Creep Behavior of Mixed SAC 405/SnPb Soldered Assemblies in Shear Loading

In this work, the creep behavior of mixed lead free (Sn4%Ag 0.5%Cu) (SAC 405) and eutectic tin lead (Sn37%Pb) solders has been studied. A double lap shear configuration was used to study the creep response of the mixed solder joints, the dimensions of which ranged from 300 to 500 mum. The tested solder specimens were wafer level chip scale packages (WLCSP) bumped with preformed solder spheres. The volume ratios of the two solders were controlled by using preformed solders spheres in the size range of 300 to 450 mum. All the specimens were reflowed at a profile with 260degC peak temperature resulting in a complete mixing of the two solder alloys. The creep tests were done at constant stress levels by applying a constant load to the specimen. The displacement of the joints was recorded as a function of time from which the steady state strain rates were determined. In the present experiments, these ranged from lE-3/s to lE-9/s. while the applied stress levels ranged from 5 MPa to 40 MPa. The tests were repeated for 3 different temperatures: 40degC, 70degC and 100degC. The stress - strain rate data seems to fit well to the SINH creep model and the obstacle controlled model. The fitting to the power law shows different slopes above and below 10 MPa stress levels. In addition, the effect of isothermal aging at 125degC on the creep behavior was also studied. In general, the mixed solder joints creep faster than SAC 405 and their deformation rate lies in between that of the eutectic SnPb and SAC 405 solders.