The thermal reliability of Sn-3Ag-0.5Cu/Au/Pd(P)/Cu solder joints was evaluated in this study. After reflow and subsequent solid-state aging (180°C), the reaction product species at the interface included Cu₆Sn₅ [or (Cu,Pd)₆Sn₅] and Cu₃Sn, and their growth was strongly dependent on the Pd(P) thickness, δ _Pd(P). As δ _Pd(P) increased, the growth of Cu₆Sn₅ was significantly enhanced, while that of Cu₃Sn was suppressed. Computer coupling of phase diagrams and thermochemistry (CALPHAD) analysis showed that minor incorporation of Pd (~2 at.%) into the Cu₆Sn₅ phase decreased the Gibbs free energy of Cu₆Sn₅ from −7339 J/mol to −9191 J/mol. This effect might enhance Sn diffusion in Cu₆Sn₅ but diminish Cu diffusion in Cu₃Sn, thereby facilitating the growth of Cu₆Sn₅ but retarding that of Cu₃Sn. High-speed ball shear (HSBS) test results showed that the mechanical properties of the solder joints were slightly enhanced by an increase in δ _Pd(P). These findings suggest that direct deposition of Au/Pd(P) bilayers over the Cu pads can effectively modify the mechanical reliability of solder joints.