Electromigration Effects upon Cu/Ni/Sn-Ag and Cu/Ni/Sn-Ag-Cu Interfacial Reactions

Electromigration refers to the motion of atoms under the influence of an electric field. The electromigration effect most often is a result of momentum transfer from electrons, but it can be also caused by electrostatic force. The electromigration effect was discovered by Gerardin in 1861 [1]. The electromigration effect upon interfacial reactions and the resulted polarity phenomenon were first reported by Chen et al. [2,3], although electromigration effect upon the microstructures of a conduction line, such as void and whisker formation, had been intensively investigated much earlier. Understanding of electromigration effects upon interfacial reactions have become even more important nowadays because of thinner conduction lines, more interconnection joints, and higher electric current densities in the modern electronic products, such as products of 3D packaging. The interfacial reactions with and without passage of electric currents in the Sn-Ag/Ni/Sn-Ag, Sn-Ag/Ni/Cu/Ni/ Sn-Ag, Sn-Ag-Cu/Ni/Sn-Ag-Cu and Sn-Ag-Cu/Ni/Cu/Ni/Sn-Ag-Cu sandwich-type samples were examined at 150, 180 and 200oC. The main focus of this study is to understand the possible effects of Cu diffusion upon the Ni/Sn-Ag and Ni/Sn-Ag-Cu interfacial reactions. Two types of substrates were used, and they were 500mm-thick Ni foil and 500 mm-thick Cu foil plated with 50 mm-thick Ni. Two kinds of electricity-bias were conducted. One kind was with DC current and the other kind was with DC current but reversing the direction in the middle of the reaction time. Cu6Sn5 and Ni3Sn4 are observed in the couples with Sn-Ag-Cu solder. Electromigration effects are significant, but no Cu diffusion effects upon the solder/Ni reactions are observed. [1] H. B. Huntington in “Diffusion in solids: recent developments”, edited by A. S.Nowich and J. J. Burton, Academic Press, New York, pp. 303-352, (1975). [2] C.-M. Chen and S.-W. Chen, 1999, “Electric Current Effects Upon the Sn/Ag Interfacial Reactions”, Journal of Electronic Materials, Vol. 28(7), pp. 902-906. [3] S.-W. Chen, C.-M. Chen, and W.-C. Liu, 1998, "The Electric Current Effects Upon the Sn\Cu and Sn\Ni Interfacial Reactions", Journal of Electronic Materials, Vol. 27(11), pp. 1193-1198.