The Sandwich Coating Between Conductive Layers for the Prevention of Silver-Migrations on a Phenolic Board

Work on a functional two-layer coating construction which has the capability of withstanding both silver-migration inhibition and humidity has been developed for application to a phenolic board. This system also plays an additional role of isolating printed conductors from the board material. First, undercoating-resin layers, applied prior to the printed conductors, serve as the isolation layers to keep the silver conductors from touching the laminate directly. Secondly, the overcoating layers, both to conductors and to insulators, serve as barriers of moisture absorption from the surroundings. Sandwich coating between conductor layers, which includes both undercoatings and overcoatings, makes it possible to attain both stable circuit performance and high reliability; and thereby silver migration along potential differences, electric breakdown between conductors, adhesion and humidity endurances of printed resistors, scratching of the conductors caused by the handling, can all be alleviated. Moreover, by the adoption of this construction, many problems of hybrid-packaging constructions consisting of both printed copper and silver conductors accompanied by printed resistors, have been solved. The details of this new technique for hybrid packaging are presented and experimental results for varying factors such as the thickness of coating, the number of layers, and the material of resins, etc., are given.