Analysis of catastrophic field failures due to conductive anodic filament (CAF) formation

Under certain environmental conditions, printed wiring boards (PWB) respond to applied voltages by developing subsurface deposits of copper salts extending from anode to cathode along separated fiber/epoxy interfaces. The formation of these deposits, termed conductive anodic filaments (CAF) require high humidity (80%RH) and high voltage gradient (5 V/mil). The humidity exposure during the storage environment may cause the failure in the use environment. CAF formation is enhanced by the use of certain hot air solder leveling (HASL) fluids and/or water soluble flux constituents. In this work, two catastrophic field failures were analyzed. Both failures were related to boards produced in a manufacturing process, which included HASL. One CAF failure occurred between a component through-hole and power plane held at a potential difference of 40 V with a 0.005 inch nominal spacing. The other occurred on an inner layer of a multi-layer board (MLB) between a via and ground plane held at a potential difference of 320 V with 0.015 inch nominal spacing. The nature of the CAF was analyzed using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS). Ion chromatography (IC) was used to identify residue extracted from a failed board. The failure phenomena known as CAF poses serious long-term reliability concerns in electronic products exposed to adverse and hostile environments, especially those with closely spaced conductors.