When storing or operating in a harsh environment, electronic devices can be highly impacted by dust, humidité, mold, and other pollutants, causing poor performance and a shorter lifespan. Dans ce cas, PCB coating is a very effective way to protect them from those adverse factors, thereby improving the reliability of electronic products. Dans ce guide ultime, we’ll walk through everything you need to know about PCB coatings. Avec la bonne stratégie de revêtement, vous pouvez prolonger considérablement la durée de vie des PCB dans vos produits. Commençons!
What Are PCB Coating and Conformal Coating?
Revêtement PCB, also known as revêtement enrobant, 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 Composants electroniques and circuit boards from the working environment to avoid corroding and extend their service life.
5 Different Types of PCB Coating
Au niveau des matériaux utilisés, Les revêtements de PCB peuvent être divisés en cinq 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. Également, there is no shrinkage after curing. pourtant, 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. pourtant, 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 | Acrylique | Polyuréthane | Époxy | Silicone | Parylène |
| 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 | Oui | Oui | Oui | Oui | Non |
| Repairability | Excellent | Bien | Pauvre | Modéré | Bien |
Techniques to Apply a Circuit Board Coating
Il existe différentes méthodes pour appliquer le revêtement des circuits imprimés, ils sont:
Manuel Spraying – Cette méthode convient aux cycles de production à faible volume car il s'agit d'un processus qui prend du temps. Normalement, nous utilisons une bombe aérosol ou un pistolet pulvérisateur à main pour appliquer le revêtement, et avant de pulvériser, les zones qui ne nécessitent pas de revêtement doivent être couvertes. The coating effects would be a little different between different batches due to the manual operation.
Sélectif Coating – Il fait référence à un processus de revêtement automatique qui applique un revêtement sur des zones spécifiques des cartes de circuits imprimés à l'aide de buses de pulvérisation robotisées programmées., et il n'est pas nécessaire de couvrir les zones qui ne doivent pas être pulvérisées. Ce processus est caractérisé par une efficacité et une précision élevées, adapté à la production à grand volume.
Plongement – Pour cette méthode, Les PCB seraient d'abord immergés dans la solution de revêtement, puis retirés. De nombreux facteurs affecteraient l'effet de revêtement, tels que la vitesse d'immersion et de retrait, temps de trempage, etc. Un masquage important est requis avant le processus de revêtement, il convient donc aux PCB qui nécessitent un revêtement des deux côtés.
Le brossage – Un pinceau est utilisé pour appliquer un revêtement sur des zones spécifiques, et c'est une méthode principalement utilisée pour réparer et retravailler. Le processus prend beaucoup de temps et nécessite beaucoup de travail, l'effet de revêtement final dépend de la compétence de l'opérateur.
How to Measure the Thickness of PCB Coating?
PCB coating is usually very thin and doesn’t add extra weight to the circuit board. Donc, 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” et le “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.
Micromètre
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. ensuite, 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. pourtant, 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 non, 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. Maintenant, 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, épaisseur du revêtement, and curing method. We’ll then go over the four main curing techniques.
Evaporative Curing Mechanism
Pour le dire simplement, 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. pourtant, 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. Premier, 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. pourtant, 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?
Lorsque le circuit imprimé a besoin d'être réparé ou remplacé par un composant, nous devons retirer le revêtement conforme du circuit imprimé. Ci-dessous, nous énumérons les méthodes couramment utilisées pour éliminer les vernis de protection.:
Élimination des solvants – Use specific solvents to dissolve the coating, mais assurez-vous que le solvant que vous choisissez est approprié et n'endommagera pas les composants électroniques. Généralement, Les revêtements acryliques sont les plus faciles à dissoudre, tandis que les revêtements de silicone et d'uréthane sont plus difficiles à enlever.
Peeling – For some elastic coatings such as silicone conformal coating, nous pouvons les retirer en les décollant du circuit imprimé avec un couteau. Mais ce procédé nécessite un soin particulier et un contrôle précis de la part de l'opérateur., sinon cela pourrait endommager les composants.
Thermal/Burn-through – During the repair process, vous pouvez utiliser un fer à souder pour simplement brûler le revêtement, mais vous devez l'utiliser avec précaution. Cette méthode convient à presque tous les types de vernis de protection.
Microbillage –The process involves using a micro-sandblaster, qui utilise un mélange concentré d'abrasif doux et d'air comprimé, pour enlever efficacement le revêtement. Cette méthode convient pour éliminer les revêtements parylène et époxy.
Meulage/grattage – You need to use a drill to grind off the unnecessary coating, qui convient à certains revêtements plus durs tels que la résine époxy et le polyuréthane. pourtant, si l'opérateur ne fait pas attention, il y a une possibilité d'endommager le circuit imprimé, donc cette méthode n'est pas une priorité.
Common PCB Coating Defects and Solutions
| Les défauts | 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 Conseil Normes de revêtement
In conformal coating, il existe une série de normes de revêtement de PCB qui exigent son utilisation dans certaines conditions, comme dans le milieu militaire, voiture, usage domestique, etc. Le plus souvent, les revêtements conformes se qualifient pour la spécification MIL-I-46058C ou IPC-CC-830B qui est étroitement liée à MIL-I-46058C.
MIL-I-46058C: Une norme de revêtement conforme commune dans l'industrie, également connu sous le nom de composé isolant militaire. Il nécessite des tests de tous les laboratoires agréés MIL et est toujours utilisé même après la désactivation depuis 1998 pour de nouveaux designs. Ce test nécessite une liste de produits qualifiés standard (QPL).
Def Stan 59/47: Une norme similaire à 46058C utilisée pour le revêtement d'appareils haut de gamme à usage militaire, mais le ministère britannique de la Défense doit d'abord les approuver.
CEI 61086: Une norme basée sur l'autocertification par le fournisseur avec des exigences similaires à 46058c. La Commission électrotechnique internationale la régit.
IPC-CC-830B: Norme activement utilisée et constamment mise à jour similaire à 46058C, introduit lorsque 46058C reste inactif. Matériau normalisé pour 46058C qui suit ces spécifications. Aucun test n'est disponible car aucune QPL n'est maintenue.
UL94V0: Se rapporte à la propriété de revêtement conforme d'auto-extinction sur un substrat FR4. V0 est la catégorie la plus élevée possible avec V1 et V2 comme successeurs.
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, Prenez contact avec nous.
