Length-Scale Effects in Average Viscoplastic Behavior of Sintered Silver Materials: Empirical Exploration With Indentation Methods

Sintered silver materials (with and without epoxy matrices) are used in microelectronics, as high-temperature interconnect materials, and also as conductor trace materials in printed electronic circuitry. The sintering process results in an interconnected assemblage of discrete agglomerated particles. This results in intrinsic length-scale effects under the action of different stress gradients. In other words, the effective homogenized average continuum-scale material behavior changes with the local magnitude of the stress gradients. Consequently, regions of sharp, localized stress concentrations have to be modeled with different effective continuum material properties, compared with the properties that are relevant for regions that have a uniform stress field. In this study, the focus in on the effective creep behavior, in particular. This length-scale effect is empirically explored in this study using nanoindentation with indenters of different tip radii, causing different stress gradients. Properties estimated by each indenter are compared to demonstrate the dependence of the effective continuum properties on the local length scale effects (generated by the ratio of the tip radius to the characteristic discrete dimension of the sintered particles).