Study of microstructure evolution in novel Sn-Zn/Cu bi-layer and Cu/Sn-Zn/Cu sandwich structures with nanoscale thickness for 3D packaging interconnection

The intermetallic compounds (IMC) distribution/development and microstructure evolution in Sn-Zn/Cu bi-layer and Cu/Sn-Zn/Cu sandwich structures with nanoscale solder thickness under reflow processes were studied. It was found that the IMC growth characteristics (IMCs and inter-phase transfer) in these sub-micrometer solder layer samples were quite special: solder layer was totally consumed in the reflow process, and only Cu₆Sn ₅ and Cu₃Sn phases were observed clearly at the interface. Also, the distribution of IMC phases in this research was very different with s pure Sn solder samples. In Cu/Sn-Zn bi-layer samples, the consumption of the solder materials was clearly observed from both top-view and cross-section view. Comparatively, in Cu/Sn-Zn/Cu sandwich samples, solder layer disappeared faster than in the pure Sn samples. Zn containing IMC phases, such as Cu ₅Zn₈, could not be clearly observed in SEM images. The Zn atoms were distributed relatively evenly in the IMC phases and the near-interface zones, with a peak at the middle of the IMC layer. Kirkendall voids were observed clearly, with a diameter range of 40-100 nm. The fast formation and development of the Kirkendall voids were discussed. Diffusivity and the limitation of solder materials were proven to be the key cause of voids. © 2014 Elsevier B.V. All rights reserved.