Here, we present an advanced experimental procedure for determining the properties of a SnAg3.5 solder alloy in the strain range of primary creep under cyclic load and isothermal conditions. The challenge in this experiment is the accurate high-resolution measurement of sample elongation used for a closed-loop control, as well as avoiding the influence of sensor and specimen clamping. We realized reproducible strain rate control within a total specimen elongation of 60 μm. The tensile-compression experiment comprises strain rate variation for three strain amplitudes with integrated relaxation stages followed by a measurement of cyclic fatigue. The strain rate at every strain stage was varied in the range of 1E-3 to 1E-6 per second. At the end of every strain stage a time-limited relaxation experiment is performed, where the specimen's length is kept constant, while the stress evolution is recorded. Finally, the specimen is subjected to cyclic fatigue until a drop of 50 % of the initial materials strength is reached. The total procedure is performed in a temperature range from -40 to 150 °C. We prove the capability of common creep models to map the observed cyclic stress-strain hysteresis as well as stress dependency on strain rate. The results reveal substantial limitations of common stationary creep models and strongly suggest the application of advanced visco-plastic material models for an accurate description of the solder alloy properties. The experimental data presented can be used for the calibration of unified visco-plastic constitutive models initially proposed by Chaboché et al. and further extended during the past two decades

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During the past decade the demand for high performance automotive electronics is steadily increasing. An efficient development of such products requires the use of durability assessment techniques throughout the whole design optimization process. Since typical components comprise a large number of different materials and complex geometrical structures, Finite Element (FE) analysis is preferably used for durability evaluation and continuously replaces analytical calculations. However, a direct lifetime calculation by means of FE-techniques is still not achieved, partly due to the lack of material models capable of mapping the intrinsic material degradation under the relevant thermo-mechanical loads. Here, we propose a material model for a tin-based solder alloy which describes the non-linear mechanical behavior at the beginning of deformation as well as during continuous cyclic aging. We investigated the evolution of the mechanical properties and microstructure of the solder alloy Sn96:5Ag3:5 by cyclic strain-rate controlled fatigue- and creep-tests on as-casted standardized specimens. Material modeling is focused on the description of the complex interplay between viscoplastic, fatigue and creep processes observed in the experiment. Finally, a very good agreement is obtained between the measurements and the numerical model, which can offer new opportunities for lifetime simulations of lead-free solder joints

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