Investigation Of The Wettability And Interfacial Reactions Between Sn-0.7Cu (-0.5Ni) Solder And Cu Substrate With Thermoelectric Coupling Field

The rapid development of the electronics manufacturing industry has put forward the new requirements for the research progress on lead-free solders. Sn-Cu serials solder has been the focus of scientific researches and commercial developments because of its high performance/cost ratio among the many types of lead-free solders. How to improve the reliability of solder joints is a hot topic. The solder wettability on the surface of the substrate and the interfacial reactions are the two key factors of the solderability and the joints reliability. So it has a significant meaning for investigating how to improve the reliability of the solder joints. As it shown in the literatures, the electromigration occurring in the molten solders is different from that in the solid state. Research about liquid electromigration can help investigate the evolution of the organization and in-depth understanding of the soldering process mechanism solid electromigration. So it has a positive influence on the design of new lead-free solder and brazing process improvement to study the interface reaction、 soldering process、 wettability and electromigration under the thermoelectric coupling.Firstly, the experiments for studying the wetting behaviors and interfacial reactions between the Sn-0.7Cu lead-free solder and pure Cu substrate were carried out. Then the wetting angles between the solder and Cu substrates were measured by Optical Contact Angle Measuring Device. Also the electron scanning microscope was used to observe cross-sectional morphology of the joints. The microscopic mechanism of wettability and wetting process of solder and Cu substrate were analyzed, especially the roles of the electronic currents were discussed. The results show that the direction and density of the current have a great influence on the wettability between the solder and substrate. When the substrate is an anode pole, the currents enhance the solder wettability on the substrate, but when the substrate is a cathode pole the result is opposite; meanwhile the polarity of the substrates are the same, the amplitude of the enhanced the wettability of the solder increase with the current densities increasing.Secondly, the interface reactions between the Sn-0.7Cu solder and pure Cu substrates were also studied, and the growth pattern of IMC was analyzed. So the current roles played in the processes were investigated by analyzing the comparative results of similarities and differences in the IMC growth in two conditions. Meanwhile, the impacts of the solder joints structures on IMC growth was also studied. Investigation shows that the morphology of IMC layer are related to the solder time, while the IMC inner the solder joints are typical hollow hexagonal prism when no current is applied. However, when the currents are applied, IMC layer adhered to the cathode interface takes the shape of fine prismatic and the layer is thin, while IMC layer adhered to the anode interface is thicker and smoother. IMC internal solder joints formed of elongated IMCs with the current. IMCs formed inner solder joints of Cu/Sn-0.7Cu/Cu sandwich structures do not appear to be elongated morphology under the current.Finally, in this paper the influences of the addition of0.5wt.%Ni in the Sn-0.7Cu solder on the interfacial reactions were investigated, in view of the Sn-0.7Cu solder added a third component Ni element will effectively improve wettability, oxidation resistance, mechanical properties, the element migration phenomenon and its mechanism under the effects of thermoelectric coupling effects after the addition of Ni were selective analyzed. The results show that the addition of0.5%Ni element in the Sn-0.7Cu solder, the IMC layer turned out to be smoother and more compact. If the directions of the thermal and electrical current are the same, the two effects could promote each other, and the migrations of the elements are enhanced; and if the directions are opposite, the results are also on the contrary.