Effects of Zn, Ge doping on electrochemical migration, oxidation characteristics and corrosion behavior of lead-free Sn-3.0Ag-0.5Cu solder for electronic packaging

Due to lead-free pressure, Sn-Ag-Cu solder is regarded as the potential substitute. However, higher melting point and faintish soldering restrict its wide application. Metal or rare earth metal doping can improve its soldering. In this thesis, effects of Zn, Ge metal doping on electrochemical corrosion behavior of lead-free Sn-3.0Ag-0.5Cu (SAC) solder in 3.5wt.% NaCl solution were investigated by potentiodynamic polarization. The electrochemical migration(ECM)attributed to dendrites growth were employed by the salt immersion experiment under direct current electrical field, and the oxidation characteristic was researched by thermo gravimetric analysis(TGA). The surface morphology and elemental composition of various elements in the doped solder were determined by analyzing the corrosion product or dendrites formed on the specimen by SEM, EDAX, XRD techniques. The corrosion results showed that the dendrite growth can be accelerated after Zn doping comparing to Sn-3.0Ag-0.5Cu solder when Zn percent larger than 1%, the same function suitable to Ge doping when Ge percent less than 1%. The shapes of dendrites were completely different after Zn, Ge doping, the fore looked like as rose flowers, the latter looked like cabbage palm, all dendrites grown from different kind of material can be well differentiated by the help of their fractal dimension. EDAX and XRD results showed that the content on dendrites of 95(Sn-3.0Ag-0.5Cu)-3Zn-2Ge solder were only Sn. TGA result showed Ge doping can improve the anti-oxidation capacity of SAC solder, however, Zn caused to the contrary effect. Thus provides a good technical support to develop a new lead-free solder substituting for Sn-37Pb.