The Effects of Bi Doping and Aging on Viscoplasticity of Sn-Ag-Cu-Bi alloys

In this study, the effects of aging on viscoplasticity of Bi doped Sn-4.0Ag-0.5Cu (SAC405) alloys were investigated. Samples of four types of alloys, SAC405, SAC405+1%Bi, SAC405+2%Bi and SAC405+3%Bi were casted into 1mm x 2mm rectangular cross section bars for tensile testing. Samples of each composition in as-cast condition as well as after subjecting to aging at 125°C for 30 days were subjected to tensile testing at constant strain rates of 0.1s
^-1
, 0.01s
^-1
, and 0.001s
^-1
at each of the testing temperatures of 25°C, 90°C and 145°C. The stress-strain data generated for each sample was processed to find out the Anand parameters which quantify viscoplasticity of each alloy in as-cast and aged conditions. The stress-strain data showed the effect of viscoplasticity which resulted in a higher tensile strength for higher strain rates for all compositions. The tensile strength for each of the testing temperature and strain rate combination was found to be higher for higher Bi content. Moreover, 2 to 3 wt.% Bi doping increased the tensile strength for each of the testing temperature and strain rate combination after aging for 30 days at 125°C. SAC405-1%Bi did not show a significant change in the tensile strength after aging at 125°C for 30 days while SAC405 showed a decreased tensile strength after aging. The material parameters of Sn-4.0Ag-0.5Cu-xBi alloys and the effect of Bi doping on tensile strength found out in this study will prove to be useful to ascertain the reliability of microelectronic packages consisting of Bi doped SAC solder.