Effects of solder ball size and reflow cycles on properties of Sn-3.0Ag-0.5Cu/Cu joints

Mounting several chips on one semiconductor package can achieve a high integration density. Therefore, this decreases the solder size, owing to which, several bonding processes are performed when implementing such a package. In this study, Sn-3.0Ag-0.5Cu solder balls with 200, 300, 450, and 600 mu m in diameter were mounted on ball grid array substrate. The properties of the solder joints were evaluated during multiple reflow soldering for up to 15 cycles. Smaller solder balls formed a coarser intermetallic compound (IMC), which became thicker during multiple reflows. Furthermore, in the small solder ball joint, coarser and larger Cu6Sn5 IMC grains were formed at the interface during multiple reflows. Shear tests were used to evaluate the mechanical properties of the solder joints. The shear strength increased in the first three cycles and then decreased. However, that of the smaller solder joints increased again after 15 cycles, owing to rapid coarsening/growth of the IMC (Cu6Sn5 and Ag3Sn) that contacted the shear tool during the shear test. The same trend was shown in the shear force-displacement curves: the shear force and displacement increased again at 15 cycles for 200- and 300-mu m solder joints. At 15 cycles, brittle fractures occurred at the solder joints following the shear test, and larger solder balls produced more brittle fractures. These results confirmed that the number of reflow cycles and solder ball size can affect the interfacial properties of solder joints, which in turn affect the microstructure and mechanical properties.