The effectiveness of Ni microalloying on the microstructure and mechanical properties of low temperature In-35 wt%Sn/Cu solder joint

The effects of 0.05 wt%Ni additions on In-35 wt%Sn (In–35Sn) alloy on the ex-situ interfacial reactions, in-situ microstructures formation during solidification, and mechanical properties during reflow soldering on Cu substrate were systematically studied. Reaction of In–35Sn with Cu resulted in the formation of two intermetallic Cu3(Sn,In) layers were found at the interface. When Ni is added, Ni is participated in the interfacial (Cu,Ni)3(Sn,In) layer formed at the In–35Sn-0.05Ni/Cu interface, causing the morphology of the IMC layer at the solder side to become more refined. Adding Ni resulted in the degree of undercooling In–35Sn decreasing close to zero. The in-situ real-time synchrotron x-ray imaging confirmed that the presence of Ni is responsible for the spontaneous β-In3Sn grain nucleation and formation of equiaxed grain in the solder joint. The grain refinement on the (Cu,Ni)3(Sn,In) IMC layer in In–35Sn-0.05Ni/Cu solder joint leads to minor increment in the shear strength. This study demonstrated the effectiveness of Ni microalloying in low temperature In–35Sn/Cu solder joint in modifying the β-In3Sn dendrite growth at the bulk solder and the microstructure refinement at the interface of the solder joints, which could potentially enhance the overall joint strength.