Study on the IMC Growth Mechanism of Cu/Sn-58Bi/Cu Joint Under Electromigration with Alternating Current

With the ongoing miniaturization of solder joints in three-dimensional integrated electronic packaging, electromigration reliability has become a pressing concern. This study systematically examines the interfacial intermetallic compound (IMC) growth behavior of Cu/Sn-58Bi/Cu joint under electromigration (EM) with a symmetrical square-wave alternating current (AC). Electron backscatter diffraction (EBSD) was employed to perform statistical spatial analysis of Sn grain orientations within the joints to reveal the growth mechanism of interfacial IMC. Results demonstrate that the AC field markedly enhances the anisotropy of IMC growth in Cu/Sn-58Bi/Cu joints, exhibiting two phenomena: uniform growth on both sides and rapid growth (polar growth) on one side of the interfacial IMC. Among them, the IMC thickness difference characterization quantity ΔIMC reached as high as 45.56% for the latter. This is attributed to the directional regulation of atomic migration rate by Sn grain orientation (the angle θ between the c-axis and the electron flow) and is further amplified by the altered atomic diffusion pathways imposed by the Bi phase distribution. Specifically, the Sn grains exhibit a pronounced preferential orientation mode along the current path (horizontal direction), with an orientation gradient of 0.915 μm−1. The arrangement of Bi-rich phases alters the distribution of Sn grains in Cu/Sn-58Bi/Cu joints, thereby reshaping the internal electron transport pathways and significantly intensifying the orientation-dependent effect of IMC growth. Moreover, Sn grains adjacent to the Bi-rich phase boundaries (phase boundary grains) display a stronger tendency for c-axis orientation parallel to the current direction, exhibiting an average effective orientation parameter 1.948 times greater than that of bulk grains, which establishes a well-defined spatial orientation gradient.