Evaluation of creep properties for aged Pb-free solder joints/(Ni-P/Au) UBM with small addition Cu using shear punch creep testing method

The creep mechanical and physical properties of solder joints play an important role to ensure the service life of electronics products. In this paper, we investigated the creep properties of Pb-free Sn-4Ag and Sn-4Ag-0.5Cu (small additions Cu) solder joints on Ni-P/Au under bump metallization (UBM) using the shear punch creep test (SPCT) method at 30 °C. These results have been compared with the Pb-based Sn-37Pb solder joint after isothermal aging (150 °C) according to different times. In order to analyze the result of SPCT, the specific methods (i.e., Power-law relationship, Larson-Miller law and Monkman-Grant correlation) were used. Through the Power-law relationships, the stress exponents (n) of the Pb-free solder were higher than the Pb-based solder and generally, (n) value decreased with increasing aging time. Also, according to Larson-Miller law, the Pb-free solder joints revealed superior mechanical properties. The Monkman-Grant exponent (m) value of three solder joints which analyze by Monkman-Grant correlation is proximately 1. Furthermore, the failure fracture surface behavior of solder was investigated using scanning electron microscopy (SEM). Elongated dimple-like structures were detected on the fracture surfaces of all as-soldered (aging time 0 h) samples, but these gradually decreased with increasing aging time. The results indicated that the creep properties of Pb-free solder were better than Pb-based solders, and the Sn-4Ag-0.5Cu solder joint showed the best creep mechanical properties.