Diffusion-Controlled Growth of Phases in Metal-Tin Systems Related to Microelectronics Packaging
The electro–mechanical connection between under bump metallization (UBM) and solder in flip–chip bonding is achieved by the formation of brittle intermetallic compounds (IMCs) during the soldering process. These IMCs continue to grow in the solid–state during storage at room temperature and service at an elevated temperature leading to degradation of the contacts. In this thesis, the diffusion–controlled growth mechanism of the phases and the formation of the Kirkendall voids at the interface of UBM (Cu, Ni, Au, Pd, Pt) and Sn (bulk/electroplated) are studied extensively. Based on the microstructural analysis in SEM and TEM, the presence of bifurcation of the Kirkendall marker plane, a very special phenomenon discovered recently, is found in the Cu–Sn system. The estimated diffusion coefficients at these marker planes indicate one of the reasons for the growth of the Kirkendall voids, which is one of the major reliability concerns in a microelectronic component. Systematic experiments using different purity of Cu are conducted to understand the effect of impurities on the growth of the Kirkendall voids. It is conclusively shown that increase in impurity enhances the growth of voids. The growth rates of the interdiffusion zone are found to be comparable in the Cu–Sn and the Ni–Sn systems. EPMA and TEM analyses indicate the growth of a metastable phase in the Ni–Sn system in the low temperature range. Following, the role of Ni addition in Cu on the growth of IMCs in the Cu–Sn system is studied based on the quantitative diffusion analysis. The analysis of thermodynamic driving forces, microstructure and crystal structure of Cu6Sn5 shed light on the atomic mechanism of diffusion. It does not change the crystal structure of phases; however, the microstructural evolution, the diffusion rates of components and the growth of the Kirkendall voids are strongly influenced in the presence of Ni. Considering microstructure of the product phases in various Cu/Sn and Cu(Ni)/Sn diffusion couples, it has been observed that (i) phases have smaller grains and nucleate repeatedly, when they grow from Cu or Cu(Ni) alloy, and (ii) the same phases have elongated grains, when they grow from another phase. A difference in growth rate of the phases is found in bulk and electroplated diffusion couples in the Au–Sn system. The is explained in AuSn4 based on the estimated tracer diffusion coefficients, homologous temperature of the experiments, grain size distribution and crystal structure of the phase. The growth rates of the phases in the Au–Sn system are compared with the Pd–Sn and the Pt–Sn systems. Similar to the Au–Sn system, the growth rate of the interdiffusion zone is found to be parabolic in the Pd–Sn system; however, it is linear in the Pt–Sn system. Following, the effect of addition of Au, Pd and Pt in Cu is studied on growth rate of the phases. An analysis on the formation of the Kirkendall voids indicates that the addition of Pd or Pt is deleterious to the structure compared to the addition of Au. This study indicates that formation of voids is equally influenced by the presence of inorganic as well as organic impurities.
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