Conformal coatings are regularly employed to protect the surface of a soldered printed circuit board assembly (PCBA) from moisture, chemicals in the PCBA’s service environment, and foreign objects or debris. Conformal coatings are nonconductive and therefore cannot be placed on any location where electrical contact will be required, such as connector pins, test points, and sockets. Conformal coatings are also not permitted on any mechanical interface location, such as mounting holes or brackets, to assure the proper fit between items in the final assembly. In order to apply conformal coatings to an assembly and comply with the restrictions on keep-out areas, masking is employed to protect those surfaces.
There are two basic strategies for masking of conformal coatings. The first strategy is to use a temporarily installed material that prevents conformal coatings from bonding to locations that forbid coating application. Polyimide tape can be used as a temporary masking agent. The tape must be cut to the proper shape to ensure the tape only covers the area where conformal coatings are not desired. Polyimide tape has a few basic advantages as a masking material. The first advantage is that polyimide tapes are able to withstand a typical oven cure cycle for conformal coatings without damage. Another major advantage is that polyimide tapes have a resistance to tearing that exceeds many other types of adhesive tapes, which allows easy removal after the coating has been cured. Tapes can be cut and formed to cover very precise areas on an assembly. A final advantage is that the assembly is ready for coating immediately after tape is applied.
The second basic strategy for masking an assembly is to implement purpose-built masking fixtures. Masking is a common requirement for processes that apply conformal coatings to PCBAs.
