Influences of the initial thickness of the interfacial IMC layer on electromigration behavior of Cu/Sn/Cu microscale joints

The rapid development of modern electronic devices and products has proposed a great demand for the continuous scaling down in the dimension of solder joints and interconnect pitches in packaging; accordingly the current density in solder interconnects gets higher and higher. The higher current density can easily result in electromigration (EM) which has been regarded as a serious reliability issue. It is common that the thicknesses of the interfacial IMC layers at both interfaces of an as-reflowed solder interconnect are different. In this study, the effect of the initial IMC thickness on the EM behavior of solder interconnects was investigated by SEM/EDX/FIB. In order to clarify the influence of the initial thickness of interfacial IMC layers on EM behavior, the joint samples were divided into two types, A and B, i.e., the Type-A joints have a thick interfacial IMC layer at the cathode interface, while the Type-B joints have a thick one at the anode interface. The results show that after current stressing the thickness of the thick IMC layer at the cathode interface of Type-A joints decreases obviously, while the thin IMC layer at the cathode interface of Type-B joints hardly changes. The results of the FIB/SEM analysis show that Kirkendall voids occurred near the IMC/Cu interface at the anode of Type-A joints, whereas hardly seeing the voids in Type-B joints. The analyzed results show that the faster dissolution of IMC layer at the cathode of Type-A joints is caused by the higher joule heating and the higher EM flux; the Kirkendall voids at the anode of Type-A joints are caused by the higher backstress gradient.