Size and constraint effects on mechanical and fracture behavior of micro-scale Ni/Sn3.0Ag0.5Cu/Ni solder joints

Solder joints are generally regarded as the weakest part in packaging systems and electronic assemblies in modern electronic products and devices. In this study, both experimental and finite element methods were used to characterize the mechanical behavior of micro-scale Ni/Sn3.0Ag0.5Cu/Ni sandwich-structured joints with different thickness-to-diameter ratios (R varying from 1/3 to 1/12) under quasi-static tension loading using a dynamic mechanical analyzer (DMA). Experimental results show that crack initiation and propagation in the solder matrix occur in a typical ductile manner. Compared with Cu/Sn3.0Ag0.5Cu/Cu sandwich-structured solder joints, Ni/Sn3.0Ag0.5Cu/Ni solder joints have much higher tensile strengths due to the dispersion strengthening effect through the fine Ag₃Sn particles. With decreasing R, both stiffness and tensile strength of solder joints increase obviously with decreasing coefficient of stress state and damage equivalent stress. Moreover, results of quantitative fractographic analysis by SEM and EDS display three fracture modes with decreasing R. Joints with R≥1/4 all fail by ductile fracture, those with R=1/6 fail by either ductile fracture or mixed ductile and brittle fractures, and for joints with R=1/12, brittle fracture is dominant. Furthermore, results obtained have also shown that the crack growth driving forces, K_I and K_II, as well as the strain energy release rate, G_I, in the Ni₃Sn₄ layer and at the Ni₃Sn₄/Ni interface, increase significantly with decreasing R. Hence, under tensile loading the fracture mode of solder joints changes from ductile to brittle as R is decreased.