Conformal coating-methods-Selective coating
Selective coating by machine
This method is the best choice for high volume applications as it is a fast and accurate way of applying the desired thickness coating, to precisely the areas of the board where it is required.
It works by using a needle and atomised spray applicator, non-atomised spray or ultrasonic valve technologies that can move above the circuit board and dispense / spray the coating material in selective areas. Flow rates and material viscosity are programmed into the computer system controlling the applicator so that the desired coating thickness is maintained. This method is highly effective at large volumes as long as the PCBs are designed for the method. However, there are limitations in the select coat process like all the other processes, such as potential capillary effects around low profile connectors which "suck" up the coating accidentally, it also requires a skilled operator trained to use the machinery.
The process quality of dip or dam-and-fill coating and non-atomised spray technology can be improved when necessary by applying and then releasing a vacuum while the assembly is submerged in the liquid resin. This forces the liquid resin into all crevices, eliminating uncoated surfaces in interior cavities.
The differences in application methods can be seen in a comparison presentation. Choice of method is dependent on the complexity of the substrate to be conformally coated, the required coating performance, and the throughput requirements.