| Aluminum and it’s alloys have very good electrical and thermal conductivities, which are close to that of copper, as well as possess low density and low cost. In recent years, in electronic and electrical industries there is a trend toward replacing copper with cost-effective aluminum and its alloys. Accordingly, there is increasing demand for the high-performance soldering materials used to join aluminum and its alloys at low temperature. However, there are still some limitations impeding the further applications of aluminum and its alloys by the soldering process, which mainly include: 1) lack of appropriate soldering fluxes capable of removing the stable compact oxide layer from surface of aluminum substrate; 2) the large difference in electrochemical potentials between the solder and aluminum, which may result in low reliability of the solder joints. This thesis study aims to develop high-performance fluxe and the flux-cored lead-free solder wire used for soldering aluminum at low temperature.First, the influence of adding activators, film-forming agents and carriers to the flux on the spreading performance of Sn-0.7Cu solder on Al substrate and the electrochemical corrosion resistance of solder joints were studied. The results show that the product of the organic amine A reacting with halogen acid A exhibits the strongest ability of removing the oxide film. The addition of zinc salt A and zinc salt B to the flux can significantly improve both the flux activity and electrochemical corrosion resistance of solder joints. Further, the activity of the flux can be improved significantly with the addition of stannous fluoride. The flux shows very high activity and the solder joints possess very good reliability when the flux consists of the following compounds in the following percentages by weight: 4~6% zinc salt A, 6~8% zinc salt B, ~2% stannous fluoride A. In particular, addition of 5~20% PEG4000 to the flux can effectively increase the activity and adjust the physical status of the flux.Then, SEM and EDS are used to analyze the morphology and composition changes of aluminum substrate after reacting with the flux and the cross-sectional area of joints soldered at temperature of 260 ℃. The results show that zinc salt A and zinc salt B may lead to the formation of a flat layer of zinc over large areas on aluminum substrate, thus the new interfacial structure of Al/Zn/Sn-0.7Cu is formed, which may facilitate the soldering process.Furthermore, the role of the activator, film-forming agent and carrier playing in the flux in influencing the solderability of aluminum substrate was investigated. The effects of each of constituents in the flux on the activity, solder joint reliability, flux residue after soldering and spattering characteristics of the flux-cored solder wire were evaluated. Based on the above systematical study, a new type of flux-cored lead-free Sn-0.7Cu solder wire, which is suitable for soldering aluminum and its alloys at low temperature, was developed successfully.Finally, the designed flux has been successfully used to produce, at pre-production scale, flux-cored lead-free Sn-0.7Cu solder wire with a diameter of 1.0 mm and a flux content of 2.5%. In comparison with a commercial product, the present flux-cored Sn-0.7Cu solder wire shows the good performance of high activity, no pungent smell and less spattering during soldering, low-residue and non-corrosive after soldering, and good resistance to electrochemical corrosion. The wire may be used in applications requiring fast sodering, high-safety operation, and high reliability of solder joints; in particular, it is expected that the developed flux-cored lead-free solder wire may be used in electronic and electrical industries where the aluminum-for-copper substitution is highly demanding. |
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