Microstructure evolution and shear behavior of Pb–16Sn–7.5Sb–xAg/Cu joints

The aerospace industry requires a reliable solder with a melting point of about 240 °C that does not produce tin whiskers. The Pb–16Sn–7.5Sb-based solders, with a melting point of about 240 °C, would be the candidate solders. This paper studied the microstructure and shear strength of the Pb–16Sn–7.5Sb–xAg/Cu solder joints. The Cu₃Sn formed at the interface of Pb–16Sn–7.5Sb–xAg/Cu after soldering at 260 °C. The thickness of Cu₃Sn IMC is proportional to the square root of time until the SbSn phase in the solder is exhausted. Adequate Ag addition in Pb–16Sn–7.5Sb solder increases the activation energy of Cu₃Sn, inhibiting the rapid growth of Cu₃Sn. Therefore, after soldering for 5 min, the thickness of Cu₃Sn layer in Pb–16Sn–7.5Sb/Cu solder joint is 2.2 μm, which is reduced to 1.2 μm in Pb–16Sn–7.5Sb–1Ag/Cu solder joint. The shear strength of the solder joint increases with the thickness of the Cu₃Sn layer when the thickness of the Cu₃Sn layer is below 1.2 μm, then decreases when the Cu₃Sn layer is too thick. The solder joint, with a thin and flat Cu₃Sn IMC layer, shows high shear strength. After soldering for 5 min, the shear strength of the Pb–16Sn–7.5Sb–1Ag/Cu solder joint achieves 32 MPa, about 60% higher than the Pb–16Sn–7.5Sb/Cu joint.