Effect of crystal anisotropy and IMCs on electromigration resistivity of low temperature flip chip interconnect

Abstract

In recent years, global warming prevention and reduction of energy consumption are increasingly required. And technical point of view, low stress bonding is required. A low temperature bonding using Sn-Bi solder is candidate technique to solve these problems. In this report, improving the reliability of electro-migration at flip chip interconnect in Sn-Bi solder, which is a low-temperature solder, are focused. A combination of six types of Sn-Bi solder and surface finish of interposer pad were studied, these are, combination of Sn30wt.%Bi (Sn30Bi) solder, Sn57wt.%Bi (Sn57Bi) solder, electroless Ni/Au on Cu pad and Ni/Pd/ Au on Cu pad. The effects of Bi concentration, trace element and inter-metallic compounds formation on electromigration resistivity were evaluated. The current density of 40 kA/cm2 was applied at 125 degrees C. Analysis of the microstructure and crystal orientation of the interconnected bumps were performed in all combinations by electron backscatter diffraction and electron probe microanalyzer. Mean time to failure of Sn57Bi/NiAu(thick), that of Sn30Bi/NiAu(thick) and that of Sn30Bi/NiPdAu were19, 141 and 600 hours, respectively. And maximum resistance increases from initial of Sn57Bi/NiAu(thick), that of Sn30Bi/NiAu(thick), Sn30Bi/NiPdAu were 60.6%, 19.5%, 13.2% after 2500 hours. Reducing the total amount of Au and Pd atoms in the solder can delay increase of resistance after current stressing. In cases of NiAu(thick) and NiAu(thin) pads, two types of failure mode were observed after current stressing. One was growth of large (Cu, Ni)6Sn5 with thick Ni(P rich) layer. The other was the mode in which (Cu, Ni) 6Sn5 hardly grow and unreacted Sn layer remains. In case of c-axis of the beta-Sn grain aligned with the electron flow, Ni diffusion from Ni pad to the solder was accelerated. On the other hand, in the case of NiPdAu pad, only one failure mode was observed after current stressing. It is a mode in which the unreacted Sn layer remains. Even if c-axis of the beta-Sn grain aligned with the electron flow, Ni diffusion from Ni pad was not observed. Focusing on the intermetallic compounds (IMCs) formed on the interposer pad after 10% resistance change, in cases of NiAu(thick) and NiAu(thin) pads, un-uniform and needle-type (Cu, Ni) 6Sn5 were formed. Large Ni diffusion from their grain boundary into IMCs were observed after current stressing. On the other hand, in the case of NiPdAu pad, thin (Ni, Cu)3Sn4 and uniform and dense scallop-type (Cu, Ni)6Sn5 were formed after 10% resistance change. They were still stabled after current stressing. The significant increase in resistance in the early stage is influenced by Bi concentration in solder. And Au atoms play a role of accelerating diffusion and increase in resistance at early stage on electro-migration test. Dense scallop-type (Cu, Ni)6Sn5 layer on Ni pad showed as an effective barrier diffusion of Cu or Ni. It was found that if a uniform and dense IMCs were formed on the interposer pad before current stressing, it would show high electro-migration resistivity even with low temperature solder.