Effect of intermetallic compounds on fracture behaviors of Sn3.0Ag0.5Cu lead-free solder joints during in situ tensile test

In this paper, in situ tensile tests under various amounts of deformation were performed on Sn3.0Ag0.5Cu lead-free solder joints subjected to multi-reflow and isothermal aging processes by using a scanning electron microscope. Microstructure evolution and deformation behavior of the solder joints were observed. Effects of the intermetallic compound (IMC) Cu₆Sn₅ on fracture behaviors of the solder joints were investigated. Results showed that the Sn3.0Ag0.5Cu lead-free solder joints contained only a few Sn grains, and the sequence and degree of plastic deformation varied for the different grains in the same solder joint due to the strong anisotropic properties of Sn grains. Further experiments revealed that plastic deformation occured primarily in the form of slip bands in the solder joints during the in situ tensile test. Various fracture modes including intergranular and phase boundary fractures were observed. The fracture behaviors of solder joints were significantly affected by morphologies and distributions of the Cu₆Sn₅ IMCs. It was found that Cu₆Sn₅ particles located at the grain boundaries are apt to become crack sources, and that the long rod shaped Cu₆Sn₅ were easily broken. However, spherical Cu₆Sn₅ hardly deformed during the tensile test, resulting in dynamic recrystallization. In this case, fracture occured at the sub-grain boundaries.