An insight into dynamic properties of SAC305 lead-free solder under high strain rates and high temperatures

The development of high density and high integration of electronic chips puts forward stricter requirements on electronic packaging structures. Under the premise of lead-free rules, Sn-3.0Ag-0.5Cu (SAC305) has developed into a competitive solder material. However, the reliability of its mechanical properties has been attracting much attentions especially under extreme conditions. In this paper, the high-strain rate experiments are performed by a split-Hopkinson pressure bar (SHPB) to realize different plastic strain rate, while the high temperature environment is achieved by incorporating a heating furnace. By combining these two devices, the dynamic responses of solder specimens can be measured in the strain rate range from 833 s⁻¹ to 1961 s⁻¹ and the temperature conditions of 70 °C and 120 °C. It is worth noting that as a typical strain-rate hardening process, the maximum stress undergoes an initial ascending stage as the applied strain rate increases. Nevertheless, the hardening process is followed by a descending stage to indicate a softening process of maximum stress due to a further temperature increase during the rapid material deformation at higher strain rates. To gain a further insight into this maximum stress transition, the microstructure characterizations of the impacted solder specimens are carried out by a scanning electron microscope (SEM). It is found that the β-Sn phases and eutectic phases appear to experience refinement and then coarsen as the plastic strain rate continues to increase. A finite element (FE) model is created in the commercial nonlinear FE software ABAQUS/Explicit by considering all the experimental details. Since none of the existing material models in the material library is available for such a maximum stress transition with different ranges of strain rate, the mechanical properties of solder material are emphasized by proposing an improved Johnson-Cook model which can be readily utilized in the numerical evaluations of packaging structures under extremely high strain-rate conditions.