Finite Element Analysis Of Lead-free Solder Interconnect Reliability Based On Shape And Grain Orientation

Lead-free solder interconnect, as a mechanical connection and electrical signaltransmission channel, can largely determine the reliability of device or system. Dueto environment temperature change, electronic devices in the service process willcause temperature fluctuation. During service, coefficient of thermal expansion(CTE) mismatch between different materials in electronic devices can lead to stressand strain concentration, and the creep and fatigue damage will accumulate, leadingto final failure of solder joints. The main constituent of Pb-free solder joint is β-Sn,which is body-centered tetragonal metal. There is big difference in CTE and elasticmodulus along different directions of β-Sn, showing strong anisotropy. Therefore,solder joints with different orientations show quite different thermo-mechanicalresponses and failure modes.The finite element simulation method is widely used in structural optimization.Traditional finite element simulation method is generally simplification in shape orthe structure of solder. Simplified finite element model of the structure, will oftenmake the results are inaccurate, far away from the reality. Therefore, a modelconsidered the shape and grain anisotropy is needed.In this study, Surface Evolver was adopted to simulate the three-dimensionalshape of the solder joint. Shape file was imported into ANSYS to clean, add chipand pins structure. For QFP, SOJ, BGA assemblies, electric heating and thermalcycling was computed to get the stress-strain distribution and the stress change withtime. Nanoindentation load-displacement curve of typical grain orientations werecalculated to get the elastic modulus, nano-hardness and other material information.BGA assemblies were subjected to thermal cycling, and the orientation of the solderjoints was characterized by EBSD to track the orientation evolution in differentsolder joints. Based on the shape and grain structure of real lead-free solder joints,the thermal stress and strain distribution in BGA assemblies under thermal loadingwere computed. Sub-model based on grain numbers and orientation distribution issolved to get the strain distribution of the three typical solder joints.There are huge differences in elastic modulus of typical orientations, but thereare almost the same in hardness. The experimental and simulation results show thatgrain orientation significantly influences the solder joint reliability and failure mode.For single-grained solder joints, stress and strain concentration is located in thesolder bulk near the interface, where recrystallization accompanied with initiationand propagation of cracks. However, for multi-grained solder joints, the distribution of stress and strain depends on grain orientation. Recrystallization and cracking tendto divert from the interfacial region into the solder bulk along the pre-existing grainboundary. Some special solder joints with grain boundary perpendicular to theinterface are not favorable for deformation, exhibiting higher reliability. When thegrain boundary inclined at45°to the pad, the original grain boundaries producelarge stress and strain concentration under combined action of shear stress andanisotropy of Sn grains, accelerating the crack initiation and propagation. This leadsto solder joints recrystallization and fracture occurred along the original grainboundaries, increasing the probability of early failure.