Conductive filament formation: a potential reliability issue in laminated printed circuit cards with hollow fibers

E-glass fibers are used as a reinforcement material in the manufacture of laminates used in printed circuit cards and multichip module laminated substrates (MCM-L's). The principal advantages of using E-glass fibers include high strength, high chemical resistance, and excellent insulating properties at reasonable cost. Although most fibers are solid, hollow fibers can be produced if there is insufficient process control during the manufacture of E-glass fibers. This creates a potential reliability problem in laminates since hollow fibers provide a path for conductive filament formation (CFF) between two differently biased points, which can result in short circuit failure modes. The probability of CFF is a function of temperature, moisture content, the voltage bias and other environmental conditions and physical factors. This paper presents relevant information on E-glass laminate manufacture and the causes of hollow fibers, details of experiments performed to observe hollow fibers, the reliability issues in terms of the CFF mechanism, and analysis as to the opportunity for failure and recommendations for improvement.