Shear deformation of lead free solder joints

The deformation behavior of lead free solder joints was characterized under shear loading. The test vehicle was a 0.5mm pitch wafer level chip scale package (WLCSP). Five alloys were compared: Sn₆₃Pb₃₇, Sn_95.5Ag_4.0Cu_0.5, Sn_96.5Ag_3.0Cu_0.5, Sn_96.5Ag_3.5, and Sn_99.3Cu_0.7. The strain rate range was between 1.0/sec and 10^-7/sec, and test temperature was 25C. Both un-aged and thermally aged samples were tested. The aging condition was 24hrs at 125C. At strain rates of 10^-2/sec and above, all of the alloys had comparable strength, except for Sn_99.3Cu_0.7, which had 20% to 30% lower strength. Sn₆₃Pb₃₇ was the most strain rate sensitive, and hence had the lowest strength below 10^-4/sec strain rate. All alloys showed a 10% to 30% reduction in strength with aging. The solder joints exhibited very high ductility. In some cases strains of 100% were achieved before the joints started to fail. The ductility of Sn₆₃Pb₃₇ increased with aging, but the ductility of the lead free alloys all decreased with aging. Sn_96.5Ag_3.0Cu_0.5 showed the largest decrease in ductility with aging. The steady state creep behavior observed in the current study is fairly consistent with other published work. However, the absolute value of the joint strength is a little higher than data on bulk samples. All of the alloys exhibited significant primary creep before steady state conditions were achieved. This behavior is not always reported in the literature, and hence it deserves more investigation.