This study systematically investigated the thermomigration behavior of Sn–35Bi solder joints under both temperature gradient and isothermal aging conditions, with particular emphasis on the comparative effects of Cu and Ni substrates as hot‐end materials on microstructural evolution. Through comprehensive finite element modeling (FEM) and detailed microstructural characterization, the results revealed that replacing Cu with Ni as the hot‐end substrate significantly influenced the thermomigration phenomena. During solid‐state thermomigration, the Ni substrate effectively reduced the temperature gradient from 914 to 794°C/cm, thereby suppressing the asymmetric growth of interfacial intermetallic compounds (IMCs) and retarding Bi segregation. In liquid‐state thermomigration, the Ni substrate demonstrated a similar beneficial effect, decreasing the temperature gradient from 159 to 143°C/cm and reducing the IMC growth rate at the cold end. Furthermore, under isothermal aging conditions, the Ni substrate maintained excellent phase stability by preserving the uniform distribution of the Bi phase throughout the solder joint. These findings underscore the superior performance of Ni substrates in mitigating thermomigration‐induced microstructural degradation, offering valuable insights for enhancing the reliability of Sn–35Bi solder joints in advanced electronic packaging applications.
Lin et al. (Sat,) studied this question.