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

摘要

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℃), the reaction product species at the interface included Cu_6Sn_5 [or (Cu,Pd)_6Sn_5] and Cu_3Sn, and their growth was strongly dependent on the Pd(P) thickness, δ_(Pd(P)). As δ_(Pd(P)) increased, the growth of Cu_6Sn_5 was significantly enhanced, while that of Cu_3Sn was suppressed. Computer coupling of phase diagrams and thermochemistry (CALPHAD) analysis showed that minor incorporation of Pd (～2 at.%) into the Cu_6Sn_5 phase decreased the Gibbs free energy of Cu_6Sn_5 from -7339 J/mol to -9191 J/mol. This effect might enhance Sn diffusion in Cu_6Sn_5 but diminish Cu diffusion in Cu_3Sn, thereby facilitating the growth of Cu_6Sn_5 but retarding that of Cu_3Sn. 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.