Conductive Anodic Filaments in Reinforced Polymeric Dielectrics: Formation and Prevention

The purpose of this paper is to report on a mode of failure observed during the life testing of epoxy-glass printed-circuit boards (PCB). Upon exposure to elevated humidity, circuits maintained under a dc bias have been observed(I-3) to develop either permanent or intermittent loss of insulation resistance. Visual examination has shown that this characteristic electrical behavior is accompanied by the formation of growths emanating from the positively biased conductors (anodes) and growing along the reinforcing glass fibers. We refer to these growths as conductive anodic filaments (CAF). In another paper presented at this meeting3, Lahti et al. have reported that this failure mode, which occurs at high humidity, is often nearly temperature independent. This failure mode is also of great concern since current trends in PCB applications, especially in the telecommunication field, involve operation in more humid environments, such as outdoor installations. Further, the trend toward lower power dissipation by components, which results in a low operating temperature, will also tend to make the operating humidity of the PCBs higher. In addition, the increased use of light-weight, water penneable housings is aggravating the situation. These trends have begun relatively recently and therefore, even though the number of field failures so far attributed to CAF is small, the situation is potentially serious. This paper addresses the following questions: 1. What material characteristics of PCB substrates make them susceptible to this mode of failure? 2. What is the mechanism of failure? 3.