Estimating the constitutive behaviour of sintered silver nanoparticles by nanoindentation

With the regulation of lead-free packaging materials and the rapid miniaturization of electronic packaging device, power density significantly increases and therefore thermal management becomes a limiting factor. Among the material candidates, the sintered silver nanoparticles (AgNPs) own superior electrical and thermal conductivities. More importantly, due to scale effect, the reduction of particle size down to nanoscale will significantly increase the surface energy of Ag particles and decrease the melting and sintering temperatures of AgNP paste. This will facilitate the sintering process at low temperatures but the sintered material is readily serving for high-temperature applications. Nevertheless, due to the limitation of prepared specimens and experimental equipment, it is challenging to evaluate the constitutive behaviour and mechanical properties of sintered AgNP material. In this study, a three-sided Berkovich diamond indenter with a tip radius of 50 nm is used to perform nanoindentation accompanied by the continuous stiffness measurement technique. Young's modulus and hardness are continuously measured with a function of penetration depth. By assuming a power-law for sintered AgNP to describe the plastic properties, the representative stress is calculated according to the indentation work done in the loading curve, and the work hardening exponent is calculated based on the contact stiffness in the unloading curve. Therefore, the stress-strain curve can be estimated from each indentation response despite the non-uniform morphology. By analyzing the stress-strain curves based on repeated indentation on the same material sample, reliable estimates regarding the constitutive behaviour of sintered AgNP can be made. Lastly, the limitations of the proposed approach are also addressed.