Effect of nano-dopant on intermetallic compounds growth in Sn-3.0Ag-0.5Cu-xBi solder joints during aging process

The effect of nano-Bi particles on the growth of intermetallic compound (IMC) between Sn-3.0Ag-0.5Cu-xBi (x=0.0, 0.8, 1.5, 2.5, 3.5, and 4.5 wt.%) solder and Cu substrate during aging process at temperatures of 120, 150, and 190°C has been investigated in this study. Scanning electron microscopy (SEM) was used to observe the microstructural evolution of the solder joints and measure the thickness of IMC layer. Energy dispersive X-ray (EDX) was adopted to identify the composite of the IMC phase. The activation energies and growth rates of the IMC layer were determined. Results show that adding nano-Bi into Sn-3.0Ag-0.5Cu solder can increase activation energy and thus reduce the atomic diffusion rate, so as to suppress the excessive growth of the IMC layer. The solder joints containing about 0.8wt.% nano-Bi has the highest activation energy and the lowest growth rate. SEM images reveal that with an increase in nano-Bi to 0.8 wt.%, the number of small particles precipitated along grain boundary reaches maximum. Based on the observation of the microstructural evolution of the solder joints, a grain boundary pinning mechanism for inhibition of the IMC grow due to nano-Bi addition is proposed.