Development and Evaluation of Direct Deposition of Au/Pd(P) Bilayers over Cu Pads in Soldering Applications

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 Cu 6 Sn 5 [or (Cu,Pd) 6 Sn 5 ] and Cu 3 Sn, and their growth was strongly dependent on the Pd(P) thickness, δ Pd(P) . As δ Pd(P) increased, the growth of Cu 6 Sn 5 was significantly enhanced, while that of Cu 3 Sn was suppressed. Computer coupling of phase diagrams and thermochemistry (CALPHAD) analysis showed that minor incorporation of Pd (~2 at.%) into the Cu 6 Sn 5 phase decreased the Gibbs free energy of Cu 6 Sn 5 from −7339 J/mol to −9191 J/mol. This effect might enhance Sn diffusion in Cu 6 Sn 5 but diminish Cu diffusion in Cu 3 Sn, thereby facilitating the growth of Cu 6 Sn 5 but retarding that of Cu 3 Sn. 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.