Abstract
BGA joints of Sn-3.5Ag and Sn-3.5Ag-0.75Cu lead-free solders on the substrates plated by Electroless Nickel/Immersion Gold were studied. Solder ball pull strength was measured by hot bump pull method, and it is found that the pull strength of Sn-3.5Ag-0.75Cu was higher than that of Sn-3.5Ag. P-rich layer was observed at the jointed interface by cross-section analysis using EPMA. The P-rich layer of Sn-3.5Ag-0.75Cu was thinner than that of Sn-3.5Ag. In the case of Sn-3.5Ag-0.75Cu, intermetallic compounds (Cu, Ni) 6Sn5 were formed, and they prevented the diffusion of Ni to solders, and inhibited the concentration of P at the jointed interface. As the result, P-rich layer was hardly formed, and this is considered to be a reason for high pull strength of Sn-3.5Ag-0.75Cu solders.
