PhD Student, Indian Institue of Science Bangalore, Indian
Diffusion–controlled growth of phases in metal–tin systems
Ni–Sn system find applications in microelectronics industry especially with respect to flip–chip or direct chip attach technology. Here the region of interest is under bump metallization (UBM) and solder bump (Sn) interface due to the formation of brittle intermetallic phases there. Understanding the growth of these phases at UBM/Sn interface is important, as in many cases it controls the electro–mechanical properties of the product. Cu and Ni are the commonly used UBM materials. Cu is used for good bonding because of fast reaction with solder and Ni often acts as a diffusion barrier layer due to its inherently slower reaction kinetics with Sn–based solders. Investigation on the growth kinetics of phases in Ni–Sn system is reported in this study. Just for simplicity, Sn being major solder constituent is chosen. Ni–Sn electroplated diffusion couples are prepared by electroplating pure Sn on Ni substrate. Bulk diffusion couples prepared by conventional method are also studied along with Ni–Sn electroplated diffusion couples. Diffusion couples are annealed for 25–1000 h at 50–215 °C to study the phase evolutions and growth kinetics of various phases. The interdiffusion zone was analysed using field emission gun equipped scanning electron microscope (FE–SEM) for imaging. Indexing of selected area diffraction (SAD) patterns obtained from transmission electron microscope (TEM) and composition measurements done in electron probe micro−analyser (FE–EPMA) confirms the presence of various product phases grown across the interdiffusion zone. Time-dependent experiments indicate diffusion controlled growth of the product phase. The estimated activation energy in the temperature range 125–215 °C for parabolic growth constants (and hence integrated interdiffusion coefficients) of the Ni3Sn4 phase shed light on the growth mechanism of the phase; whether its grain boundary controlled or lattice controlled diffusion. The location of the Kirkendall marker plane indicates that the Ni3Sn4 phase grows mainly by diffusion of Sn in the binary Ni–Sn system.
Abstract: The presence of bifurcation of the Kirkendall marker plane, a very special phenomenon discovered recently, is found in a technologically important Cu–Sn system. It was predicted based on estimated diffusion coefficients; however, could not be detected following the conventional inert marker experiments. As reported in this study, we could detect the locations of these planes based on the microstructural features examined in SEM and TEM. This strengthens the concept of the physico–chemical approach that relates microstructural evolution with the diffusion rates of components and imparts finer understanding of the growth mechanism of phases. The estimated diffusion coefficients at the Kirkendall marker planes indicates that the reason for the growth of the Kirkendall voids is the non–consumption of excess vacancies which are generated due to unequal diffusion rate of components. Systematic experiments using different purity of Cu in this study indicates the importance of the presence of impurities on the growth of voids, which increases drastically for <img height="12" border="0" style="vertical-align:bottom" width="12" alt="Full-size image (<1 K)" title="Full-size image (<1 K)" src="http://origin-ars.els-cdn.com/content/image/1-s2.0-S1359645417302616-egi10JSFN1GB2C.jpg"> 0.1 wt.% impurity. The growth of voids increases drastically for electroplated Cu, commercially pure Cu and Cu(0.5 at.%Ni) indicating the adverse role of both inorganic and organic impurities. Void size and number distribution analysis indicates the nucleation of new voids along with the growth of existing voids with the increase in annealing time. The newly found location of the Kirkendall marker plane in the Cu3Sn phase indicates that voids grow on both the sides of this plane which was not considered earlier for developing theoretical models.
Pub.: 27 Mar '17, Pinned: 02 Aug '17