Thermomigration has become a critical reliability issue in consumer electronic products because of Joule heating. To conduct heat away, it requires temperature gradient. Just 1°C difference across a microbump of 10 μm in diameter produces a temperature gradient of 1000 °C/cm, which can cause thermomigration, especially in low melting eutectic SnBi solder joints. However, the study of thermomigration in SnBi soler joints is hardly seen, not to mention the effect and the details. In this study, a Cu/Sn-58Bi/Cu joint with a 42 μm solder height, bonded by a reflow process, is examined for thermomigration with a thermal gradient of 1309 °C/cm. We report here that Bi atoms move from the hot end to the cold end, following the temperature gradient, it is the dominant diffusing species. The Sn atoms move from the cold end to the hot end. Under the assumption of constant volume, the Sn atoms are being squeezed by the Bi atoms at the cold end and have to make room for the Bi atoms, so they move to the hot end. Moreover, the formation of Cu-Sn intermetallic compound (IMC) layers at the cold and hot end site were symmetrical, unaffected by thermomigration. Additionally, finite-element-method (FEM) simulations showed that the phase separation of Bi and Sn has reduced current crowding regions, which affects the electromigration of the eutectic SnBi solder joints.