In the production and manufacturing of printed circuit boards (PCBs), the PCBs of some products need to be coated with conformal coating according to the requirements of different products.
In the production and manufacturing of printed circuit boards (PCBs), the PCBs of some products need to be coated with conformal coating according to the requirements of different products.
Actually, conformal coating is a type of protective coating. It is a special polymer film-forming product that can protect PCBs, components and other electronic equipment from adverse environmental conditions. These coatings are not limited by PCB structures or other environmental factors, and they provide higher dielectric resistance, thereby protecting PCBs from corrosive environments, humidity, and contaminants such as dirt and dust.
According to the needs of PCBs, conformal coatings can be made of acrylic resin, silicone resin, polyurethane resin, and other more application-specific compounds (such as epoxy resin). The coatings can be applied by brushing, spraying, or dipping the PCBs into the coating material.
Among them, acrylic resin provides good elasticity and meets general protection requirements; acrylic conformal coatings are renowned for their high dielectric strength, good moisture resistance and wear resistance.
Silicone conformal coatings offer excellent protection over an extremely wide temperature range, with good chemical resistance, moisture resistance and salt spray resistance, and they are highly flexible. Silicone conformal coatings are not wear-resistant due to their rubbery nature, but this characteristic does enable them to resist vibrational stress. Silicone coatings are usually used in high-humidity environments and can be applied on LED lamps without discoloration or loss of strength.
Polyurethane resin is famous for its outstanding moisture resistance and chemical resistance, and it is also highly wear-resistant. Combining these factors with its solvent resistance, it forms a conformal coating that is very difficult to remove, which is generally used in aerospace applications.
It should be noted that we should not confuse conformal coating with PCB surface finishing. Conformal coating is different from PCB surface finishing because the coating is applied after the PCB assembly, that is, after the SMT production process; while PCB surface finishing is a part of the PCB manufacturing process. Both processes can protect the PCB, but the methods are different.
PCB surface finishing is a solderable coating, mainly used to protect the copper foil from corrosion before PCB assembly; while conformal coating is mainly used to protect the PCBA during the operation of the PCB. The standard surface finishing process is Hot Air Solder Leveling (HASL), which involves dipping the PCB into molten solder and then leveling it with hot air. Of course, there are also immersion surface finishing processes for tin, silver and gold, etc.
Note that we should also not confuse PCB and PCBA. A PCB is just a bare board with no components on it; while a PCBA is an assembled finished product with the required electronic components inserted and mounted on the PCB.
At present, there are three main methods for applying conformal coating:
The first is manual spraying. Conformal coating can be sprayed using an aerosol can or a hand-held spray gun, which is generally suitable for situations without fixed equipment and is usually used for small-batch production. For example, in the prototype production line, which is used to produce prototypes in the design and R&D stage, this method is generally adopted.
Of course, this method can be time-consuming because it is necessary to avoid coating areas that do not require the coating. Therefore, the final result largely depends on the operator, and thus the coating differences between boards may be relatively large.
The second is automatic spraying. This is a programmed spraying system that moves the PCBs on a conveyor belt, and a spray head will perform the coating operation when the PCBs move to a specific position.
The third is
selective coating, which is also an automated conformal coating process. It can apply the conformal coating to specific areas on the PCB, which is suitable for mass production and is somewhat similar to selective wave soldering.
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