A study of dynamic testing of single solder joints and its application to finite element modelling

Abstract

Intermetallic compounds, impurities, voids, and other inhomogeneities are formed in solder joints of integrated-circuit packages. Consequently, the mechanical response of these interconnections may not be well modelled if the properties of bulk solder alloy alone are used. This investigation explores the feasibility of eliciting the dynamic response of single-solder-joint specimens through split-Hopkinson-bar (SHB) testing. As solder joints are extremely small, contact between such specimens and the input-output bars of an SHB results in high stress localization. Numerical simulation of SHB testing of a small specimen, the size of a solder joint, is undertaken using Abaqus to validate the assumption of one-dimensional plane wave propagation in the input-output bars, as well as the accuracy of results derived from the strain signals associated with the midpoints of the bars. Uniaxial compression and tensile tests on single-solder-joint specimens are also conducted at deformation rates ranging from 0.000 15 mm/s to 1.4 m/s. The rate sensitivity is observed and force-deformation curves are obtained, as the actual solder joints possess a barrel-like geometry. In order to apply the force-deformation responses in finite element method (FEM) modelling, a numerical study is also undertaken to investigate the possibility of idealizing the barrel-like solder joint geometry by a cylinder. Subsequently, experimentally measured force-deformation responses of single solder joints are converted into 'normalized' or 'equivalent' stress-strain material properties. Equivalent stress-strain curves for uniaxial tension at strain rates of 0.0005-1000 s-1, and uniaxial compression at strain rates of 0.0005-5000 s-1 are derived. The variation in the equivalent yield stress with the strain rate is identified and a curve fit is applied. Initial implementation of the equivalent stress-strain curves, together with an idealized cylindrical solder connection, into FEM modelling demonstrates that this is able to predict the mechanical response of solder joints in tension, compression, and shear with reasonable accuracy.