The effect of epoxy/glass interfaces on CAF failures in printed circuit boards

Reductions in printed circuit board line spacing and via diameters and the increased density of vias with higher aspect ratios (ratio between the thickness of the board and the size of the drilled hole before plating) are making electronic products increasingly more susceptible to material and manufacturing defects. One failure mechanism of particular concern is conductive anodic filament (CAF) formation, which typically occurs in two steps: degradation of the resin/glass fiber bond followed by an electrochemical reaction. Bond degradation provides a path along which electrodeposition occurs due to electrochemical reactions. The path can result from poor glass treatment, from the hydrolysis of the silane glass finish, or from mechanical stresses. Once a path is formed, an aqueous layer, which enables the electrochemical reactions to take place, can develop through the adsorption, absorption, and capillary action of moisture at the resin/fiber interface. This paper describes the concerns with CAF and the methods used for analyzing low-resistance failures. A case study is then given which highlights problems that arose on a commercial circuit board material used by a major telecommunications provider.