Microstructure of Conductive Anodic Filaments Formed during Accelerated Testing of Printed Wiring Boards

Under certain environmental conditions, printed wiring boards (PWBs) respond to applied voltages by developing sub-surface deposits of copper salts extending from anode to cathode along separated fibre/epoxy interfaces. These deposits are termed conductive anodic filaments (CAFs) and, in this work, the dimensions and growth patterns of a CAF have been determined by serial sectioning. The CAF growth pathway is characterised and the spatial distribution of the copper salts is quantified with scanning electron microscopy (SEM) using backscattered electrons. The chemical composition of the CAF is determined using energy dispersive X-ray analysis (EDS). Prior research using high-resolution non-destructive X-ray microtomography is correlated with the serial sectioning data. The failure phenomenon known as CAF may pose serious long-term reliability concerns in electronics applications exposed to adverse and hostile environments.