Experimental and finite element analysis of the shear speed effects on the Sn-Ag and Sn-Ag-Cu BGA solder joints

An experimental investigation was combined with a non-linear finite element analysis using an elastic-viscoplastic constitutive model to study the effect of ball shear speed on the shear forces of BGA solder joints. Two solder compositions were examined in this work: Sn-3.5Ag and Sn-3.5Ag-0.75Cu. The Cu substrates had been surface finished electrolytically with a 7 μm thick Ni diffusion barrier followed by an 0.5 μm thick Au layer to enhance solderability. Ag₃Sn and a few AuSn₄ intermetallic compound (IMC) particles were found inside the two solders. Only a continuous Ni₃Sn₄ layer was observed at the interface between the Au/Ni plated layer and the Sn-3.5Ag, while a continuous (Ni_1-xCu_x)₃Sn₄ layer and a small amount of discontinuous (Cu_1-yNi_y)₆Sn₅ particles were formed at the interface between the substrate and the Sn-3.5Ag-0.75Cu. The IMC was identified using energy dispersive spectrometer (EDS) and electron probe micro analysis (EPMA). Shear tests were carried out over a shear speed range from 10 to 700 μm/s at a shear ram height of 50 μm. The shear force was observed to linearly increase with shear speed and reach a maximum value at the highest shear speed in both the experimental and the computational results. All test specimens fractured in a ductile mode. The failure mechanisms were discussed in terms of von Mises stresses and plastic strain energy density distributions.