结构变化对Cu/Sn-58Bi/Cu微焊点电迁移行为和组织演变的影响

INFLUENCE OF SOLDER JOINT CONFIGURATION ON ELECTROMIGRATION BEHAVIOR AND MICROSTRUCTURAL EVOLUTION OF Cu/Sn-58Bi/Cu MICROSCALE JOINTS

利用SEM观察、聚焦离子束(FIB)微区分析和有限元模拟对比研究了直角型和线型Cu/Sn-58Bi/Cu微焊点在高电流密度下(1.5×10~4A/cm^2)的电迁移行为,从原子扩散距离和微区域电阻变化及阴阳极物相变化的角度研究了焊点结构变化对电迁移影响的机理.结果表明,2种焊点通电112和224 h后均发生了Bi向阳极迁移并聚集及Sn在阴极富集的现象;直角型焊点阳极由于Bi聚集后膨胀而产生压应力进而导致小丘状凸起和微裂纹出现,而阴极存在拉应力引发凹陷和微裂纹,且沿界面呈非均匀变化.微区组织分析表明,电迁移作用下焊点内部Bi原子的扩散速度大于Sn原子的扩散速度.观察分析和模拟结果还表明,具有结构不均匀性的直角型焊点中电子流易向电阻较小区域聚集而产生电流拥挤效应,这是引起直角型焊点电迁移现象严重的根本原因.

With the increasing miniaturization of electronic devices and systems,the pitch size and dimension of solder interconnects become smaller,accordingly the current density in solder interconnects gets higher and this results in severe electromigration（EM） effect that may deteriorate the performance of solder interconnects.The studies on flip chip interconnects have shown that the configurable change of solder joints can significantly affect the electromigration behavior.In this study,the microscale Cu/Sn-58Bi/Cu joints with different geometrical configurations,i.e.,right angle-type and line-type solder joints,were designed and the electromigration behavior of joints under a direct current of a density 1.5×10~4 A/cm^2 were investigated by SEM observation,microanalysis based on focused ion beam（FIB） and finite element simulation.The focus was placed on clarifying the influence of the solder joint configuration on the electromigration mechanism of the joint in terms of atomic diffusion distance,microregional resistance change and the change of phases in anode and cathode.Results showed that for both types of solder joints after current stressing for 112 and 224 h,Bi migrated to the anode side and congregated there,while Sn tended to enrich near the cathode side;in particular for the right angle-type solder joint the microscale hillocks and microcracks occurred at the anode side caused by the compressive stress which was attributed to Bi congregation and the consequent volume expansion of the phase,while the microscale concave valleys and microcracks appeared at the cathode side caused by the tensile stress,and it was worth noticing that the phenomenon above happened non-uniformly along the interface in right angle-type joint.The microanalysis results revealed that the diffusion velocity of Bi atoms was faster than that of Sn under current stressing in the solder joint.Furthermore,observations and finite element simulation results showed that for the solder joint with an asymmetrical configuration like the right angle-type solder joint the electrons flowed toward the bottom corner of the joint where the resistance was smaller and thus the current crowding effect occurred,and this was the primary factor for causing the severe electromigration.