Investigation On Microstructure And Corrosion Behaviour Of Sn-0.3Ag-0.9Zn Solder Solidified With Different Cooling Rates

Considering the harmful influence of the lead in traditional Sn-Pb solders on human body and the environment, it is essential to develop the lead-free solder alloy systems. And considerable attention has been paid on the Sn-Ag and Sn-Zn alloy systems, owing to their excellent mechanical performances and moderate melting points. Besides, the reliability of solder joints can also be affected by the corrosion resistance of solder alloys. In this work, the microstructure evolution of casted Sn-0.3Ag-0.9Zn alloy under different cooling rates is investigated, as well as the electrochemical behaviour of Sn-0.3Ag-0.9Zn in a 3.5 wt.% NaCl solution.The Sn-0.3Ag-0.9Zn alloys solidified with different cooling rates were obtained by changing the cooling media. The phase transformation and microstructure evolution of Sn-0.3Ag-0.9Zn alloy at each cooling rate were studied through differential scanning calorimeter and scanning electron microscope. By means of electrochemical detection and surface analysis techniques, the electrochemical corrosion behavior of Sn-0.3Ag-0.9Zn alloy at each cooling rate was also explored. The conclusions can be listed as follows:(1) The microstructure of furnace-cooled and air-cooled Sn-0.3Ag-0.9Zn alloys consisted of ?-Sn, granular ?-AgZn3 and needle-like Zn-rich phase, while the microstructure of the water-cooled alloy is just made up of ?-Sn and ?-AgZn3 phases. The precipitation of Zn-rich phase in Sn-0.3Ag-0.9Zn alloy was inhibited by the high cooling rate imposed.(2) The size of the ?-Sn dendrite and the intermetallic compounds(IMCs)(?-AgZn3 and Zn-rich phases) decreased with the increase of the applied cooling rates, which reduced the galvanic corrosion between the cathodic IMCs and the anodic Sn matrix.(3) The content of SnO and oxygen vacancy in the passive film formed on the surface of Sn-0.3Ag-0.9Zn alloy decreased with the increase of the cooling rates, which improved the stability and protective ability of the passive film.(4) The major corrosion product on the surface of the Sn-0.3Ag-0.9Zn alloys was Sn3O(OH)2Cl2 and the corrosion process exhibited a classic local corrosion feature.