Effect Of Microstructure On Mechanical Properties Of SAC305 Solder In Corrosive Environment

Lead-free solders are widely used in electronic devices as packaging materials that meet the needs of modern human health and environmental protection.However,electronic products often face complex service environments in the process of production and use,and lead-free solder joints,as the main bearing parts,bear the mission of electrical connection and mechanical connection,so the reliability of lead-free solder cannot be ignored.Especially in recent years,the corrosion problem is becoming more and more prominent.Lead-free solder joints may be directly exposed to corrosion medium,the corrosion defects and corrosion products will directly affect the reliability of solder joints.In addition,when the corrosion environment and applied stress together may cause extremely serious damage to solder joints,which directly affects the performance of electronic devices.Based on this background,the effects of different microstructure morphologies on the mechanical and corrosion properties of materials were investigated.In addition,the electrochemical corrosion behavior,corrosion fatigue and stress corrosion behavior of lead-free solder in simulated marine environment were studied simultaneously.The interaction between corrosion and stress was also explored.The research contents and main conclusions are as follows:(1)The materials of different microstructure were obtained by controlling the different cooling rates,manifested as the morphologies and sizes of the second phase Ag₃Sn.That resulted in different corrosion and mechanical properties.In the immersion tests,when the Ag₃Sn appear in a larger size,it was easy to form galvanic corrosion and accelerated the dissolution of the surrounding matrix Sn.The electrochemical tests shown a lower self-corrosion potential and a higher self-corrosion current density and a smaller impedance radius,which all illustrated the decrease of corrosion resistance.In addition,the larger size of Ag₃Sn will reduce the mechanical properties,and the irregular deformation of SAC305 is characterized by microscopic analysis.(2)Based on the digital image technology,an in-situ mechanical test system for on-line observation in corrosive environment was constructed by using microscope.The polarization curve shown that the loading reduced the corrosion resistance of SAC305,which made the self-corrosion potential negative decrease and the corrosion current density increase.In addition,it is pointed out that the rupture of corrosion products film was the cause of corrosion resistance decline through on-line observation tests.And SEM was used to compare the effects of different loading forms on the growth,rupture and distribution of corrosion products.In addition,the strain field under different loading conditions were analyzed by DIC technique to visualize the strain and provided the basis for microscopic observation.(3)Through the synchronous analysis of the corrosion current density and strain curve,it is found that the cyclic loading condition made the corrosion current density fluctuate periodically.The applied potential affected the fluctuation range of corrosion current density,and the corrosion current density value was the highest under the action of anodic potential,indicating that the dissolution effect was the largest.In addition,the interaction of stress and corrosion reduces the fatigue life of the material.Electrochemical corrosion caused the corrosion products formed on the surface,and the stress caused the corrosion product film to be destroyed.The region of large deformation was severely destroyed,the lattice defects were more,and the atomic activity was higher.The fresh tissue was constantly exposed and the corrosion extended to the substrate,which accelerated the failure of the material.