Environmental aging effects on the durability of electrically conductive adhesive joints

The mechanical behavior of electrically conductive adhesive (ECA) joints exposed to elevated temperature and relative humidity conditions has been investigated, and failure mechanisms of conductive adhesive joints have been determined. Three silver-filled, epoxy-based model adhesive systems have been studied in conjunction with printed circuit board (PCB) substrates with metallizations of Au/Ni/Cu and Cu. Double cantilever beam (DCB) tests have been adopted to investigate the effects of environmental aging on ECA joints. This study reveals that conductive adhesives as well as substrate metallizations both play important roles in the durability of conductive adhesive joints. The rate of water attack on the interface of conductive adhesive joints with Cu-plated PCB substrates is faster than for those with Au/Ni/Cu metallization. A possible explanation of this phenomenon is based on considerations of surface free energy and interfacial free energy. Following drying of the aged conductive adhesive joints, the fracture energy recovered to some extent. This recovery in the fracture energy could be attributed to the reversible effect of plasticization of the bulk adhesives, as well as the regaining of bond strength between the adhesive and the substrate during drying at 150°C. XPS analysis of DCB failure surfaces suggested that diffusion of Cu to the Au surface might have occurred on the Au/Ni/Cu-plated PCB substrates during aging. Copper oxide was detected on the substrate surface upon exposure of the conductive adhesive joints to the hot/wet environment.