Study On Electromigration Damage Behavior Of Sn-based Lead-free Solder Joints

Microelectronics manufacturing technology was continuously along the road to conform to the development of the society,toward lead-free,miniaturization.It makes the components volume reduced gradually and increases packing density,which means that the size of the solder joints are getting smaller,and the electrical,mechanical and thermal load can be increased.It is a higher requirement for the reliability of solder joints.Due to that Sn grains in lead-free Sn-based solder joints are anisotropic,grain orientation seriously affects the resistance to electric migration of solder joints.In addition,the undercooling,melting point and other basic properties of solder also affect the grain orientation of solder joints.Therefore,combined with the basic properties of solder and grain orientation,it is of great significance to further understand the electromigration damage behavior of lead-free linear solder joints.In this thesis,Sn3.5Ag0.5Bi8.0In solder,Sn44Bi0.32Ag solder and SAC305solder are mainly used as research subjects.The basic properties of different solder are studied by reflowed solder balls and linear solder joints.The results show that the melting point of Sn3.5Ag0.5Bi8.0In solder decreases by 20℃and the undercooling decreases by about 15℃compared with that of SAC305 due to the addition of Bi and In elements with low melting point.Due to the addition of Bi and In,the microhardness of solder ball is higher.Compared with SAC305 solder,Sn3.5Ag0.5Bi8.0In solder has lower undercooling,higher nucleation difficulty,slower nucleation rate and lower release of latent heat of crystallization during rapid cooling.The latent heat of crystallization is not enough to completely melt the other nucleated grains in the solder joint.The number of grains which are not completely melted in the solder joint will increase,eventually forming a"Crossing"orientation solder joint.With the increase of reflowed temperature,the release time of latent heat of crystallization in the solidification process of solder joints is prolonged.The crystal orientation in solder joints will rotate to the position with the lowest energy of the whole system,and eventually form"Multiple double-Twin"oriented solder joints.In addition,single crystal Cu as solder pad can affect the formation of grain orientation of SAC305 and promote the formation of multi-grain structure of solder joint.In addition,the electromigration experiments of lead-free linear solder joints are also carried out,including that the changes of crystal orientation electromigration of solder joints,the changes of microstructure in solder joint and the growth behavior of intermetallic compounds（IMC）at the solder joint interface are analyzed.The“Crossing”orientation obtained by reflowed Sn3.5Ag0.5Bi8.0In solder is easy to change during electromigration.Due to the small size of the grain,the equilibrium state of the grain is broken,the grain rotates,the grain orientation of the solder joint changes.The surface of the solder joint is raised or depressed due to the influence of the current stress,back stress and thermal diffusion force during the electromigration process.The grain size of the solder joints of SAC305 and Sn44Bi0.32Ag solder is large,which can resist a large total torque,and the grain is not easy to rotate.However,the IMC accumulation will occur at the grain boundary due to different grain orientations and different Diffusion rates of Cu atoms during the electromigration process.The morphology of solder joints at the interface will change greatly when the electromigration is long time.At the anode interface,IMC increases greatly due to the aggregation of Cu atoms,resulting in IMC accumulation,and the growth rate of IMC was expressed by Fick’s second law.At the cathode interface,Cu atoms on the cathode side rapidly diffused into the solder joint,and IMC was dissolved to form dendrites at the early stage of electromigration,resulting in the formation of isolated Cu₆Sn₅ grains at the cathode interface.With the increase of electromigration time,IMC disappears completely and Cu pad begins to dissolve,cathode dissolution occurs.