Study Of The Microstructure And Mechanical Properties Of SAC305-nano Cu Composite Solder Paste

Sn-Ag-Cu(SAC) lead-free solder because of its environmental characteristics and excellent welding performance is widely used. With the electronics industry continue to miniaturized development, put forward higher requirements of the micro solder joint reliability. In order to improve the microstructure and properties of SAC305 lead-free solder. This paper to SAC305 alloy powder for solder matrix and Cu nanoparticles for reinforcements, the mechanical mixing method be used to produce SAC305-nano Cu composite solder paste, and investigated the melting characteristics, wettability, solder joint porosity, shear properties, microstructure and interfacial IMC evolution of the composite solder paste.The results show that the addition of Cu nanoparticles into SAC305 solder paste resulted Cu nanoparticles and the solder matrix in the reaction of Sn formed tiny Cu6Sn5 compound. The Cu6Sn5 compound as nucleation point dispersed in the solder matrix, which refined the microstructure of composite solder paste.The refining effect with the Cu nanoparticles adding amount increasing is more significant. At the same time, part of tiny Cu6Sn5 compound particles adsorbed on the surface of Ag3 Sn compounds, which hindered Ag3 Sn growth, so that the Ag3 Sn compounds are refined.The addition of Cu nanoparticles, which had a minor effect on the solder interface IMC morphology. However, the interfacial IMC thickness increased,and with the adding dosage increases. When the adding amount is 0.5 wt%, the interfacial IMC thickness than without the addition of SAC305 solder paste increased 0.91 ?m. In 150 ? high temperature aging process, due to the Cu nanoparticles are added to increase the concentration of Cu element in the solder matrix. Therefore, Cu concentration gradient was reduced on the interfacial IMC,which reduced the diffusion rate of Cu element. Thereby, the growth of interfacial IMC was inhibited, and the inhibition effect with Cu nanoparticles addition is more remarkable.Through the porosity test of the composite solder joints discover that: the porosity increased with Cu nanoparticles content increases. Small amount of nanoparticles had a minor effect on solder joint porosity. When the amount is too large, the porosity of solder joint will increase highly, and engendered large pores.The reason was that the Cu nanoparticles and solder of Sn reaction formed Cu6Sn5 compounds with high melting point in the welding process. Cu6Sn5 compounds decreased the fluidity of liquid solder, which hindered the pores discharge. Because of the surface free energy of Cu6Sn5 compound is much higher than pore, so the Cu6Sn5 compound easily gathered on the surface of the pores and inhibited the excretion of pores.In the fine grain strengthening and dispersion strengthening effect, the shear strength of the composite solder paste was increased to a certain with the addition of Cu nanoparticles. Meanwhile, the increase of the porosity resulted in the mechanical properties of joint deterioration. The combination of these two factors, The solder joints had excellent mechanical properties when the addition was 0.5 wt%, and the shear strength improved 10 % than SAC305 solder paste.