Effect of micro structure on fatigue characteristics of lead free solder joints

This paper presents the influence of the micro structure on the crack propagation in lead free solder joint. The author´s group have studied the Manson-Coffin´s law for lead free solder joint by using the isothermal fatigue test and FEM analytical approaches to establish the practicable evaluation of thermal fatigue life of solder joints, for example, for the Sn-Cu solder, because this solder is attracted from the aspect of the decrease of solder leach in the flow process and material cost. However, even if the same loading is given to the solder joints of BGA test piece, there was a large dispersion in the fatigue life. Even though the effect of the shape difference has been considered, the range of the dispersion could not been explained sufficiently. In the study, the fatigue crack propagation modes in the solder joints were investigated, and an internal fatigue crack mode and an interfacial fatigue crack mode were confirmed. And the tendency of a shorter on fatigue life in the interfacial fatigue mode was confirmed. To clarify the mechanism of these fatigue crack modes, the crystal grain size in the solder joints was investigated before the fatigue test and also after the test. Furthermore, the verification of the mechanism using FEM models considering the crystal grain size was carried out. First of all, each element in FEM models matching to the average crystal grain size was made. Second, the inelastic strain ranges in each FEM models were studied. As a result, it was shown that the influence of the crystal grain size to the fatigue crack progress can be evaluated.