Metallurgical Perspective on Alloying Modification of Sn-Ag-Cu Solders

This study investigates the effect of alloying additives on the solidification behavior and microstructural characteristics from a metallurgical perspective. Results show that alloying of Mn and Ti resulted in dramatically reduced undercooling coarse eutectic structure and extended volume fraction of proeutectic Sn of which the dendritic size was refined. Those thermal and microstructural changes might be ascribed to the formation of MnSn₂ and Ti₂Sn₃ intermetallic compounds (IMCs) appeared respectively in the Mn-doped and Ti-doped samples. Instead of forming heterogeneous IMCs, adding of the low temperature solute elements, Bi and Ga, decreases the melting point, extends the solidus/liquidus range and causes a mixed normal-coarse structure with a varying solute concentration. Unlike Bi which is only detected in the Sn matrix, Ga also dissolves in eutectic Ag₃Sn and even transforms it into a new intermetallic phase, Ag₇₂Ga₂₈ within the coarse eutectic cells. It is believed that this could give rise to a specific two-stage nonequilibrium eutectic solidification feature.