Due to the fact that the paths on PCBs are becoming narrower and more densely packed, the components smaller, more advanced, and more sensitive, assembled PCBs require coating. The process of conformal coating involves applying a thin layer of material that conforms to the shape of the components on the board and protects them from environmental influences, corrosion, dust, or dirt. Coating allows for a reduction in the distance between the traces, pads, and components.
Applying protective coatings is done, among other reasons, because electronic circuits on the board are becoming increasingly delicate structures. Coating also reduces the risk of the growth of so-called Whiskers, which are tin whiskers. Thus, it eliminates the risk of dendrite formation, corrosion, and short circuits, which in turn extends the device's lifespan. This is extremely important from the perspective of the quality of products offered by electronics manufacturers.
In order for the aforementioned advantages to be achieved, the coatings must meet certain conditions, such as flexibility, resistance to temperature changes and vibrations. It is very important for them to have resistance to moisture or punctures, as well as good insulation properties, good chemical resistance, and permeability to the substrate. The ability of the coating to wet the surface of the board and the components placed on it is also very important. However, this wetting ability cannot be too low or too high. If it is too low, the coating will tend to chip off and will not have sufficient adhesion. On the other hand, if the wetting ability is too high, it can cause the coating to creep and contaminate areas where coating is undesirable.
There are several methods for applying conformal coating, unfortunately, each method has its advantages and disadvantages. The following methods are distinguished:
Dip coating is a simple and efficient coating process. This method requires masking. Although the coating has protective properties, it tends to flow into crevices and create runs.
Spray coating is an easy and enjoyable process of applying conformal coatings to PCBs. Unfortunately, in addition to the disadvantages of dip coating, it is characterized by high emission of volatile compounds (aerosol, overspray). Only a small portion of the coating reaches the board, significantly reducing the efficiency of this process. As a result, even with good ventilation, a large portion of the coating ends up in places other than the board surface.
The most cost-effective and efficient method is microspray coating. It eliminates many of the disadvantages of spray coating, but unfortunately, in most cases, it requires masking. The advantage of microspray is that almost all of the coating can be applied to the surface of the printed circuit board. The microspray with a needle is precise enough to coat certain individual details without masking. It is most commonly used for precise application of fluxes.
The dispensing method has many disadvantages, including imprecise application of low-viscosity liquids. In cases where we want to apply a small amount of liquid, the dispensing tip must approach the surface onto which the liquid is to be applied. This is very difficult when the boards are densely packed. The dispensing tip can easily damage the board or itself can be damaged or bent. This type of tool is not very efficient, and applying protective coatings with it results in build-ups at the beginning and end caused by opening and closing the valve. However, this can be almost completely eliminated through proper software, albeit at the expense of efficiency.
A non-contact method that provides a safe distance between the components and the valves is the curtain coating method. It is a method of selectively applying conformal coating with high efficiency, speed, and precision. Moreover, it does not require masking. This method can be used to cover narrow areas. Precise edge cutting of the coated area is characteristic of this method. The drawback of curtain coating is that its width depends on the viscosity and pressure of the material. When the viscosity is constant, the pressure is also constant. The operator of the tool must react to any changes in viscosity by adjusting the pressure. To eliminate this inconvenience, a closed-loop feedback system has been introduced. Viscosity control can also be achieved through thermal stabilization of the coating, which can be applied separately or in combination with the closed-loop feedback system.
A non-contact method commonly used for applying solventless coatings is the spiral coating method. Solventless coatings have higher viscosity and cannot be applied using the curtain coating method. The spiral method allows for clean cutting of coating boundaries without splashing or shading. Similar to curtain coating, this method requires control of the coating pressure.
Jet coating is a process similar to needle coating. The selective coating valve has a needle, but it does not touch the surface when applied from a distance of a few millimeters. On the other hand, the coating material is not squeezed out like with needle coating, but ejected using a rapid electromagnetic mechanism. As a result, there are no build-ups caused by valve opening and closing in this method. Coatings can be applied at high speed, in narrow gaps, and in limited areas.
In the process of applying protective coatings, both the method and the type of lacquer are important. Lacquers can be divided into groups based on their chemical base. The following types of lacquers are distinguished:
The most popular types of lacquers on the market are acrylic lacquers. These types of lacquers are commonly used for electronics enclosed in casings. Due to their low viscosity, these lacquers can be applied using various methods. Acrylic lacquers are characterized by their popularity and moisture resistance. The curing process involves solvent evaporation, making it relatively fast and can be accelerated by baking the boards in an oven. However, acrylic lacquers are not chemical-resistant and are not used in conditions where the board may come into contact with chemicals or their vapors. However, chemically damaged boards can be repaired, and in such cases, the lacquer can be easily removed using a solvent.
Both acrylic lacquers and polyurethane lacquers are one-component products. They exhibit excellent chemical resistance. Only mechanical means or high temperatures can remove cured lacquer, which carries the risk of damaging the board or its components. Compared to acrylic lacquers, polyurethane lacquers have a much longer drying time, but the curing is achieved relatively quickly. Full chemical resistance is achieved after approximately 30 days in some cases.
Silicone lacquers are heat-resistant. They are typically solvent-free, which results in their high viscosity. Additionally, silicone lacquers require the use of different dispensing systems. Separate dispensing systems or equipment are necessary for these types of lacquers. They have good insulation properties. However, their chemical resistance is not as good. Due to the difficulty in removing silicone particles, they can interfere with the curing of other lacquers.
Epoxy lacquers are known for their abrasion resistance. They can withstand temperatures up to 150ᵒC. They provide excellent protection to the board and its components against corrosion, especially from the condensation of water vapor. This type of lacquer can be removed using high temperatures. The curing time for epoxy lacquers is relatively long.
UV lacquers have the advantage of rapid curing, making them ideal for large-scale production. This is not only due to their fast curing time but also because they occupy a small surface area in the curing oven. UV lacquers offer good moisture resistance, excellent electrical insulation, chemical resistance, and, importantly, resistance to temperatures above 150ᵒC.
Appliances, such as those used in kitchens or bathrooms, are exposed to high levels of humidity. Therefore, they benefit from conformal coating technology. It is worth mentioning that apart from industrial and domestic applications, conformal coating is also used in automotive and military industries.