Protective coating for printed circuit boards
Protective coating is used to enhance the performance and reliability of printed circuit board assemblies, enabling them to be used in harsh environments such as underwater, aerospace and military applications. Manufacturers of consumer electronics are increasingly using protective coatings as an economical way to improve product reliability.
When a printed circuit assembly board without protective coating is exposed to humid air, a thick film of water molecules will form on its surface, reducing the surface insulation resistance (SIR) of the circuit board. The lower the surface insulation resistance, the more severe the deterioration of the transmission performance of the electrical signal. The typical consequences are crosstalk, electrical leakage and transmission interruptions, which may lead to permanent signal interruption, i.e. short circuit.
The moisture film on the printed circuit board without protective coating also provides favorable conditions for metal growth and corrosion, which in turn affects the insulation strength and high-frequency signals. Dust, dirt and other environmental pollutants that fall on the assembly board continue to absorb moisture, thereby expanding its negative effects. Conductive particles such as metal fragments can also cause electrical bridging.
Conformal coating is a plastic film wrapped around the printed circuit assembly board. The thickness of this film is 0.005in. It seals dust and environmental contaminants outside the circuit board. Of course, the protective coating may also seal in contaminants that were not removed during the pre-cleaning process, so it is very important to clean the circuit board surface before applying the protective coating.
The functions of the protective coating are as follows:
1) Protect the circuit from extreme environments and avoid it from moisture, fungus, dust and rust;
2) Prevent the circuit board from damage during the manufacturing, assembly and use process, reduce mechanical stress on components and protect them from thermal shock;
3) Reduce wear during use;
4) Because the protective coating increases the insulation strength between conductors, it enhances the performance of the circuit board and allows for greater component density.
1 Materials for protective coatings
Many materials can be used to make protective coatings, each of which has its own properties and application areas. The main properties of different types of protective coatings are summarized as follows:
1) Acrylic protective coating: It is easy to apply, can be cured in a few minutes at room temperature, has satisfactory electrical and physical properties, is antibacterial and does not shrink. This type of coating has a long shelf life, does not shrink, and does not release heat or releases very little heat during the curing process, which avoids damage to heat-sensitive components. Its main disadvantage is that it is sensitive to solvents, but this feature makes maintenance easier.
2) Epoxy protective coating: It is usually used as a two-component system compound with moisture resistance, wear resistance and chemical resistance. Epoxy protective coatings are difficult to remove by chemical methods during processing. This is because any removal equipment will decompose the epoxy resin coated components on the circuit board while removing the coating, and may even damage the epoxy fiberglass printed circuit board itself. Therefore, component replacement requires a knife or soldering iron to penetrate the epoxy protective coating.
3) Polyurethane coating: It can be used in both single-component and two-component systems, has excellent moisture and chemical resistance, and has excellent insulation properties for a long time. Polyurethane coatings are easily burned through by soldering irons, so component replacement is relatively easy. Cleaning is an essential and important step before applying any protective coating, especially polyurethane, which is more sensitive to moisture. It will bubble in wet conditions and eventually cause circuit failure. In terms of form, polyurethane should be applied with more caution, and the coating and curing environment should be carefully controlled.
4) Silicone protective coating: This coating is particularly suitable for high temperature environments around 200’t. It can provide excellent moisture and wear resistance and good heat resistance, which makes silicone protective coating more suitable for printed circuit boards containing high heat dissipation components. Typical electrical and thermal properties of various protective coatings are shown in the following table (Warγold and Lawrence, 1991). 2 Methods of using protective coatings There are four basic methods for applying protective coatings, which are discussed below. (Waryold et al., 1998)
- Dipping: In this method, the masked assembly is dipped into a reservoir of liquid coating material and then removed. The process ensures uniform coverage and smoothness of the coating. The rate of dipping and removal is an important parameter that needs to be controlled so that the viscous liquid material completely fills every hole in the assembly board. Typical dipping rates are 2-12in/min to ensure that the protective coating completely excludes air around the components.
When applying protective coating by dipping, the solvent in the liquid evaporates quickly, which causes the viscosity of the bath to increase rapidly. For this reason, it is very important to continuously monitor the bath to maintain the appropriate viscosity.
- Spraying: Spraying is the most common and efficient method for applying protective coatings. With the appropriate diluent, moderate nozzle pressure and pattern, a stable and strong coating can be obtained. Spraying can be done manually or automatically by integrating a computer control system into an existing wave soldering and cleaning line. The main disadvantage of this method is that the coating is very thin or cannot be applied to the parts under the components or covered by the components.
When spraying the coating on the assembly board, clean and dry air should be used to spray at the minimum pressure required to obtain the best atomization effect. When spraying, the spray gun should be kept at a 45-degree angle to the assembly board and reciprocated back and forth, and the assembly board should be rotated 90° after each time.
- Brushing: It uses manual operation. This is the least efficient method for applying protective coating. It is difficult to obtain a uniform and consistent coating. This method is only suitable for coating a small number of printed circuit boards.
Each coating method has its advantages and disadvantages. The combination of dipping and spraying is better than using a single method.