Bismuth pyramid formation during solidification of eutectic tin-bismuth alloy using 4D X-ray microtomography

Next-generation electronic packaging strategies like heterogeneous integration packaging necessitate low melting temperature solder alloys. The Sn-58Bi alloy is notable candidate for its low melting point, but the development of coarse Bi particles during solidification adversely affects the joint’s mechanical properties. The mechanisms determining the morphology of these Bi particles remain unexplored. Here, we employ a 4D investigation of the solder solidification process. We observe the growth of novel pyramidal morphology of precipitating Bi in-situ during the solidification. We decipher the growth mechanisms that lead to the pyramidal shape of Bi crystals. The crystallographic nature of the pyramid facets and the inaccuracies in the Jackson factor prediction of interface stability for semimetals is investigated in detail. An alternative way of analyzing the atomic configuration for a stable solid-liquid interface is proposed. Finally, the effect of grain boundary defect formation on the growth morphology of Bi crystals is studied. Tin-bismuth alloy solders are used in electronic packaging, but the formation of bismuth particles is known to be detrimental to the mechanical performance of joints. Here, 4D x-ray microtomography is used to study the formation of pyramidal bismuth crystals during the solidification of eutectic Sn-58Bi.