Isothermal Ageing of SnAgCu Solder Alloys: Three-Dimensional Morphometry Analysis of Microstructural Evolution and Its Effects on Mechanical Response

Due to the high homologous temperature and fast cooling rates, the microstructures of SnAgCu (SAC) solders are in a meta-stable state in most applications, which is the cause of significant microstructural evolution and continuous variation in the mechanical behavior of the joints during service. The link between microstructures evolution and deformation behavior of Sn-4.0Ag-0.5Cu solder during isothermal ageing is investigated. The evolution of the microstructures in SAC solders are visualized at different scales in 3D by using a combination of synchrotron x-ray and focused ion beam/scanning electron microscopy tomography techniques at different states of ageing. The results show that, although the grain structure, morphology of dendrites, and overall volume fraction of intermetallics remain almost constant during ageing, considerable coarsening occurs in the Ag3Sn and Cu6Sn5 phases to lower the interfacial energy. The change in the morphometrics of sub-micron intermetallics is quantified by 3D statistical analyses and the kinetic of coarsening is discussed. The mechanical behavior of SAC solders is experimentally measured and shows a continuous reduction in the yield resistance of solder during ageing. For comparison, the mechanical properties and grain structure of beta-tin are evaluated at different annealing conditions. Finally, the strengthening effect due to the intermetallics at different ageing states is evaluated by comparing the deformation behaviors of SAC solder and beta-tin with similar grain size and composition. The relationship between the morphology and the strengthening effect due to intermetallics particles is discussed and the causes for the strength degradation in SAC solder during ageing are identified.

Microstructure-based modeling of the ageing effect on the deformation behavior of the eutectic micro-constituent in SnAgCu lead-free solder

John Botsis, Joël Cugnoni, Milad Maleki

The eutectic micro-constituent in SnAgCu solder governs the deformation behavior of the joint as it shows better deformation resistance than the Sn dendrites and occupies a high volume percentage of the whole solder. The main scope of this study is to develop a three-dimensional (3-D) homogenization model taking into account the microstructural evolution in the eutectic micro-constituent of SnAgCu solder in order to simulate the change in mechanical behavior of the joint caused by isothermal ageing. For this purpose, 3-D configurations of Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs) in near-eutectic SnAgCu solder are visualized in the as-soldered condition and after ageing by focused ion beam/scanning electron microscopy tomography. The tomographic images are used to generate feature-preserving finite element meshes of the actual microstructures. The representative volume element size and constitutive behavior of the eutectic mixture in the two conditions are determined by a numerical homogenization procedure. The results show a considerable reduction in the yield stress level of the eutectic micro-constituent after ageing of the solder joint. It is shown that the increase in the interparticle spacing and decrease in the aspect ratio of IMCs due to ageing cause a significant change in the strain distribution in the tin matrix, which leads to a lower contribution of IMCs in load-sharing and yield strength of aged solder. The elastic plastic properties of as-soldered and aged eutectic mixtures are determined by nanoindentation. The results of homogenization are validated through comparison with experimental results and prediction of the dislocation detachment theory. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Elsevier2013

Multi-scale modeling of elasto-plastic response of SnAgCu lead-free solder alloys at different ageing conditions: Effect of microstructure evolution, particle size effects and interfacial failure

John Botsis, Joël Cugnoni, Milad Maleki

In microelectronics applications, SnAgCu lead-free solder joints play the important role of ensuring both the mechanical and electrical integrity of the components. In such applications, the SnAgCu joints are subjected to elevated homologous temperatures for an extended period of time causing significant microstructural changes and leading to reliability issues. In this study, the link between the change in microstructures and deformation behavior of SnAgCu solder during ageing is explained by developing a hybrid multi-scale microstructure-based modeling approach. Herein, the SnAgCu solder alloy is seen as a three phase metal matrix composite in which Ag3Sn and Cu6Sn5 hard intermetallics play the role of reinforcements and Sn the role of a ductile matrix. The hardening of the Sn matrix due to fine intermetallics in the eutectic mixture is modeled by incorporating the mean field effects of geometrically necessary dislocations. Subsequently, a two level homogenization procedure based on micromechanical finite element (FE) models is used to capture the interactions between the different phases. For this purpose, tomographic images of microstructures obtained by Focused Ion Beam (FIB) and synchrotron X-Ray in different ageing conditions are directly used to generate statistically representative volume elements (RVE) using 3D FE models. The constitutive behavior of the solder is determined by sequentially performing two scales of numerical homogenization at the eutectic level and then at the dendrite level. For simplification, the anisotropy of Sn as well as the potential recovery processes have been neglected in the modeling. The observed decrease in the yield strength of solder due to ageing is well captured by the adopted modeling strategy and allows explaining the different ageing mechanisms. Finally, the effects of potential debonding at the intermetallic particle-matrix interface as well as particle fracture on the overall strength of solder are discussed based on simplified unit cell FE models and experimental observations. (C) 2016 Elsevier B.V. All rights reserved.

Elsevier2016

Damage evolution in coated cemented carbides under compressive contact loads

Maria-Simona Moldovan

This work deals principally with two important issues and their interrelation: the evolution of damage in coated cemented carbide tools used for cold forming, and the assessment of mechanical behavior of cemented carbides under compressive contact loads. Damage evolution in ironing tools is qualitatively investigated by planar and cross-sectional microscopical observations. Several mechanisms of damage are identified by taking into account the circumferential distribution along the surface of the ironing tools: coating wear and/ fracture, plastic deformation and fracture of the substrate. In the context of damage location in the substrate, the emphasis is put on the study of damage following all processing steps in substrate and coating preparation. This analysis aims at determining whether the substrate is weakened or not by the mechanical and chemical pre-treatment prior to coating. Spherical indentation testing is used as an experimental procedure to reproduce the compression stress state during cold forming. Large indenter tips (300 microns) are employed for testing cemented carbides in an effort to minimize the effects of material structural inhomogeneities. The behavior of cemented carbides under compressive loads is governed by the reversibility of deformation induced by indentation. A unique feature revealed by analysis of experimental load-displacement indentation curves is the positive energy balance, in that a gain of energy is evidenced after a loading-unloading cycle. Two hypotheses are developed for interpreting such a behavior: the strain induced martensitic transformation of the cobalt ductile phase and the thermal residual stresses in the composite. Substrate weakening subsequent to mechanical and chemical pretreatment prior to coating is expected to have a particular role on the indentation behavior of cemented carbides. The impact of substrate damage on the indentation parameters is identified by a parametric study using finite element simulations in which the damage is considered uniform. In reality, the damage distribution over the surface is rather irregular. In this context, a statistical approach is used for assessment of the material indentation parameters. The failure of coating/substrate systems under compressive contact loads is investigated by finite element simulations. The first failure events are identified by considering that system failure occurs either by substrate plastic deformation or coating fracture. The response of coated systems to spherical indentation is synthesized in damage diagrams which can predict whether the coating or the substrate fails first. The possibility for using such diagrams for assessing the evolution of damage in coated cemented carbide tools is highlighted.

EPFL2008

Multi-scale modeling of elasto-plastic response of SnAgCu lead-free solder alloys at different ageing conditions: Effect of microstructure evolution, particle size effects and interfacial failure

John Botsis, Joël Cugnoni, Milad Maleki

Elsevier2016

Damage evolution in coated cemented carbides under compressive contact loads

Maria-Simona Moldovan

EPFL2008