The effect of platinum contact metallization on Cu/Sn bonding

In this work, formation and evolution of microstructures in CuSn/Pt bonding were investigated after 320 °C reflow process as well as after high temperature storage test at 150 °C. Sputtered thin film platinum on silicon wafer and high purity platinum sheet were applied as contact metallizations for electroplated copper-tin based bonding metallurgy. As bonded microstructure showed PtSn₄ intermetallic compound growth at the Pt/Sn interface, and both Cu₆Sn₅ and Cu₃Sn phases formed at the Cu/Sn fiinterface. Both hexagonal and monoclinic Cu₆Sn₅ were found to coexist after 1000 h high temperature storage test. Platinum was discovered to dissolve into the Cu₆Sn₅ phase during soldering process and form (Cu, Pt)₆Sn₅ intermetallic compound exhibiting hexagonal allotropy. Meanwhile, under annealing, monoclinic Cu₆Sn₅ phase layer without platinum was observed to form between (Cu, Pt)₆Sn₅ grains and tin. Thermodynamic analysis was performed in order to reason the effects of Pt on the phase equilibria and phase stabilities. Results show that platinum has a significant impact on the stability of hexagonal Cu₆Sn₅.