Study Of Shearing Strengthen And Interfacial Microstructure Of Sn-9Zn/Cu Joints

Electronic products are leading the tendency of miniaturization andlightweight, which requires a much smaller joints size of the electronic products,causing the bearing of mechanics and the heat load become heavier and thereliability of BGA （Ball Grid Array） joints is in the highlight. Therefore, basedon the formation mechanism of voids in the BGA joints, the influence of thereflow temperature curve on the formation of voids was studied in this paper. Byanalyzing the Sn-9Zn/Cu joints with the BGA diameter700,1000and1300m,the changes of the shearing strength and interfacial microstructure caused by thesolder diameter are revealed.The research of voids in BGA joints shows that it can reduce the formationof voids in Sn-9Zn/Cu joints when the reflowing temperature is255℃, staying100s, and the voids in joints can be accepted by IPC-7095A standards.The study of joints’ shearing strength demonstrates that after welding, theaverage shearing strength of the joints is49.9MPa, and after aging at150℃for160h, the average shearing strength is21.5MPa. However, there is a littledifference in shearing strength among the joints in different diameter under thesame aging treatment.The study in fracture surface micro-morphology of joints reveals that afterwelding, the fractured model of the joints in different diameter is ductile fractureoccurred in the solder, and after aging, there are second-phase particles in thebottom of fractures’ dimple, and the fractured model is a mixed ductile and brittlefracture occurred in the interface.The result of IMC （Intermetallic Componds,IMC） layers’ thickness of jointsstates that with the same aging treatment, the thickness of IMC layers increase asthe joints diameter increasing, with the same diameter of joints, joints’ IMClayers thickness is larger after aging than after welding. Study of the type and distribution of joints’ IMC layers indicates that theshearing strength can be reduced if the compounds Cu₅Zn₈are too thick or in freestate. After aging, the mixed phase of Cu₅Zn₈and Cu₆Sn₅exists in the IMC layers,which can also reduce the shearing strength of joints.