Microstructure evolution and shear strength of Sn-3.5Ag-RE lead-free BGA solder balls

The interfacial microstructure evolution and ball shear strength of Sn-3.5Ag and Sn-3.5Ag-0.5RE (in wt%) solders alloys attached on Au/Ni ball grid array (BGA) metallization pad were investigated after thermal aging at 150°C up to 1000 hr. In the as-reflowed Sn-3.5Ag solder, acicular AuSn₄ and platelet Ag₃Sn intermetallic compounds (IMCs) were randomly distributed in the β-Sn matrix. However, the sizes of β-Sn grains and Ag₃Sn IMCs were decreased by the addition of 0.5 wt% of rare earth (RE) elements. RE-bearing phase was also observed near the interface. At the interface, a Ni-Sn intermetallic layer (IML), Ni₃Sn₄, was formed in both as-reflowed Sn-3.5Ag and Sn-3.5Ag-0.5RE solders. With thermal aging, large platelet (Au,Ni)Sn₄ phases were found inside the solder ball and close to the interface in both solders. The IML was grown slightly upon aging. The ball shear strengths of Sn-3.5Ag-0.5RE were higher than those of Sn-3.5Ag in the whole thermal aging duration. An enhancement of the as-reflowed shear strength by adding RE elements was attributable to the microstructure refinement. The solders with RE elements added also had relatively higher and more stable ball shear strength than those of the Pb-bearing solders upon aging. The fracture mode of the Sn-3.5Ag and Sn-3.5Ag-0.5RE solders was completely ductile with failure occurred inside the bulk solder. The RE elements also had an effect on the morphology of the Ni₃Sn₄ IMC.