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Growth of zinc whiskers

Abstract : Whiskers, conductive metallic filaments that grow from metallic surfaces, are a very importantissue for reliability of electronic components. Through recent years, there has been a renewedindustrial interest on whisker growth, mainly due to the miniaturization of electronic devices andthe environmental regulations forbidding the use of lead.While most of the research has been focused on tin whiskers, there is still little reference tozinc whiskers. Electroplated zinc coatings are actually used as anticorrosive protection for lowalloy steels in diverse industries such as automotive, aerospace or energy, as well as for supportstructures or raised-floor tiles in computer data centers. In order to mitigate, prevent and predictthe failures caused by the zinc whiskers, the mechanisms of growth must be understood.By accelerated storage tests and Scanning Electron Microscopy (SEM) observation, kinetics ofgrowth of zinc whiskers was studied on low alloy chromed electroplated carbon steel.Quantitative characterization of both whisker and hillocks (density, volume and growth rate) wasrelated with the parameters temperature, electroplating electrolyte, presence of chrome, steelsubstrate thickness, zinc coating thickness and residual stress, in order to understand themechanisms of growth.Additionally, both microstructure and crystallography of zinc coating, whisker roots and actualwhiskers were studied by Electron Backscatter Diffraction (EBSD), Transmission ElectronMicroscopy (TEM), Energy-dispersive X-ray spectroscopy (EDX) and local grain orientationwith ASTAR setup, using Focused Ion Beam (FIB) for samples preparation. Recrystallization aswell as dislocations were observed in both whiskers and hillocks; no intermetallic compoundswere seen in neither electroplated nor whiskers.It is found that compressive residual stress relaxation and whiskers growth are two differentbut strongly interconnected phenomena both thermally activated, an each of them follows adifferent mechanism; apparent activation energies of the two phenomena are calculated, andgrain boundary diffusion is established as the main diffusion mechanism for whiskers growth.Whiskers growth kinetics, both analytical and phenomenological is proposed. Goodestimation of whiskers growth and whiskers growth rate at temperatures close to operationconditions is obtained when compared with experimental data.
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Juan Manuel Cabrera-Anaya. Growth of zinc whiskers. Materials. Université de Grenoble, 2014. English. ⟨NNT : 2014GRENI039⟩. ⟨tel-01298169⟩

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