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Sintered copper nano particles are being considered as alternatives to solder and/or sintered silver in different applications. Like for the alternatives, interpretation of accelerated fatigue test results does however require modeling, typically involving prediction of stresses and strains vs. time and temperature based on constitutive relations. This poses a challenge as the inelastic deformation properties depend strongly on both the initial particles and details of the processing, i.e. unlike for solder general constitutive relations are not possible. The present work provides a mechanistic description of the early transient creep of relevance in cycling, including effects of sintering parameters and subsequent oxidation. Inelastic deformation is dominated by diffusion, rather than dislocation motion. Generalized constitutive relations are provided to the extent that quantitative modeling of a specific structure only requires the measurement of a single creep curve for that.

The inelastic deformation properties of sintered metal nanoparticle joints are complicated by the inherent nanocrystalline and nanoporous structures as well as by dislocation networks formed in sintering or under cyclic loading. Creep rates of sintered nanocopper structures were found to be dominated by the diffusion of individual atoms or vacancies, while dislocation motion remained negligible up to stresses far above those of practical interest. Rapid sintering of one material led to unstable structures the creep of which could be strongly reduced by subsequent annealing or aging. Longer sintering of another material led to more stable structures, but creep rates could still be strongly enhanced by subsequent work hardening in mild cycling.