When storing or operating in a harsh environment, electronic devices can be highly impacted by dust, humedad, moho, and other pollutants, causing poor performance and a shorter lifespan. En este caso, PCB coating is a very effective way to protect them from those adverse factors, thereby improving the reliability of electronic products. En esta guía definitiva, we’ll walk through everything you need to know about PCB coatings. Con la estrategia de recubrimiento adecuada, puede extender significativamente la vida útil de los PCB en sus productos. Empecemos!
What Are PCB Coating and Conformal Coating?
Revestimiento de PCB, also known as revestimiento de conformación, 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 componentes electrónicos and circuit boards from the working environment to avoid corroding and extend their service life.
5 Different Types of PCB Coating
En cuanto a los materiales utilizados., Los recubrimientos de PCB se pueden dividir en cinco tipos:
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. también, there is no shrinkage after curing. sin embargo, 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. sin embargo, 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 | Acrílico | Poliuretano | Epoxy | Silicona | Parileno |
| 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 | si | si | si | si | No |
| Repairability | Excelente | Bueno | Pobre | Moderado | Bueno |
Techniques to Apply a Circuit Board Coating
Existen diferentes métodos para aplicar el recubrimiento de placas de circuito impreso., ellos son:
Manual Spraying – Este método es adecuado para tiradas de producción de bajo volumen, ya que es un proceso que requiere mucho tiempo.. Normalmente, usamos una lata de aerosol o una pistola rociadora manual para aplicar el recubrimiento, y antes de pulverizar, aquellas áreas que no requieren recubrimiento deben cubrirse. The coating effects would be a little different between different batches due to the manual operation.
Selectivo Coating – Se refiere a un proceso de recubrimiento automático que aplica un recubrimiento a las áreas específicas de las placas de circuitos mediante el uso de boquillas rociadoras robóticas programadas., y no hay necesidad de cubrir áreas que no se van a rociar. Este proceso se presenta con alta eficiencia y precisión., adecuado para la producción de alto volumen.
Inmersión – Para este método, Los PCB se sumergirían primero en la solución de recubrimiento y luego se retirarían. Muchos factores afectarían el efecto del recubrimiento, como la inmersión y la velocidad de extracción., tiempo de inmersión, etc.. Se requiere un enmascaramiento extenso antes del proceso de recubrimiento., por lo que es adecuado para aquellas PCB que requieren recubrimiento por ambos lados.
Cepillado – Se utiliza un cepillo para aplicar un recubrimiento en áreas específicas., y es un método utilizado principalmente para reparar y reelaborar. El proceso lleva mucho tiempo y requiere mucha mano de obra., el efecto de recubrimiento final depende de la competencia del operador.
How to Measure the Thickness of PCB Coating?
PCB coating is usually very thin and doesn’t add extra weight to the circuit board. Así, measuring coating thickness usually needs professional tools. Here are some primary measurement methods.
Wet Film Thickness GRAMOauge
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” y el “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.
Micrómetro
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. Luego, 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 PAGrobes
The eddy current probe is a non-destructive and highly accurate measurement tool. It emits an oscillating electromagnetic field to measure the coating thickness. sin embargo, 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. Si no, 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. Ahora, 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, espesor del recubrimiento, and curing method. We’ll then go over the four main curing techniques.
Evaporative Curing Mechanism
Para hacerlo mas simple, 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. sin embargo, 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. primero, 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. sin embargo, 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?
Cuando la placa de circuito necesita reparación o reemplazo de componentes, Necesitamos quitar el revestimiento conformado de la placa de circuito.. A continuación enumeramos los métodos comúnmente utilizados para eliminar los recubrimientos conformados.:
Eliminación de solventes – Use specific solvents to dissolve the coating, pero asegúrese de que el disolvente que elija sea apropiado y no dañe los componentes electrónicos.. Generalmente, Los recubrimientos acrílicos son los más fáciles de disolver., mientras que los recubrimientos de silicona y uretano son más difíciles de eliminar.
Peladura – For some elastic coatings such as silicone conformal coating, Podemos quitarlos despegándolos de la placa de circuito con un cuchillo.. Pero este proceso requiere un cuidado especial y un control preciso por parte del operador., De lo contrario podría dañar los componentes..
Thermal/Burn-through – During the repair process, Puedes usar un soldador para simplemente quemar el recubrimiento., pero debes operarlo con cuidado. Este método es adecuado para casi todos los tipos de recubrimientos conformados..
microgranallado –The process involves using a micro-sandblaster, que emplea una mezcla concentrada de abrasivo suave y aire comprimido, para eliminar el recubrimiento de manera efectiva. Este método es adecuado para eliminar revestimientos de parileno y epoxi..
Moler/raspar – You need to use a drill to grind off the unnecessary coating, que es adecuado para algunos recubrimientos más duros como resina epoxi y poliuretano. sin embargo, si el operador no tiene cuidado, existe la posibilidad de dañar la placa de circuito, entonces este método no es una prioridad.
Common PCB Coating Defects and Solutions
| Defectos | Possible Causes | Soluciones |
| 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 |
tarjeta de circuito impreso Junta Estándares de recubrimiento
In conformal coating, Hay una serie de estándares de recubrimiento de PCB que requieren su uso bajo ciertas condiciones, como en el ámbito militar., automóvil, uso doméstico, etc.. Por lo general, los recubrimientos de conformación califican para la especificación MIL-I-46058C o IPC-CC-830B, que se relaciona estrechamente con MIL-I-46058C..
MIL-I-46058C: Un estándar de revestimiento conformado común en la industria, también conocido como compuesto aislante militar. Requiere pruebas de cualquier laboratorio autorizado por MIL y todavía se usa incluso después de la desactivación ya que 1998 para nuevos diseños. Esta prueba requiere una lista de productos calificados estándar (QPL).
Def Stan 59/47: Un estándar similar al 46058C utilizado para recubrir dispositivos de alta gama para uso militar, pero el Ministerio de Defensa del Reino Unido debe aprobarlos primero.
IEC 61086: Un estándar basado en la autocertificación del proveedor con requisitos similares a 46058c. La Comisión Electrotécnica Internacional lo gobierna.
IPC-CC-830B: Estándar utilizado y actualizado de forma activa similar al 46058C, introducido cuando 46058C permanece inactivo. Material estandarizado para 46058C que sigue estas especificaciones. No hay pruebas disponibles ya que no se mantiene QPL.
UL94V0: Se relaciona con la propiedad de revestimiento conforme de autoextinguible en un sustrato FR4. V0 es la categoría más alta alcanzable con V1 y V2 como sus sucesores..
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, Póngase en contacto con nosotros.
