Study Of Electrcmigration Behaviour Of Electronic Solder Joints In Vehicles Under High And Low Temperature Loading

With the continuous development of China’s new fuel efficient vehicle industry,automotive electronics is developing rapidly in the direction of diversification,integration and intelligence,and the proportion of electronic products in the vehicle has increased significantly.At present,there is no lead-free solder with high reliability that can be maturely applied to automotive electronics in China,while international related enterprises develop commercial products by adding multiple alloying elements to improve the service reliability of solder,so it is necessary to study the microscopic mechanism of multiple alloying elements synergistically affecting the reliability of solder joints.In addition,research on the reliability of lead-free solder joints has mainly focused on the problems of electromigration and thermal fatigue of solder joints under the action of a single field,while the failure behaviour of solder joints under the action of multiple fields is relatively rare.Therefore,it is of great theoretical significance and practical value to carry out research on the electromigration behaviour and thermal fatigue reliability of brazing joints for automotive electronics at high current densities,and to reveal the failure behaviour of solder joints under real service environments,in order to improve the research and design of brazing joints for automotive electronics in China.In this thesis,we take LF516(Sn-3.4Ag-0.7Cu-3.2Bi-3.0Sb-0.05Ni-0.006B),a solder for automotive electronics developed independently in China,and Innolot(Sn-3.8Ag-0.7Cu-3.0Bi-1.5Sb-0.15Ni),the most widely used commercial multi-alloy solder abroad,as the object of study.The SAC305(Sn-3.0Ag-0.5Cu)solder was used as a control to demonstrate the feasibility of the application of this solder by studying its solderability.Electromigration experiments were carried out on one-dimensional linear butt joints to analyze the formation and growth of intermetallic compounds at the interface of the joints and to investigate the mechanism by which the addition of elements such as Sb,Bi and Ni affects the electromigration damage and failure behaviour of the joints.Electrical-thermal coupled field loading tests were carried out on the solder joints of the ball grid array package structure to analyze the microstructure evolution of solder joints with different crystal orientations and to investigate the failure behaviour and mechanism of solder joints when coupled with high current density loading and temperature cycling loading.The results show that the melting point and subcooling of LF516 and Innolot are lower than those of SAC305,and that Bi lowers the melting point and subcooling of the solder and improves the wetting performance between the solder and the substrate;Bi,Ni and Sb elements improve the hardness of the solder joint through second phase dispersion strengthening and solid-solution strengthening.During electromigration,the Intermetallic compounds thickness at the interface between the positive and negative electrodes of LF516 and Innolot joints is always less than that of SAC305 joints,and the cavities and obvious Intermetallic compounds dissolution at the negative electrode appear later,indicating that the addition of Ni,Sb and Bi enhances the electromigration resistance of the joints.cavities and Intermetallic compounds dissolution at the negative electrode of LF516 joints appear earlier than those of Innolot joints,and Bi is present in LF516 electromigration process produced bias,difficult to form solid solution with Sn,weakening the role of inhibiting atomic migration.Under the electric-thermal field,single crystal joints are thermally stressed and dislocations slip along small angular grain boundaries,causing grain rotation.The polycrystalline joints are subjected to thermal stresses that are released along the grain boundaries and the surface bumps are thus extended along the large angular grain boundaries.The addition of alloying elements increases the strength of the Sn matrix in LF516 and Innolot joints,which increases the ability to resist plastic deformation and increases the ability of the solder joint matrix to withstand lattice distortion energy,resulting in a concentration of stress at large angular grain boundaries.The Sn matrix in SAC305 is not strong enough to resist plastic deformation and the Sn matrix releases lattice distortion energy through plastic deformation.The fatigue failure of the solder joint is exacerbated by the thermal effect of the high current density.In addition,the thermal effect of the current accelerates the failure process of polycrystalline solder joints due to the uneven distribution of the current at the grain boundaries.