Study On Microstructure And Properties Of Low Silver Lead-free Solder Joints Containing Bi Under Thermal Fatigue Conditions

With the rapid development of the electronics industry,research and development of new lead-free materials is essential to improve solder quality.Sn-Ag-Cu material is a promising lead-free material,but its high cost limits its wide application.Therefore,it is imperative to research and develop the second generation of low-silver-tin-silver-copper solder materials.In this thesis,the performance of a new low-silver type lead-free solder material was investigated by adding different contents of Bi elements.First,the microstructure of Sn-1Ag-0.5Cu-x Bi alloy was observed by means of metallographic microscope(OM)and scanning electron microscope(SEM),and the microstructure of different solder joints was treated by thermal aging.It is found that the addition of Bi element significantly reduces the size of Sn matrix grains and intermetallic compounds.When the content of Bi element reaches 5wt.%,an obvious Bi diffusion barrier layer can be formed on the surface of intermetallic compounds at the solder joint interface,which hinders the migration of atoms.The shear strength of Sn-1Ag-0.5Cu solder joints was significantly improved by Bi element through fine grain strengthening,solid solution strengthening and precipitation strengthening.The shear strength of 5wt.% solder joints was the highest,and the shear fracture showed obvious brittle characteristics,followed by 2wt.%.The lowest shear strength was Sn-1Ag-0.5Cu solder joints,but its fracture showed obvious ductile fracture.However,excessive Bi element is not conducive to the thermal fatigue performance of solder joints.Secondly,using electron backscatter diffraction(EBSD),the change in grain orientation of the Sn-based brazing material after thermal fatigue treatment was observed,and the effect of different solid solution conditions on the grain orientation of Sn and the diffusion of Bi elements was investigated to gain insight into the effect of Bi elements on the thermal fatigue performance of the low-silver solder joints.The thermal fatigue failure modes of micro-solder joints with different Bi contents are obviously different,when there is no Bi element the joints exhibit a typical through-crystal fracture mode,after the addition of Bi element the joints exhibit a along-crystal fracture mode,but the Bi particle clusters formed by excessive Bi element become brittle points for the through-crystal fracture of the joints.Based on the EBSD data analysis,the recrystallized grains of low silver Pb-free solder joints containing Bi evolve under the combined effect of thermal-mechanical stress through the continuous recrystallization process of subgrain rotation and the discontinuous recrystallization process of deformation storage,and the formation of recrystallization and crack extension significantly deplete the dislocations within the solder joints.The addition of excessive Bi element provides a heterogeneous nucleation point for the formation of a large number of fine recrystallized grains,which is detrimental to the thermal fatigue of the solder joints.This is detrimental to the thermal fatigue properties of solder joints.The addition of Bi elements significantly increased the hardness and modulus of SAC105 solder joints,thus improving the creep resistance of the brazing material,but the thermal fatigue caused a decrease in the hardness and modulus values of the solder joints.