Morphologies and nucleus models of β-Sn grains in SAC305-xIn freestanding solder balls and solder joints

Due to the significant anisotropy of β-Sn in physical and mechanical properties, the reliability of Sn-based solder joints is closely related to β-Sn grain features. In this study, the morphologies of β-Sn grains in Sn-3.0Ag-0.5Cu-xIn (SAC305-xIn, x = 0, 0.5, 1, 2, 5, 10 wt%) freestanding solder balls and SAC305/Cu and SAC305/Au/Pd/Ni solder joints in two sizes were characterized. Seven β-Sn grain morphologies that all contained twinning relationship were found in the SAC305-xIn solder balls and joints. When the content of In addition was larger than 2 wt%, the β-Sn grains were refined significantly, regardless of solder ball size and substrate type. Four different cyclic twinning structures were established to explain the mechanism of the twin structures, i.e., {101} sixfold, {301} sixfold, {101} threefold and {301} threefold cyclic twin structures. In addition, the nucleus & growth models for the seven β-Sn morphologies were successfully established by combining the β-Sn cell or the cyclic twin structure with the preferred growth direction of β-Sn dendrites.