Interfacial reactions and mechanical properties of the ball-grid-array (BGA) solder joints using monolithic eutectic SnPb and Cu-cored solder balls after reflow and solid-state annealing were investigated. The Cu cores of three different sizes were used in the solder joints. The incorporation of a Cu core into the BGA solder joint effectively inhibits the (Au1-xNix)Sn-4 regrouping and the (Cu1-x-yAuxNiy)(6)Sn-5 phase is formed at the joint interface instead. Growth of the intermetallic compounds formed in the monolithic and Cu-cored solder joints approximately obeys the parabolic law. In the Cu-cored solder joints, the larger the Cu core is, the slower the intermetallic compounds grow. The size effect of the Cu core on the intermetallic compound growth results from the inconsistent amount of the outer solder layer. Shear and tensile strengths of the Cu-cored solder joints decrease with increasing solid-state annealing time, and do not have a noticeable relationship with the Cu-core size. Shear and tensile tests also show that the mechanical strength of the Cu-cored solder joint is better than that of the monolithic solder joint