The Effect of Thermal History on the Microstructure of SnAgCu/SnBiAg Mixed Assemblies

The effects of reflow parameter values on the microstructure of SnAgCu/SnBiAg mixed assemblies were examined. The variation of the volume fraction of the hypoeutectic SnBiAg phase with respect to the peak temperature during reflow, and the initial volumes of the SnAgCu and SnBiAg phases, was characterized. A simple theory was developed to predict the volume of the SnAgCu and SnBiAg phases after reflow, as a function of the peak temperature during reflow and the initial volume of the SnBiAg phase. This theory was based upon a one-dimensional Sn/eutectic SnBi mixed assembly. Concentration gradients in the hypoeutectic SnBiAg phase after reflow were characterized and compared to results from the Scheil equation.