Abstract
The 3D microstructure around a tin whisker, and its evolution during heat treatment were studied using scanning 3DXRD. The shape of each grain in the sample was reconstructed using a filtered-back-projection algorithm. The local lattice parameters and grain orientations could then be refined, using forward modelling of the diffraction data, with a spatial resolution of 250 . It was found that the tin coating had a texture where grains were oriented such that their c-axes were predominantly parallel to the sample surface. Grains with other orientations were consumed by grain growth during the heat treatment. Most of the grain boundaries were found to have misorientations larger than 15, and many coincidence site lattice (CSL) or other types of low-energy grain boundaries were identified. None of the grains with CSL grain boundaries were consumed by grain growth. During the heat treatment, growth of preexisting Cu6Sn5 occurred; these grains were indexed as a hexagonal phase, which is usually documented to be stable only at temperatures exceeding 186 . This indicates that the phase can exist in a metastable state for long periods. The tin coating was found to be under compressive hydrostatic stress, with a negative gradient in hydrostatic stress extending outwards from the root of the whisker. Negative stress gradients are generally believed to play an essential role in providing the driving force for diffusion of material to the whisker root.
Highlights
Coating electronic components with a layer of tin (Sn) is an extensively used method to protect them from corrosion
We investigated the evolution of the microstructure after a heat treatment
In the whisker growth model proposed by Smetana [33], it is assumed that some recrystallisation mechanism forms grains with oblique grain boundaries; the stress gradient drives diffusion of atoms towards the newly formed grain, which will grow upwards and become a whisker due to grain boundary sliding
Summary
Coating electronic components with a layer of tin (Sn) is an extensively used method to protect them from corrosion. The most common solders consisted of tin alloyed with lead. Due to the toxicity of lead, the European Union enacted the Restriction of Hazardous Substances (RoHS) act in 2006. This legislation aims to eliminate the use of lead in consumer electronics. The elimination of lead from solders has resulted in the reappearance of an old problem, namely the formation of conducting tin whiskers. Whiskers are known to cause short-circuits and catastrophic failures of electronic components [2]. The first observations of tin Materials 2019, 12, 446; doi:10.3390/ma12030446 www.mdpi.com/journal/materials
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