Enhancing adhesion between mold compound and substrate in BGA packaging

An UV induced graft copolymerization method has been developed to modify the solder resist surface on BGA substrates to enhance its adhesion to epoxy mold compounds. Experimental results showed that with graft copolymerization the adhesion strengths of the solder resist to the mold compounds were increased by at least four times over untreated samples. These adhesion strengths were also found to be superior after various temperature-humidity pre-conditions. The adhesion enhancement is attributed to the formation of covalent bonds between the epoxide groups of the grafted polymer at the solder resist surface and the curing agents in the mold compound. The formation of graft polymer with epoxide functional groups was identified and observed by X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). The graft copolymerization was confirmed to be compatible with BGA assembly processes. The surface activation of treated solder resist surface can be maintained for a period in excess of 4 months.