The Selection of Coatings for Printed Wiring

It is pointed out that printed wiring has given rise to a serious surface leakage problem, in contrast to standard wiring where volume leakage is predominant. Suitable insulating coatings are credited with protection against contamination and moisture entry, as well as with control of corona at high altitudes. A coating material suitable for use on printed wiring boards is described as needing to possess high insulation resistance, superior humidity resistance, good adherence to both metals and plastics, chemical and physical stability under variable environments, and good mechanical properties. It is stated that practical consideration requires that the material be transparent so that component-part coding can be seen, that the cure temperature be tolerable to the boards and component parts, that application may easily and uniformly be made, and that factory and field repair be possible. Test procedures used in evaluating coatings for these properties are outlined and discussed. Examples of coated boards are shown. A total of 48 coating materials, consisting of solvent proprietary blends and laboratory formulations, are evaluated. From the test results, a sprayable epoxy-polyamide formulation is found acceptable for use on printed wiring. The importance of the volume-resistance of a coating is indicated.