| This thesis focused on investigating the effect of thermal and chemical factors which influence solidification of Sn in near eutectic Sn-3.0Ag-XCu (X=0, 0.27, 0.59, 0.86, 1.1, 1.4 wt.%) family alloys.; Pure Sn tends to undercool hundreds of degrees from the melt before solidification. Even in SnAgCu alloys, Sn is observed to undercool tens of degrees. Because of this Sn undercooling, equilibrium nucleation and growth of SAC alloy does not occur. This study examined how the degree of Sn undercooling influences the metastable nucleation and growth processes of Sn-3.0Ag-XCu alloys and their final microstructures. The effect of purity of Sn, Cu substrate, reflow temperature and other factors on the degree of Sn undercooling in SnAgCu alloys were carefully analyzed. Factors that influence the growth of large intermetallic compounds such as Ag3Sn and Cu6Sn5 were also investigated. This study demonstrated that Sn-3.0Ag-0.59Cu (wt.%) is an optimized composition of Sn-Ag-Cu alloy to replace conventional Pb-Sn solders, which can effectively avoid the growth of large Ag3Sn and Cu6Sn 5 IMCs in the SAC solder both at fast and slow cooling conditions. |
