Effect of third element addition on joint strength of low-Ag lead-free solder

Effects of additions of small amount of Bi, Ni and Ge in Sn-1.0Ag-0.7Cu (mass%) low-Ag lead-free solder on the strength and microstructure of the solder ball joint were investigated for Cu and electroless Ni-P/immersion Au (ENIG) electrodes. In the case of the Cu electrode, Cu6Sn5 and Cu3Sn layers form at the joint interfaces in the joints with Sn-1.0Ag-0.7Cu and Sn-1.0Ag-0.7Cu-2.0Bi (mass%). (Cu, Ni)6Sn5 and (Cu, Ni)3Sn layers form at the joint interfaces in the joints with Sn-1.0Ag-0.7Cu-0.07Ni-0.01Ge (masss%) and Sn-1.0Ag-0.7Cu-2.0Bi-0.07Ni-0.01Ge (mass%). In the ball shear test at a low shear speed of 0.001 m/s, fracture mainly occurs in solder and the addition of Bi is effective to improve ball shear force. Similar tendency was observed in the joints with ENIG electrodes. In the ball shear test at a high shear speed of 1 m/s, the addition of Ni is effective to improve ball shear force. After aging at 120°C for 3 weeks, ball shear force in the joint with Sn-1.0Ag-0.7Cu-2.0Bi-0.07Ni-0.01Ge was excellent. In the case of the ENIG electrode, Ni-Sn-Cu phases form at the joint interfaces in all solder investigated. In the ball shear test at the high shear speed, the effect of added elements on ball shear force is negligible. Although IMC fracture mainly occurs in the joint with solder including Ni, the effect of IMC fracture on ball shear force is negligible. Furthermore, it was found that absorption energy in the ball shear test decreases when fracture occurs in the IMC layer regardless of the electrode type.