Influence of metal-oxide/salt content in the aluminum soldering flux on solderability and corrosion resistance of Sn-0.7Cu/6061Al joints

A water-soluble soldering flux, aiming at making the flux-cored Sn-0.7Cu solder wire for soldering aluminum and its alloys, has been developed in this study, whose active matrix contains different amounts of zinc and tin metal oxides or salts. The solderability of Sn-0.7Cu solder on 6061 aluminum alloy (6061Al) substrate and corrosion resistance of Sn-0.7Cu/6061Al joints using the developed flux are evaluated systematically. Results show that an increase in content of both zinc-oxide and zinc-salt in the flux can significantly improve the solderability of Sn-0.7Cu solder and chemical corrosion resistance of Sn-0.7Cu/6061Al joints, in particular the increase of zinc-salt content shows a more effective role in increasing the corrosion resistance of the joints. There are a large number of micro-sized Zn-containing particles and network-like Zn-containing phase on the surface of 6061Al arisen from the reaction between the substrate and flux containing zinc-oxide or zinc-salt. During the soldering process, the complex reactions among the flux, Sn-0.7Cu solder and 6061Al result in formation of a combined system of Sn-0.7Cu/Zn/Al, in which Zn plays a key role in forming Zn metallization on the surface of 6061 aluminum. Meanwhile, an increase in content of tin-salt in the flux can also improve the solderability of Sn-0.7Cu solder but reduce the chemical corrosion resistance of Sn-0.7Cu/6061Al joints. There is a flat Sn metallization layer formed on the surface of 6061Al. The result indicates that the role of Sn metallization is only for improving the solderability of Sn-0.7Cu solder rather than effectively enhancing the corrosion resistance of Sn-0.7Cu/6061Al joints.