Conductive filament formation (CFF), also referred to as metallic electromigration, causes electrical shorts between two conductors in MCM-L substrates or printed wiring boards.  The failure is caused upon an electric field by metallic ion migration from one conductor to other along glass fiber/epoxy resin interfaces, accelerated by elevated temperature and high humidity.

This paper focuses on the CFF failure mechanism, emphasizing interfacial degradation between glass fibers and the epoxy resin matrix, and the relationship between interfacial degradation and conductive filament formation of three substrate materials: flame retardant (FR-4), bismaleimide triazene (BT) and cyanate ester (CE).  The environmental scanning electron microscope (ESEM) and energy dispersive spectroscope (EDS) were used to study interfacial degradation effects and the conductive filament formation mechanism.  Deformation of the epoxy matrix at epoxy/fiber interfaces and interfacial debonding were observed in the substrates.  The threshold temperature and humidity conditions, and threshold temperature and humidity cycling numbers, that cause interfacial deformation and debonding between glass fibers and the epoxy matrix were determined, and an analytical model was proposed to assess the mean time to failure due to CFF based on the experimental observations.