| Because of the legislative concerns of environmental protection and the requirement of high-integrated microelectronics development, the research and development of new lead-free solder has become one of the critical issues in electronics and materials industry. Eutectic Sn-9Zn alloy is widely recommended as the promising lead-free solder of the next generation, owing to its low cost, favorable melting point and good mechanical properties. Howerer, Sn-9Zn alloy is poor in wettability and Zn is easy to be oxidized, which is a major obstacle to its applications. In recent years, some progress has been made to improve the wettability and other properties of Sn-Zn alloy, but it has not been fundamentally resolved.On the basis of summarizing the previous study of Sn-Zn alloys, this paper has explored and researched the using of Cr micro-alloying method to enhance the oxidation resistance and further improve the wettability of Sn-Zn-Bi alloy. In addition, the interfacial reactions and IMCs growth behaviors of Sn-Zn-Bi alloy with Cu or Ni substrate have been studied through long-term aging method. And the influence of Cr on solder joint reliability has been studied.The study shows that: adding a small amount of Cr can significantly improve the oxidation resistance of Sn-Zn-Bi alloy and restrain the growth of the surface oxide film; adding Bi can reduce the surface tension of the solder alloy. Adding Bi and Cr at the same time can significantly improve the wetting property of Sn-Zn alloy. Sn-8Zn-3Bi-0.3Cr alloy has the best wettability.Adding a small amount of Cr can significantly refine the microstructure and improve the mechanical properties of Sn-Zn-Bi alloy. Sn-8Zn-3Bi-0.1Cr alloy has the best mechanical properties, the ultimate tensile strength is reached to 76MPa and the elongation is up to 50% during 2 mm/min tensile speed in room temperature. A lot of large brittle cleavage surfaces are observed in the fracture surface of Sn-8Zn-3Bi alloy, and the fault mechanism is quasi-cleavage fracture. Whereas only very fine dimples can find in the fracture surface of Cr containing alloy, which shows the fault mechanism is ductile fracture.The IMCs grows during aging at 150℃, and the thickness in the Sn-8Zn-3Bi-0.3Cr solder joint is much thinner than Sn-8Zn-3Bi solder joint during the same time. The IMCs growth rate at the Sn-8Zn-3Bi-0.3Cr/Cu interface was about 1/2 times than Sn-8Zn-3Bi/Cu interface, while 1/4 times at Ni plating interface during aging at 150°C. During long-term aging, Cr react with Zn and form Sn-Zn-Cr phase in the solders, this solid phase transition controlled the diffusion of Zn atom to the interface, thus indirectly controlled the IMC growth rate of Cr containing solder joints. |
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