无铅焊点在热疲劳和电迁移作用下的组织演变

研究了Cu/Sn-58Bi/Cu焊点接头在室温和55℃下通电过程中阴极和阳极界面处微观组织的演变,电流密度均采用104A/cm2。结果表明,室温条件下通电达到25 d,Bi原子由阴极向阳极发生了扩散迁移,在阳极界面处形成了厚度约22.4μm的均匀Bi层,而阴极出现了Sn的聚集。加载55℃通电2 d后,焊点发生了熔融,阴极界面处形成了厚度为34.3μm的扇贝状IMC,而阳极界面IMC的厚度仅为9.7μm。在IMC层和钎料基体之间形成了厚度约7.5μm的Bi层,它的形成阻碍了Sn原子向阳极界面的扩散迁移,进而阻碍了阳极界面IMC的生长,导致了异常极化效应的出现。

Microstructure evolution of lead-free solder joints under the coupled action of the thermal fatigue and electromigration

The microstructural evolution in Cu/Sn-58Bi/Cu solder joints was investigated during electromigration at room temperature and 55 ℃.The current density was 104 A/cm2.Experimental results indicate that Bi atoms migrate from the cathode to the anode with low speed,which forms a uniform Bi layer at the anode interface with thickness of 22.4 μm after current stressing for 25 d at room temperature.However,Sn accumulation forms near the cathode.The solder joint is melted after current stressing for 2 d at 55 ℃.A thick scallop-type IMC forms at the cathode interface with average thickness of 34.3 μm,but the IMC thickness at the anode interface is merely 9.7 μm.A Bi layer with thickness of 7.5 μm forms between the IMC layer and the solder matrix.The diffusion and migration of the Sn atoms to the anode interface are hindered because of the Bi layer formation,which blocks the IMC growth at the anode interface,leading to the abnormal polarity effect.