Reaction of Sn-3.5Ag-0.7Cu-xSb solder with Cu metallization during reflow soldering

The influence of Sb on the growth kinetics of intermetallic compound (IMC) and chemical reaction between Sn-3.5Ag-0.7Cu-xSb (x=0, 0.2, 0.5, 1.0, 1.5, and 2.0) lead-free solder and Cu during reflow soldering is investigated in this study. Scanning electron microscope (SEM) is used to measure the thickness and grain size of the intermetallic layer and observe the microstructural evolution of solder joints. Results show that both thickness and grain size of IMC decrease once Sb is added into the Sn-Ag-Cu solder system, and have a significant drop at the composition of about 1.0 wt%. Beyond this composition, the thickness and grain size of IMC increase slightly. The growth exponents for both IMC layers and grains are determined by curve-fitting to study the growth kinetics of IMC in wetting reaction. The results reveal that the growth exponents of IMC range from 0.32 to 0.35, and those of IMC grains range from 0.28 to 0.32, which suggests that the growth of IMC is controlled through combined kinetic processes of atomic interdiffusion, interfacial reaction, and grain ripening. These data also show that Sn-3.5Ag-0.7Cu with about 1.0 wt% Sb solder system exhibits the smallest growth rate and gives the most prominent effect in retarding IMC growth and refining IMC grain size. Based on the phase diagram analysis and the observation of the microstructural evolution of the solder joints, a heterogeneous nucleation mechanism for retarding the IMC growth due to Sb addition is proposed.