Research On Electromigration Failure Of Lead-free Solder Joints Considering Grain Structure

At present,the mainstream lead-free solder joints are generally based on tin.Because of the fact that the content of Sn usually accounts for 95%（mass fraction）of the components of the lead-free solder alloy,lead-free solder joint performance and resistance depend on the performance of Sn itself.Based on theβ-Sn crystal structure,the life time and failure modes of the lead-free solder joints of SAC305 under electromigration were studied.The main work and results are as follows:（1）Lead-free solder electromigration experimental platform was built.The self-designed BGA packaged samples were subjected to electromigration acceleration experiments under constant temperature.In order to analyze the influence of Sn grain structure on the electromigration failure process of lead-free interconnect solder,scanning electron microscopy（SEM）and electron backscatter diffraction（EBSD）were used to observe and characterize the Sn grain structure in lead-free interconnect solder joints.（2）Through statistical analysis of the grain structure of solder joints,it was found that the number of grains contained in most solder joints is limited,and the number is usually 1².Through the electromigration acceleration test of the sample,it was found that theβ-Sn grain orientation of single grain structure solder joints with the structure of Cu/SAC305/Cu（Ni）is the decisive factor for different failure modes and different IMC expansion modes of the solder joints.When the c-axis and electron flow direction of the Sn crystal grain are nearly vertical,the failure mode of the solder joint shows that the cathode interface generates a cavity and expands into crack propagation.When the c-axis of the Sn crystal grain and the electron flow direction are nearly parallel,the failure mode of the solder joint is that the substrate is largely dissolved,and diffusion to the anode eventually causes a large amount of IMC to precipitate at the grain boundary.（3）Based on the finite element analysis software,taking into account the different grain structure of solder joints,the migration failure simulation of SAC305 lead-free solder joints was performed using the Atomic Density Integration（ADI）method.Bunge Euler angles（₁?,（37）,?₂）were used to characterize the orientation of the crystals.The results show that the normalized atom density of the single grain structure solder joints and the solder joint failure lifetime largely depend on the crystal orientation（In particular,the angle,（37）,the angle between the c-axis orientation of the crystal and the current direction,which characterizes the rotation of theβ-Sn grains around the c-axis）.With a dual grain structure solder joint with a 60°misorientation,the relative position of the current direction relative to the grain boundary affects the distribution of the atomic density within the solder joint.