Effect of thermal aging on microstructure, shear and mechanical shock failures for solder ball bonding joint

The interfacial microstructure evolution between Sn-3.5Ag-0.75Cu solder ball and Au/Ni/Cu pad was investigated under as-bonded and thermal aging conditions. Moreover, the shear and mechanical shock tests were carried out to study the failure mechanisms. Results showed that only one intermetallic compound, fine needle-like AuSn₄ was found at the solder/pad interface under as-bonded condition. After aging, Ag₃Sn appeared in the solder bulk as a particle-like or elongated structure. Meanwhile, the intermetallic compounds layer became flat and a quaternary intermetallic compound Cu-Sn-Ni-Au appeared between AuSn₄ layer and Ni(NiFe) layer. In addition, parts of AuSn₄ spalled into the solder bulk with long hour aging. In the shear test, the solder joint exhibited a ductile failure. While in the mechanical shock test, the cracks initiated at the right end of the horizontal pad and propagate through the interfaces between IMCs. Furthermore, the factors dominating the failure mode were investigated which consists of external force and stress distribution, interfacial reaction and strain rate