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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.
Micro-area analysis and finite element simulation of FIB and SEM were used to study the effect of the micro-spot and linear Cu / Sn-58Bi / Cu micro-spot welds at high current densities of 1.5 × 10-4 A / cm2 ), The mechanism of the effect of the solder joint structure on the electromigration was studied from the perspective of the atomic diffusion distance, the change of the resistance of the micro-region and the phase change of the cathode and the anode. The results showed that after the two kinds of solder joints were energized for 112 and 224 h The phenomenon of Bi migrating to the anode and concentrating and the enrichment of Sn in the cathode occurred. The anode of the right-angle solder joint produced compressive stress due to the expansion of Bi, which then led to the occurrence of hillocks and micro-cracks. The tensile stress of the cathode caused the depression And micro-cracks, and non-uniform changes along the interface.Microstructure analysis shows that the diffusion speed of Bi atoms within the solder joint is larger than that of Sn atoms under the action of electromigration.The observation and simulation results also show that the Bi atoms with structural heterogeneity Of the right-angle solder joints in the flow of electrons tend to gather smaller areas of current crowding effect, which is caused by the right-angle pad electromigration phenomenon serious root cause.