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Interfacial reactions and microstructural evolution of BGA structure Cu/Sn3.0Ag0.5Cu/Sn58Bi/Cu mixed assembly joints during isothermal aging

In order to reduce the cost of solder materials and retard the thermal shock of temperature-sensitive electronic components during packaging process, SnBi solder paste was explored to replace SnAgCu solder paste due to the low melting temperature. In this study, BGA structure Cu/solder-ball/solder-paste/Cu joints were designed and prepared to study the interfacial reactions and microstructural evolution of the mixed assembly joints, by using homogenous Sn3.0Ag0.5Cu (SAC305) and Sn-58Bi (SnBi) solders in the forms of both ball and paste, during isothermal aging. Results show that microstructural coarsening occurs in Cu/SAC305-ball/SnBi-paste/Cu, Cu/SAC305-ball/SAC305-paste/Cu and Cu/SnBi-ball/SnBi-paste/Cu joints and the thicknesses of interfacial IMCs increase with prolonging the isothermal aging time. Crack may form in the fusion region between the SnAgCu and SnBi-SnAgCu mixed solders in Cu/SAC305/SnBi/Cu joints after aging at 125 °C for 150 h due to the difference in coefficient of thermal expansion (CTE) between the SAC305 and SnBi solders, which may significantly weaken the reliability of mixed assembly joints. Moreover, in the solder matrix of Cu/SAC305/SnBi/Cu joints, a large amount of bulk Cu6Sn5 phase exists due to the limited solubility of Cu in the SnBi matrix. In addition, the growth of interfacial IMC layer at both solder-ball/Cu and solder-paste/Cu interfaces of the joints is mainly controlled by bulk diffusion.