<p>This study investigates the liquid/solid interfacial reactions between lead-free solders (LFS) and Cu-6.01 wt.% Sn-0.12 wt.% P alloy (C5191), which offers superior strength, corrosion resistance, thermal stability, and electrical conductivity. Thus, C5191 is considered to replace Cu and is used for lead-frame materials. Three LFSs, including Sn, Sn-3.0 wt.% Ag-0.5 wt.% Cu (SAC), and Sn-0.7 wt.% Cu (SC), were reacted with C5191 at 240°C, 255°C, and 270°C for various durations. The results showed that the scalloped Cu₆Sn₅ and layered Cu₃Sn phases were observed at the LFS/C5191 interfaces. Over prolonged reactions, the scallop-shaped Cu₆Sn₅ phase became more layered in morphology, particularly after 10&#xa0;h in the Sn/C5191 couple. The intermetallic compound (IMC) thickness (<i>d</i>) was increased with reaction time (<i>t</i>) and temperature. The linear relationship between <i>d</i> and <i>t</i>^1/2 was observed. It means that the IMC growth mechanism was diffusion-controlled. The activation energies in the LFS/C5191 systems were lower than those in the LFS/Cu systems. This finding reveals that C5191 is suitable for use as a lead-frame material in electronic packaging to ensure solder-joint reliability.</p>