An investigation of effects of Sb on the intermetallic formation in Sn-3.5Ag-0.7Cu solder joints

This study investigates the microstructural evolution and kinetics of intermetallic (IMC) formation in Sn-3.5Ag-0.7Cu lead-free solder joints with different percentages of Sb element, namely, Sn-3.5Ag-0.7Cu-xSb (x=0, 0.2, 0.5, 0.8, 1.0, 1.5, and 2.0). To investigate the elemental interdiffusion and growth kinetics of IMC formation, isothermal aging test is performed at temperatures of 100°C, 150°C, and 190°C, respectively. Scanning electron microscope (SEM) is used to measure the thickness of intermetallic layer and observe the microstructural evolution of solder joint. The IMC phases are identified by EDX and XRD. Results show that some of the antimony powders are dissolved in the β-Sn matrix (Sn-rich phase), some of them participate in the formation of Ag₃(Sn,Sb) and the rest dissolves in the Cu₆Sn₅ IMC layer. There is a significant drop in IMC thickness when Sb is added to 0.8 wt%. Over this amount the thickness of the IMC increases slightly again. The activation energy and growth rate of the IMC formation are determined. Results reveal that adding antimony in Sn-3.5Ag-0.7Cu solder system can increase the activation energy, and thus reduce the atomic diffusion rate, so as to inhibit the excessive growth of the IMC. The solder joint containing 0.8 wt% antimony has the highest activation energy. SEM images reveal that the number of small particles precipitating in the solder matrix increases with the increase in Sb composition. Based on the observation of the microstructural evolution of the solder joints, a grain boundary pinning mechanism for inhibition of the IMC growth due to Sb addition is proposed.