The surface energies of beta-Sn - A new concept for corrosion and whisker mitigation
P Eckold and MS Sellers and R Niewa and W Hugel, MICROELECTRONICS RELIABILITY, 55, 2799-2807 (2015).
Corrosion data provided under high-temperature and high-humidity conditions as well as tin whisker growth studies are explained by differences in the surface energy of lattice planes within the crystal structure of beta-tin. For this purpose, electrodeposited tin finishes were investigated regarding their microstructure utilizing X-ray diffraction, cross-sectional SEM and EBSD analyses. The corrosion as well as the tin whisker propensity strongly depends on the preferred orientation of the surface finishes. With an increasing texture along the (211) lattice plane a decreasing corrosion and whisker propensity were observed, on the contrary, the presence of the (101) and (112) textures results in an increased corrosion and whisker propensity. The maximum whisker length was reduced by one order of magnitude by changing the preferred orientation towards the (211) lattice plane of the tin finish. Modified embedded atom method simulations of tin surfaces demonstrate the minimization of the surface energy of (211) surfaces, whereas the surface energies of the (101) and (112) Miller planes are increased. We find a strong connection between the minimization of surface energy and the corrosion and tin whisker propensity of electrodeposited tin finishes. To our best knowledge, this is the first study connecting the influence of the electrodeposition parameters on the corrosion and whisker propensity explained by calculations of the surface energies of the corresponding crystal faces. The applied parameters for electrodeposition influence the grain orientation and thus the surface energy of the tin layers which affects both the corrosion as well as the tin whisker propensity. (C) 2015 Elsevier Ltd. All rights reserved.
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