Abstract
AC impedance spectra were measured between copper electrodes on FR‐4 epoxy/glass composite printed wiring boards (PWBs) at 80°C and 90% relative humidity. By using plated through‐holes as well as interdigitated comb electrodes on both PWB surfaces in appropriate combinations, bulk epoxy, surface, and epoxy/glass interfacial conduction paths were distinguished. Bulk impedance responses showed parallel ohmic and diffusion‐controlled ionic conduction mechanisms to contribute to conduction through the epoxy. Surface impedance was controlled by diffusion of electroactive species, and increased with exposure time as ionic contaminants were depleted or immobilized. Epoxy/glass interfacial impedance was primarily ohmic, and decayed by two orders of magnitude as moisture penetrated the PWB. Decay of all bulk and interfacial impedance parameters was controlled by the rate of water diffusion into the boards. Applying 50 V dc bias after environmental equilibration of the boards resulted in catastrophic insulation loss, but on a time scale much shorter than that required to develop the conduction paths.