Influence of Sb addition on microstructural evolution of Sn-Ag solder

This paper investigates the influence of various levels of Sb addition on the microstructure of Sn-Ag hypoeutectic solder. To be precise, the microstructural evolution and the characteristics of Sn2.58Ag solder are studied for Sb additions of 1.75, 4.75 and 8.78wt%. The experimental results indicate that part of the Sb addition solutes into the β-Sn matrix, while the remainder reacts with the Ag and Sn atoms to form ε-Ag₃(Sb,Sn) and SbSn compounds. It is noted that Ag₃(Sb,Sn) is readily formed even when the level of Sb addition is low (1.75%). This suggests that the Sb atoms are quite active in substituting some of the Sn atoms in the Ag₃Sn compound to form a nonstoichiometry compound, i.e. Ag₃(Sb,Sn). In contrast, the SbSn phase is only identified when the Sb content exceeds 4.75%. After a process of deep etching, it is observed that the morphology of the Ag₃(Sb,Sn) phase in the as-soldered condition possesses a long-strip-like appearance. However, it is shown that if the cooling rate is sufficiently slow, then the β-Sn matrix and the Ag₃(Sb,Sn) compound tend to solidify simultaneously in the as-cast condition. A coarse laminated structure is observed and the Ag₃(Sb,Sn) phase appears platelike. This results in a substantial reduction in the strength and ductility of the solder joint. Finally, it is shown that the size of the SbSn phase remains at a constant value of ca.30 μm for the two cooling rate conditions considered in the present investigation.