The sealing requirements for Printed Circuit Boards (PCBs) primarily revolve around protection against moisture, oxidation, contamination, and physical damage. The objective is to ensure that the boards maintain their electrical performance and structural integrity throughout the stages of storage, transportation, and operation.
For all PCBs, it is highly desirable-and indeed critical as circuit densities continue to increase-that the circuit patterns be completely encapsulated by a solder mask. Industry standards mandate that the solder mask thickness over any point of the conductive circuitry on the PCB must meet a minimum requirement of 25 μm. Furthermore, the degree of encapsulation achieved on the board directly determines its electrical insulation rating as well as its resistance to environmental degradation.
In the case of screen-printed inks, the quality of encapsulation is directly influenced by factors such as the choice of screen mesh, ink viscosity, and the printing temperature and speed; moreover, variations in the height of the conductive traces will result in corresponding variations in the degree of encapsulation. While liquid solder masks may exhibit pronounced, directional ridges in their coating profile, pre-formed dry films offer a distinct advantage: provided that the height of the conductive traces on the board does not exceed the thickness of the dry film itself, the resulting encapsulation will be uniform and consistent across the entire surface of the board. Consequently, the use of dry films enables the achievement of a minimum encapsulation thickness of 25 μm across the entire surface of the printed circuit board.
