Rehm Thermal Systems, Blaubeuren-Seißen (Germany)
By the protective coating process (conformal coating), printed circuit boards (PCBs) are protected from dust and environmental effects by applying a targeted protective coating, this measure additionally helps ensuring their reliability. Theoretically, any PCB assembly can be coated, although, depending on the system used, the size of the board and how it is populated may need to be reviewed beforehand. Because of the very technologically complex challenges and the extreme climatic changes involved, protective coating of electronics is essential in air and space applications, ship building, automotive engineering and military technology. But this coating is also very useful to protect the function of products that come into contact with dirt or other environmental conditions. Therefore, coating processes are used in all areas of the industry where electronics technology is being applied: such as security, telecommunications, industry, traffic and railway. medical, food and many other fields.
To implement a conformal coating line, as well as the coating system itself, handling equipment and a drying or annealing oven are also needed. It is very important to have a detailed, tightly controlled temperature oven profile to prevent blistering or other flaws in the surface of the coating layer.
The first step is choosing the appropriate coating material. Several relevant factors must be considered, including the protection required, whether the selected coating can be processed in the line considering its application, drying and cycle time and its cost effectiveness. Popular coatings are epoxy, urethane, silicone and acrylic based. Choosing the right coating material requires a wealth of experience and technical knowledge, and is an almost impossible decision for anyone except a skilled expert. After choosing the right coating material to optimally fit the mechanical, chemical and electronics requirements of the board, the core conformal coating line process, the task itself, can begin then. There are different methods for applying the coating on to the board, such as squirting, spraying, dipping, flooding, dispensing and vapour depositing. As most assembled circuit boards contain components that cannot and must not be coated, the state-of-the-art in the industry is selective spray coating.
Various parameters must be considered, as not every coating formulation can be applied with every process. One problem is, for example, that many of the nozzles in selective spraying equipment can become clogged quickly, so highly viscose coating fluids cannot be used and changing coating materials can be problematic. Another important factor is the layout of the board, which should be designed for the coating process in advance to ensure the most effective and cost-efficient coating operation. This is because the spaces in very high or tightly packed circuit boards cannot be sprayed with every nozzle. After the protective coating has been applied, the next step is the drying or annealing process. This procedure should take place at a precisely controlled temperature profile so that the results can always be reproduced and are reliable. Incorrect temperature profiles or hardening the coating too quickly can lead to several defects, such as blistering, holes or the orange peel effect. From these it becomes clear that the application of protective coating is both crucial and complicated.
