Modification of substrate surface for BGA overmold adhesion enhancement by graft copolymerization

To improve the adhesion between epoxy molding compound (EMC) and ball grid array (BGA) substrate in the packaging of microelectronics systems, the surface of the epoxy-based BGA substrate was modified first by ozone pretreatment, followed by near-UV-light induced graft copolymerization with glycidyl methacrylate (GMA). The surface structure and composition of the substrate, before and after modification, were characterized by angle-resolved X-ray photoelectron spectroscopy (XPS). The angular-dependent XPS results suggest that the grafted GMA polymer has penetrated substantially below the substrate surface. The amount of grafted GMA polymer increased with increasing temperature and time of near-UV-light illumination during graft copolymerization. The surface of the BGA substrate became completely covered with GMA polymer after 2 h of near-UV-light induced surface graft copolymerization. The changes in surface morphology after graft copolymerization were clearly revealed by the atomic force microscopic (AFM) images. Test coupons with surface modified BGA substrate exhibited a minimum of three-fold increase in adhesion strength to the epoxy overmold. The adhesion enhancement has been attributed to the formation of covalent bonds between the epoxide groups of the grafted GMA polymer and the curing agent in the overmold.