Strain rate shift for constitutive behaviour of sintered silver nanoparticles under nanoindentation

Abstract As one of the promising packaging materials, the paste of silver nanoparticles (AgNPs) reinforced by SiC particles is investigated in this study for microstructure, thermal stability and constitutive behaviour. As a common challenge in the integrated circuit industry, it is difficult to reveal the strain rate-dependent mechanical properties of thin-film materials in such a large number of sandwich-like electronic devices. In this paper, to enable finite element analysis for mechanical reliability of packaging structure, the stress-strain relationships of the sintered AgNP material at different strain rates are evaluated analytically using the nanoindentation approach with the Berkovich type of indenter. Based on dimensionless analysis, the work done and contact stiffness are identified as the critical variables from nanoindentation responses. The proposed rate factors for the representative stress and the hardening exponent attempts to bridge the strain rate gap between nanoindentation and uniaxial tests. The proposed rate factors are calibrated by a typical lead-free packaging material Sn-3.0Ag-0.5Cu based on tensile tests under the uniaxial strain rate between 10−4 s−1 and 10−3 s−1 and nanoindentation tests under the indentation strain rate between 0.01 s−1 and 0.10 s−1. This makes it possible that the proposed approach is capable of shifting the indentation strain rate by a factor of 100 with regard to the uniaxial strain rate. In this manner, the constitutive responses of sintered AgNP materials at different uniaxial strain rates are achieved and compared with the predictions by using a spherical indenter, which reveals the advantages of assessing the strain rate sensitivity of sintered AgNP by using a Berkovich indenter.