Tin whiskers: experiments and modelling

Sammanfattning: Tin whiskers are hair-like single crystals that spontaneously grow from tin-coated surfaces. Whiskers are commonly found in electronic components, where tin coatings are used in, e.g., soldering applications, and to protect components from corrosion. Whiskers are known to cause short-circuits leading to failure of electronic components. The exact mechanisms responsible for whisker formation and growth are not fully understood. It is, however, believed that whiskers grow as a way to relax stress in the tin coating and that compressive stress gradients drive the growth. The stress in the coating is mainly caused by the formation and growth of the intermetallic compound Cu6Sn5 in the interface region between the copper substrate and the tin layer.This thesis presents numerical simulations and experimental investigations related to the growth and formation of tin whiskers on tin coated copper substrates. The aim of the experimental work has been to verify the existence of a compressive stress gradient and to characterise the microstructure around tin whiskers in 2D as well as 3D. This was realised using different x-ray diffraction methods, namely scanning Laue microdiffraction, differential aperture x-ray microscopy (DAXM), and scanning 3D x-ray diffraction (3DXRD). Laue microdiffraction was used to study the evolution of the microstructure around two whiskers over a period ranging from 4 to 21 months of ageing. The hydrostatic stress field in the tin coating was estimated by assuming plane stress conditions. It was found that the stress field was highly inhomogeneous.It was possible to identify ridges of high compressive stress leading to the whisker. These ridges, which have not been observed previously, are potentially driving diffusion of tin from specific regions of the coating towards the root of the whisker.DAXM and 3DXRD were used to study the microstructure around a whisker in three dimensions. Using DAXM, through-depth variations of the deviatoric strain field were measured for the first time. Deep in the coating, where the \ce{Cu6Sn5} is present, the deviatoric strain was high. This indicates that the growth of the intermetallic phase causes plastic deformation of the tin coating. A novel scanning 3DXRD tomography technique was used to map out intragranular variations in the unit cell parameters and the grain orientations with sub-micrometre resolution. A short (4 micrometre) radial gradient in hydrostatic stress was observed around the root of the whisker.This gradient together with long-range diffusion from specific regions could provide the driving force for whisker growth.From the 3DXRD data, it was also possible to determine the location of Cu6Sn5 in the sample. A large grain of Cu6Sn5 was found right below the whisker which seems to have caused distortion of the nearby tin grains. We also studied the evolution of the microstructure during heat treatment. The heat treatment encouraged the formation of the intermetallic phase and also led to coarsening of the tin grains.The numerical simulations in this thesis are based on a multiphase field model. This model is one of few phase field models existing that includes diffusion as well as elastic and plastic deformation. The model was used to study the growth of Cu6Sn5 during room temperature ageing, specifically the effect of the curvature of the intermetallic layer on the stress and plastic deformation of the tin coating was investigated. It was found that a high curvature led to localisation of plastic deformation in the region above the highly curved grain.