Investigations On Interfacial Microstructures And Mechanical Properties Of Sn-3.0Ag-0.5Cu Solder/Cu Joints

The reliability of solder joints in the electronic products directly affects the service life of electronic products, and the reliability of interconnected solder joints is closely related to the microstructures of intermetallic compound layers and mechanical properties at the solder/substrate interface.In the present study, a series of experiments were first designed to explore the influence of reflow time on interfacial microstructures and tensile mechanical properties of Sn-3.0Ag-0.5Cu lead-free solder/Cu single crystal joints at a reflux temperature of 260 ℃. It has been found that the interfacial intermetallic compound layer of Sn-3.0Ag-0.5Cu lead-free solder/Cu single crystal joints is mainly composed of Cu6Sn5 grains at the reflow time range of 30-600 s, and the interfacial intermetallic compound layer becomes thicker and denser with the extension of reflow time due to the growth of Cu6Sn5 grains. The tensile strength of welding joints increases as the reflow time is prolonged due to the formation of dense Cu6Sn5 compound layer. There is no obvious change in the yield strength of the welding joints as the reflow time is at the range of 30-300 s, but the yield strength slightly increases at 600 s. The cracks formed during tensile deformation appear primarily at the interfaces of Cu6Sn5 compound layer and Cu substrate and their formation is related to the collision of slip bands in the Cu single crystal matrix to Cu6Sn5 compound layer.The influences of the welding angle, which is the angle between [034] tensile direction of Cu single crystal and welding surface, on the interfacial microstructures and mechanical properties of Sn-3.0Ag-0.5Cu lead-free solder/Cu single crystal joints obtained at a reflow temperature of 260℃ and time of 1200 s were studied, and it is found that there is a dense and thin compound layer of Cu3Sn near the Cu substrate, whose thickness is about 0.5 μm. The next is Cu6Sn5 compound layer. Inside the solder, there are Cu6Sn5 and granular or rod-like Ag3Sn particles. The welding angle has a significant effect on the size, shape and distribution particles in Cu6Sn5 compound layer. With the decrease in welding angle, the size of particles reduces, and their distribution becomes more uniform; meanwhile, the number of Cu6Sn5 particles, which grow fast into the solder, also decreases, especially at the welding angles of 60° and 30°. The yield strength of the welding joints increases with decreasing welding angle, while the tensile strength is higher at the welding angles of 60° and 30°, medium at 45°, and minimum at 90°. The cracks generated during tensile deformation mainly appear inside Cu6Sn5 particles and at the interfaces between Cu6Sn5 particles and the solder, and the propagation direction and number of cracks vary with the welding angle. The cracks parallel to the welding surface in Cu6Sn5 particles are dominant and the amount of such cracks is the greatest at 90°. With the decrease in welding angle, the number of cracks inside Cu6Sn5 particles decreases and the cracks mainly propagate toward the direction perpendicular to welding surface.The interfacial microstructures and mechanical properties of Sn-3.0Ag-0.5Cu lead-free solder/Cu polycrystal joints prepared at a reflow temperature of 260℃ and time of 1200 s at different welding angles were further investigated, and it is revealed that the morphologies of interface compound layer and tensile strength of welding joints are nearly independent upon the welding angle.