Evaluation of local anisotropic elasticity and thermal expansion on whisker formation sites in beta-tin thin films

Wei-Hsun Chen, Purdue University

Abstract

Tin whiskers are long filament-like crystals, growing from Sn-alloy thin films under conditions of various stress-generating mechanisms. The growth of whiskers has been observed in Pb-free electronic devices, posing a reliability risk by creating short circuits between adjacent components. Understanding the whisker formation mechanisms is necessary to develop mitigation strategies to reduce whisker risks. Although this phenomenon has been commonly identified as a local stress relaxation mechanism via mass transport, now a fundamental question is on the local effects: what local properties determine which grains will form whiskers? Since β-Sn has a body-centered-tetragonal crystal structure with highly anisotropic elasticity and thermal expansion, 2-D stress calculations using finite element analysis were performed to investigate the effects of film textures and local grain orientations on global and local film properties, associated with the preferred whisker formation sites, as well as the film's propensity to form whiskers. These results also suggest that thin film stress relaxation can be controlled by engineering specific textures, which will be different for stress generation by mechanical load or temperature change. On the other hand, thermally-cycled large-grained Sn solder films were used to identify the effects of β-Sn anisotropy and grain geometry on local stress relaxation, such as surface defect formation and grain boundary (GB) sliding. While a global stress was induced due to the coefficient of thermal expansion (CTE) mismatch between Sn films and Cu substrates, various degrees of local stresses were induced at GBs due to the grain orientations and anisotropic CTE. Our results indicate that surface defect formation and GB sliding were observed at specific GBs, which have relatively high in-plane CTE normal to GBs for inducing local high stresses during thermal cycling. In addition, evolutions of local grain orientation, relative dislocation density, and elastic strain energy density before and after stress relaxation at GBs were determined using x-ray synchrotron micro-diffraction, while specific grain geometry for initiating surface defect formation or GB sliding was identified using FIB analysis. In the end of this study, 3-D stress calculations considering β-Sn anisotropy were performed to simulate local stress distributions around surface grains, since whisker formation sites have been commonly associated with surface grains in columnar-grained microstructures. The results indicate that a local stress gradient can be created due to local grain orientations, enhancing atomic diffusion toward the whisker base to accelerate whisker growth. Based on the studies in this dissertation, deep insight into the critical conditions for determining whisker formation sites is obtained in terms of film microstructures and β-Sn anisotropy.

Degree

Ph.D.

Advisors

Blendell, Purdue University.

Subject Area

Condensed matter physics|Materials science

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