The influence of high melting point elements on the reliability of solder during thermal shocks

The exploding use of high density and high temperature electronic devices is intensifying the desire for high temperature Pb-free solders. Alloying high melting point elements with conventional SnAgCu solders is one of the potential solutions drawing attention. The influence of high melting point elements (Ni, Sb, Bi) were studied by comparing the microstructure evolution of SnAgCuNiSbBi and SnAgCu solder joints during thermal shocks mainly by SEM and EBSD (also calculated Schmid Factor). Ni can strengthen the solder by Cu-Ni-Sn IMCs at the beginning of the thermal shocks, but its advantage becomes disadvantage after the IMCs are coarsened. Cu-Ni-Sn IMCs can act as the crystal nucleus to accelerate the recrystallization of the solder. Sb and Bi elements are soluble with Sn and act as solution strengthening elements. Higher residual stress in SACNSB solder offers more energy source for recrystallization and then more grain boundaries for easier crack propagation. So the fracture mode of the solder joints changed from the trans-granular fracture of SAC to inter-granular fracture of SACNSB.