Brittle versus ductile failure of a lead-free single solder joint specimen under intermediate strain rate

The failure mode of Sn-4.0Ag-0.5Cu solder joints under the intermediate strain rates is explored. Tensile and shear tests were performed on miniature solder joints at two different displacement rates, i.e., 5 and 50 mm/s. For the uniaxial tensile test, brittle failure occurred at the intermetallic compound (IMC)/Cu pad interface on the board side under both loading rates. However, for shear loading at 5 mm/s, ductile failure along the solder matrix was detected. With the increase of the shear loading rate up to 50 mm/s, the failure mode is converted into the (ductile+brittle) mixed mode. Numerical simulation results based on the extracted elastic-plastic solder properties indicate that the maximum stress concentration is located at the interfaces of the board side, which indicates the weak position of the crack initiation. The plastic deformation of the solder alloy is significantly suppressed at 50 mm/s loading rate. The amplitude of the solder plastic deformation under shear loading is much greater than that for the tensile test. Furthermore, most plastic deformation under shear loading is in the solder alloy adjacent to solder/IMC interface of the board side and no visible plastic deformation is detected in the middle of solder ball, which results in cohesive failure at the rate of 5 mm/s and the (ductile+brittle) mixed failure mode at the rate of 50 mm/s.