Research And Development Of Tin-based Solders For Low-temperature Soldering Aluminum Assemblies In Base Station Antenna For Mobile Communications

In recent years,the mobile communication technology has been developing rapidly.The fifth generation（5G）mobile communication technology has gradually matured and become commercialized,and consequently the antennas will be densely distributed for large-scale application.Due to the advantages of lightweight,high specific strength and low cost of aluminum and its alloys,they are often used in the connection between the aluminum phase shift module and the coaxial cable.When tin-based solders are used for soldering aluminum and its alloys,there has always been a concern for poor wettability and low corrosion resistance of the solder joints,which seriously reduce the processing reliability of the solders and the service reliability of the base station antenna.Therefore,the development of low-temperature tin-based solders with good processing performance and high service reliability is significantly important for engineering applications.In this thesis study,6061 aluminum alloy is used as substrate and the conventionally used Sn3.5Ag and Sn0.7Cu solders are modified by adding Zn element.The effects of Zn content on the microstructure,melting and solidification characteristics and wettability of the bulk solders are investigated systematically.Further,the effects of Zn content on the interface morphology,mechanical properties and corrosion resistance of solder joints are studied.Finally,the Sn-Zn-Ag solders are selected for multi-component alloy design,the influence of Ag content on the mechanical properties and corrosion resistance of the solder joints is studied,and the corrosion failure mechanism of the solder joints is discussed.The main results obtained from this thesis study are summarized as follows:In the Sn3.5Ag–x Zn alloy system,Zn and Ag tend to form an Ag Zn phase.Zn lowers the melting point and improves the corrosion resistance of the solders.After adding Zn element,the interfacial IMC of the solder joint changes from Ag₂Al to Ag Al Zn.When the Zn content is1 wt.%,the wettability of the solders,shear strength and corrosion resistance of solder joints are the best.In the Sn0.7Cu–x Zn alloy system,Zn and Cu tend to form a Cu–Zn phase.Zn lowers the melting point but increases the melting range of the solders.As the Zn content increases,both of the wettability and corrosion resistance of the bulk solders increase firstly and then decrease.When the Zn content is 1 wt.%,the solder shows the best wettability and corrosion resistance.When the Zn content is 0～3 wt.%,the solder and Al substrate are connected mainly by mechanical interlock.While for a Zn content of 4 and 5 wt.%in the solders,the metallurgical bonding is obtained at the interface between the solder and Al substrate through the mechanism of formation of Al–Sn–Zn solid solution.With the increase of Zn content,the shear strength and corrosion resistance of solder joints can be improved.In the Sn–Zn–Ag ternary alloy system,the corrosion behavior occurred at the interface of the solder joints is mainly caused by the galvanic corrosion and the chloride erosion.The corrosion resistance of the solder joints mainly depends on the distribution and thickness of the interfacial Ag Al Zn and the Sn-rich interlayer.When the Ag content increases from 1.5 to 3.0wt.%,the IMC morphology changes from island chain-like discontinuous distribution to layered continuous distribution,and the thickness of the Sn-rich interlayer decreases,resulting in an accelerated corrosion mode of"large cathode and small anode"during the corrosion process.When the Ag content is 1.5 wt.%,the shear strength and corrosion resistance of the solder joints increase obviously;while when the Ag content is 3.0 wt.%,the shear strength and corrosion resistance of the solder joint decrease significantly.