Understanding the effect of electromigration on the growth of interfacial reaction layers in Cu-Sn and Cu-Ni-Sn systems

In this paper the effects of chemical and electrical driving forces on the evolution of interfacial reaction layers in lead-free solder interconnections are discussed. Firstly, the effects of alloying and impurity elements on the driving forces for diffusion in the Cu-Sn system are analyzed. These results are then used to predict the subsequent changes in the intrinsic fluxes of the elements. It is shown that by utilizing simple thermodynamic reasoning together with kinetic considerations the effect of additional elements on the growth behavior of interfacial reaction layers can be readily understood or even predicted - if enough data is available. Secondly, the effect of electric current on the microstructural evolution of the interfacial reaction layers in the Cu-Sn system is considered. Especially, the differences in the growth behavior of intermetallic compound (IMC) layers at the cathode and anode sides of the interconnections are rationalized. This is done by considering the changes in the intrinsic fluxes of elements owing to electromigration as well as taking into account the fact that the growth of Cu₃Sn and Cu₆Sn₅ are coupled via interfacial reactions. When comparing the results from the calculations to those obtained from the experiements, quantitative diffusion kinetic data from the Cu-Sn system is utilized. In this way, better understanding of the effect of electron flux on the growth of each individual layer in the Cu-Sn system can be achieved. Thirdly, the combined effect of the chemical and electrical driving forces is briefly discussed. Finally, it is emphasized that the method used in this paper is a very generic one and can therefore be utilized in a wide variety of different applications.