before coating, the circuit board must be clean, especially without moisture. However, it is a challenge to replace components or repair circuit boards, and a special stripper must be used.
3. Epoxy PCB Coating
Epoxy resin is generally a two-component material. It stays in good condition for a shorter time after formulation. Before coating, protective measures need to be taken on fragile components to reduce the impact of coating shrinkage. If it is necessary to replace components or repair circuit boards, the epoxy resin film needs to be peeled off by physical means.
4. Silicone PCB Coating
Silicone resin has excellent thermal properties and can operate at 200°C, making it suitable for high-heat generating components such as high-power resistors. This coating stays in good condition for a short time after mixing or opening. It has a large coefficient of thermal expansion so the silicone film must be peeled off when repairing circuit boards.
5. Parylene PCB Coating
Parylene needs to be coated by automatic coating equipment (vapor deposition equipment). Under vacuum at room temperature, the active monomers of free radicals form a uniform layer of poly-para-xylene on the surface of objects. It forms coatings on a variety of surface conditions and exhibits excellent protection against various environments.
PCB Coating Comparison Chart
| Aspects | Acrylic | Polyurethane | Epoxy | Silicone | Parylene |
| Volume Resistivity ρv（/ Ω·cm） | 1012~1016 | 1011~1014 | 1012~1015 | 1013~1015 | 1016~1017 |
| Relative Permittivity ε | 3.8~4.5 | 3.8~4.5 | 3.3~4.5 | 2.6~2.8 | 2.65 |
| Dissipation Factor tan δ | 3.5×10-2 | 3.4×10-2 | 2.3×10-2 | 3.5×10-3 | 8.0×10-4 |
| CTE α（×10-5·℃-1） | 5.0~9.0 | 10.0~20.0 | 4.5~6.5 | 6.0~9.0 | 3.0~8.0 |
| Heat Resistance /℃ | 120 | 120 | 120 | 200 | 350 |
| Qualified Thickness
(After Curing) |
0.03-0.13mm | 0.03-0.13mm | 0.03-0.13mm | 0.05-0.21mm | 0.01-0.05mm |
| Curing Required | Yes | Yes | Yes | Yes | No |
| Repairability | Excellent | Good | Poor | Moderate | Good |
Techniques to Apply a Circuit Board Coating
There are different methods to apply printed circuit board coating, they are:
Manual Spraying – This method is suitable for low-volume production runs as it is a time-consuming process. Normally, we use an aerosol can or handheld spray gun to apply the coating, and before spraying, those areas that do not require coating need to be covered. The coating effects would be a little different between different batches due to the manual operation.
Selective Coating – It refers to an automatic coating process that applies a coating to the specific areas on the circuit boards by using programmed robotic spray nozzles, and there is no need to cover areas that are not to be sprayed. This process is featured with high efficiency and accuracy, suitable for high volume production.
Dipping – For this method, PCBs would be immersed in the coating solution first and then withdrawn. Many factors would affect the coating effect such as immersion and withdrawal speed, dipping time, etc. There is extensive masking required before the coating process, so it is suitable for those PCBs that require coating for both sides.
Brushing – A brush is used to apply a coating to specific areas, and it is a method mainly used for repairing and reworking. The process takes much time and requires a lot of labor, the final coating effect is dependent on operator proficiency.
How to Measure the Thickness of PCB Coating?
PCB coating is usually very thin and doesn’t add extra weight to the circuit board. Thus, measuring coating thickness usually needs professional tools. Here are some primary measurement methods.
Wet Film Thickness Gauge
This tool is ideal for measuring a wet film thickness. A gauge has many teeth and notches, just like a comb. Press the thickness gauge vertically into the coating until it touches the bottom and hold for a few seconds. Then remove it vertically. Now you can read the value between the “shortest tooth without paint” and the “longest tooth with paint”, which is the wet film thickness (WFT). If you want to get an approximate dry coating thickness, multiply this value by the solids percent of the coating.
Micrometer
A micrometer is suited for measuring harder coatings, as soft coatings tend to deform under pressure. It involves measuring the thickness before and after coating at different locations on the PCB. Then, calculate the standard deviation of measurements taken at different locations to assess the uniformity of coating thickness. This is a formulation to calculate the thickness: Single-sided coating thickness= (After Curing Thickness − Before Coating Thickness) / 2
Eddy Current Probes
The eddy current probe is a non-destructive and highly accurate measurement tool. It emits an oscillating electromagnetic field to measure the coating thickness. However, this method is limited. One is that it requires a metal under the PCB coating. The other is that the probe needs to be in direct contact with the surface of the sample being measured. If not, the results will be inaccurate.
Ultrasonic Thickness Gauge
Ultrasonic thickness gauge is a non-destructive test and has advantages over eddy current probes in that it doesn’t require a metal backplane. To make good contact with the surface, it needs a conductive substance, like water, propylene glycol, etc. The transducer emits sound, which passes through the PCB coating, reaches the surface of the printed circuit board, and then reflects back to the transducer. Now, you can calculate the thickness by this formulation: Thickness = (Speed of Sound × Time Interval) / 2
Curing Methods of Circuit Board Coating
The curing time can be affected by many factors, including type of resin, coating thickness, and curing method. We’ll then go over the four main curing techniques.
Evaporative Curing Mechanism
To put it simply, only the coating resin remains when the liquid carrier evaporates. To provide a sufficient coating on the edges of their components, circuit boards typically require at least two dips. The liquid carrier in coating materials is usually solvent-based or water-based. Solvent-based is easy to process and has consistent coverage due to good wetting propriety, offering fast cure times. However, it is flammable, requiring good ventilation and an exhaust system. Water-based eliminates flammability hazards, but it needs longer curing time and is sensitive to ambient humidity.
Moisture Curing
It is commonly used to cure silicone and some polyurethane coatings. The principle is that these materials react with moisture in the environment to form a coating. Moisture curing usually works in conjunction with an evaporative curing mechanism. First, the carrier solvent evaporates. Then the resin reacts with the moisture to achieve the final cure.
Heat Curing
Heat curing can be used to process one or multi-component systems. It can be used alone or serve as a secondary curing mechanism for evaporative curing, moisture curing, or UV curing. However, it’s essential to consider heat-sensitive boards and components during high-temperature curing.
UV Curing
UV curing is a completely solid system without carrier solvents. It uses ultraviolet light to curing, offering a fast curing process. UV lights can only irradiate the visible areas of the surface. For the blocked areas (under the components or in the shadow areas), a secondary curing mechanism is required. But this curing method also has disadvantages. It requires UV curing equipment and workers need to be protected from UV radiation. The cured coatings are difficult to repair or rework.
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.
Common PCB Coating Defects and Solutions
| Defects | Possible Causes | Solutions |
| Pinholes | -Inadequate cleaning of circuit boards
-Excessive spray pressure -High ambient humidity -High temperature causing rapid solvent evaporation |
-Thoroughly clean and dry the board before coating
-Use correct spray pressure and nozzles -Maintain humidity below 65% RH -Keep the ambient temperature below 30°C |
| Air Bubbles | -Previous stirring-induced bubbles, didn’t wait to disappear to coat
-Spray nozzle too close or pressure too high -High viscosity of coating material -Rapid solvent evaporation due to high temperature -Surface contamination, including residual solvent and moisture |
-Allow sufficient standing time after mixing before coating
-Set correct spray pressure and distance -Control viscosity during coating -Avoid high temperatures -Thoroughly clean and dry the board before coating |
| Poor Adhesion | -Inadequate cleaning
-Low surface tension of solder mask layer -Improper selection of PCB coating type |
-Perform thorough cleaning of PCB and components before coating
-Choose conformal coatings with better surface wetting or change solder mask type -Choose a compatible conformal coating |
| Cracking | -Increased brittleness and poor flexibility under low temperatures
-Excess curing agent in two-component coatings causing high shrinkage -Poor adhesion of the coating |
-Select flexible PCB coatings
-Precisely control mixing ratios for two-component coatings -Improve coating adhesion |
| Orange Peel | -Low ambient humidity
-Fast-evaporating thinner -High viscosity of coating material |
-Check the production environment
-Use a thinner with a slower evaporation rate -Reduce the viscosity of coating material |
PCB Board Coating Standards
In conformal coating, there are a series of PCB coating standards that require its usage under certain conditions like the military, automobile, domestic use, etc. Most commonly the conformal coatings qualify for either MIL-I-46058C or IPC-CC-830B specification which relates closely to MIL-I-46058C.
MIL-I-46058C: A common conformal coating standard in the industry, also known as Military insulating compound. It requires testing from any MIL authorized laboratories and is still used even after the deactivation since 1998 for new designs. This test requires a standard Qualified product list (QPL).
Def Stan 59/47: A similar standard to 46058C used for coating high-end devices for military use but the UK Ministry of Defense must approve them first.
IEC 61086: A standard based on self-certification by the supplier with similar requirements to 46058c. International Electrotechnical Commission governs it.
IPC-CC-830B: Actively used and continuously updated standard similar to 46058C, introduced when 46058C remains inactive. Material standardized for 46058C that follows these specifications. No testing is available as no QPL is maintained.
UL94V0: Relates to the conformal coating property of self-extinguishing on an FR4 substrate. V0 is the highest achievable category with V1 and V2 as its successors.
PCB Coating Service at MOKO Technology
Choosing the right PCB coating should balance many factors such as working environment, level of protection, circuit board’s requirements, etc. MOKO Technology deeply understands the concerns of applying PCB coatings to your boards. We’re here, backed by nearly 20 years of PCB & PCBA experience, to help you choose the ideal coating solution. For expert help, get in touch with us.
