<p>C194 (Cu-2.35 wt.% Fe) alloy is a commonly used lead-frame material. A Ni layer serves as a diffusion barrier in electronic packaging, while Sn is the primary base material for lead-free solder. In this study, Ni layers of varying thicknesses were electroplated onto C194 substrates to investigate the interfacial reactions between Ni/C194 substrate and molten Sn. The results show that if the Ni layer is not completely consumed, only the (Ni, Cu)₃Sn₄ phase forms at the interface. When the Ni coating is completely consumed, the (Cu, Ni)₆Sn₅ phase forms at the interface. At the same reflow temperature, as the reflow time increases, the Ni layer gradually dissolves into the molten Sn and reacts with it to form an intermetallic compound (IMC). Increasing the reflow temperature further accelerates the dissolution and consumption rates of the Ni layer. The IMC growth mechanism was lattice-diffusion controlled. The activation energy of the (Cu, Ni)₆Sn₅ phase is lower than that of the (Ni, Cu)₃Sn₄ phase. The thicker Ni layer more effectively suppresses the diffusion of alloying-addition atoms from the C194 alloy dissolved in the molten Sn solder, thereby reducing their role as heterogeneous nucleation sites that intensify the interfacial reaction.</p>