Partner und Internationale Organisationen
(Englisch)
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AT, BE, BG, CH, CZ, DE, ES, FI, FR, GR, HU, IE, IT, NL, PL, PT, RO, RS, SE, SI, SK, UK
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Abstract
(Englisch)
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The reliability and durability of lead-free solder joints depends on a large number of factors, like ge-ometry, service temperature, processing parameters and size effects. In this project, the nature and influence of the plastic constraints in the solder due to elastic joining partners have been studied by parametric finite element simulation of tensile and shear solder joints with variable dimensions. An apparent hardening introduced by the plastic constraints has been observed in tension loaded joints and has been shown to strongly depend on the solder gap to thickness ratio with an inversely proportional evolution. In contrary, joints loaded in shear did not exhibit any effects of constraints due to the isochoric nature of shear deformations. Because of the interdependence of the geometrical, processing and size effects, the macroscopic stress-strain constitutive law of lead-free solder materials should be determined in the most realistic conditions. Sub-micron resolution Digital Image Correlation algorithms have been developed to measure the evolution of strain in solder joints during tensile (small deformations) and shear tests (large deformations). Experimental results of the stress-strain response of Sn-Ag-Cu solder joints have been determined for several solder gaps in tension and shear. The measured load-displacement curves have been used in an inverse numerical identification pro-cedure to determine the constitutive elasto-plastic behaviour of the solder material. The effects of plastic constraints have been studied by comparing the apparent (constrained) and constitutive (non-constrained) stress-strain relationships. The size effects have been identified by comparing the constitutive elasto-plastic parameters of tensile and shear joints with a variable solder gap. The initial macro & micro porosity was observed to have an important effect in the plastic behaviour of solder joints. Micro porosity reduces the macroscopic yield and ultimate stress while the macro pores tend to create sites for plastic damage initiation and localization. The random nature of the porosity distribution in solder joints cannot be controlled easily and should therefore be taken into account in the analysis as it appears to be responsible for the large scatter usually observed in ex-perimental data. To model these phenomena, a porous metal plasticity model (GTN model) has been identified and allows predicting the initiation and development of shear bands (localization of plastic deformation) as observed in the experiments.
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