Correlation between microstructure evolution and mechanical strength in the Sn-3.0Ag-0.5Cu/ENEPIG solder joint

This study investigated the interfacial reaction and mechanical strength of Sn-3.0Ag-0.5Cu solder jointed with electroless Ni-P/immersion Au (ENIG) and electroless Ni-P/electroless Pd/immersion Au (ENEPIG) surface finishes under various reflow times. The morphology of interfacial (Cu,Ni,Pd)6Sn5 was refined to needle-like by the Pd insertion in the ENEPIG surface finish. The impact energy of solder joints with Pd doped declined slower than that without Pd-doped after prolonged reflow. By inspecting the fracture surface, the cleavage of interfacial (Cu,Ni)6Sn5 was frequently observed in the ENIG joints after prolonged soldering, whereas ductile dimple-like failure dominated the fracture surface of ENEPIG joints. The enhanced impact strength and the transition of failure mode in the ENEPIG joints was attributed to the needle-like morphology of (Cu,Ni,Pd)6Sn5. The detailed mechanism of improved mechanical strength for solder joints with Pd dissolved will be deliberately addressed and discussed regarding the distinct microstructural evolution in the ENEPIG joint.