Deformation and Damage in Lead-free Solder Joints

For environmental reasons, lead has been banned from microelectronic solder materials since the adoption of the RoHS legislation in 2006. A large number of investigations have lead to the development of new lead-free solder materials like the SnAgCu alloy which can satisfy most of the technological requirements. However, the long-time reliability of the lead-free candidates is still a crucial issue and in several cases, failure of electronic assemblies has been attributed to that of the solder joints. To overcome the present challenge in the soldering technology, the damage mechanisms occurring along the service life of lead-free solder joints like microstructure evolution, viscoplastic deformation as well as interfacial cracking must be better understood. The main scope of this thesis is to shed light on the complex interdependency of microstructures, deformation property and the interfacial fracture of solder joints in order to explain the change in the failure mode of solder joints from ductile to brittle interfacial fracture as well as the link between microstructural evolution and the change in the mechanical property of the solders. The first part of this research focused on characterization and macroscopic modeling of the fracture at the interface of SnAgCu solder and Cu substrate with a special attention on the viscoplastic nature of the bulk solder. For this purpose, the viscoplastic behavior of the solder joint was initially identified and the material parameters related to a unified constitutive viscoplastic model were determined. The interfacial fracture behavior of the joint was characterized by performing stable fracture tests at different strain rates. The results confirmed that the failure mode of the solder joint is a function of the loading rate. It was shown that an increase in the loading rate causes a decrease in the load-bearing capacity of the joint by enhancing the tendency to develop an interfacial failure. The interfacial crack propagation in the fracture tests were simulated by developing a detailed three-dimensional (3D) finite element (FE) model containing a cohesive interface and considering the viscoplastic behavior of the bulk solder. The influence of strain rate and viscoplasticity on the load-bearing capacity and failure behavior of the joint were pointed out by analyzing the simulation results. It was shown how the normal stress generated at the interface is limited by the significant viscoplastic relaxation developed in the bulk solder at low strain rates, while a large portion of the external work is transferred to the interface at higher rates which gives rise to interfacial fracture. The second part of this thesis focused on developing microstructure-based models in order to simulate the change in the macroscopic mechanical response of the SnAgCu solder caused by the microstructural evolution during thermal ageing. For this purpose, first, the 3D configurations of microstructures in different ageing conditions were visualized using a combination of synchrotron X-ray and FIB/SEM tomography techniques. Analyses showed that although the size and orientation of crystalline grains as well as morphology of dendrites in SnAgCu solder remain stable during ageing, two main morphological changes occur in the Ag3Sn and Cu6Sn5 intermetallic particles: spheroidizing of needle-shape intermetallics and coarsening due to bulk diffusion. The mechanical property of the solder and its phases at different ageing conditions were experimentally characterized. A significant decrease in the yield strength of the solder joint was identified because of thermal ageing and it was shown that the deformation behavior of the solder is governed by the morphometrics of the dispersed intermetallics. The link between the microstructures and the deformation behavior of solder was successfully simulated by developing a hybrid microstructure-based modeling approach. The average indirect strengthening of the intermetallics was modeled by incorporating the effects of geometrically necessary dislocation on the in-situ deformation behavior of the tin matrix. Subsequently, a micromechanical FE homogenization was considered to model the load-sharing behavior of the intermetallics. For this purpose, the tomographic images were directly used to generate 3D FE meshes of the actual microstructures and then the constitutive deformation behavior of the solder were determined by adopting two levels of numerical homogenization in the eutectic phase and over the whole joint. The decrease in the yield strength of solder due to ageing was explained through analyses of indicators in the models. It was demonstrated how the change in the morphometrics of intermetallics during ageing reduces the contribution of intermetallics in load-sharing as well as the in-situ strength of the matrix by causing a significant change in the strain distribution as well as particle size effects. The simulation results were experimentally verified and the capabilities as well as limitations of the developed approach were pointed out. Overall, the results of this research led to a proper understanding of the critical reliability issues in lead-free solders during isothermal ageing, i.e., microstructural coarsening and interfacial fracture. In addition, through this work, a set of methodologies for multi-scale modeling and experimental characterization of the damage and deformation in solder joints were developed and validated which can be extremely helpful for efficient use of the SnAgCu solders in electronic industry as well as designing new solder alloys with higher strength and better reliability for a safe transition to lead-free solders in critical applications.

On the Mutual Effect of Viscoplasticity and Interfacial Damage Progression in Interfacial Fracture of Lead-Free Solder Joints

John Botsis, Joël Cugnoni, Milad Maleki

The main goal of this paper is to shed light on the effect of strain rate and viscoplastic deformation of bulk solder on the interfacial failure of lead-free solder joints. For this purpose, interfacial damage evolution and mode I fracture behavior of the joint were evaluated experimentally by performing stable fracture tests at different strain rates employing an optimized tapered double cantilever beam (TDCB) design. The viscoplastic behavior of the solder was characterized in shear, and the constitutive parameters related to the Anand model were determined. A rate-independent cohesive zone damage model was identified to best simulate the interfacial damage progression in the TDCB tests by developing a three-dimensional (3D) finite-element (FE) model and considering the viscoplastic response of the bulk solder. The influence of strain rate on the load capability and failure mode of the joint was clarified by analyzing the experimental and simulation results. It was shown how, at the lower strain rates, the normal stress generated at the interface is limited by the significant creep relaxation developed in the bulk solder and thus is not sufficiently high to initiate interfacial damage, whereas at higher rates, a large amount of the external energy is dissipated into interfacial damage development.

Springer US2011

Correlation of microstructural and mechanical properties in particle reinforced lead-free solders

Venkatesh Sivasubramaniam

Solders are widely used as interconnect material for the electronic industry. The solder interconnect provides both electrical contact and mechanical connection between the integrated circuit devices and their substrate. Thus solders with good reliability are required to facilitate successful functioning of electronic devices and components that we encounter in daily life. However traditionally used Sn-Pb solders have to be replaced by Sn rich lead-free solders because of the toxicity concerns surrounding the use of lead. Many Sn-rich lead-free solders do exists, however they are not drop-in substitutes for Sn-Pb solders as the knowledge concerning their mechanical behavior and reliability are not well understood. A particularly critical aspect is the lead-free solders' reliability at high service temperatures required in several microelectronics applications. Such applications require solders possessing good microstructural stability, strength and creep resistance. In this regard, literature reports the alloying and the composite approach to improve the properties of the lead-free solders. Among these two methods, the composite approach wherein suitable particles are added in to the solder matrix seems to be promising. However, the scientific understanding between property improvement and characteristic of the particle reinforcement is too limited. Also the processing of the composite solder is limited by stringent process parameters that require significant improvement to increase its industrial applicability. Another important issue arises from the fact that solders as such exhibit complex elasto-plastic properties. Furthermore the solder's mechanical response is greatly influenced by specimen geometry and process parameters. Therefore novel localized strain measurement techniques that could assess and understand the mechanical response of the solder without much effect from substrate are required. The goals of the current PhD thesis are identified within the framework of the issues previously mentioned. The composite approach is demonstrated to improve the microstructural and the mechanical properties of existing lead-free solders. As reinforcing particles either Cu or Ni is used with volume fraction from 0.8-4.2 vol. %, while Sn-4.0Ag-0.5Cu (SAC405) alloy is chosen as matrix. Throughout the study the results of the composite solder is compared against the un-reinforced SAC405 reference solder. The solder material in bulk form (reflowed in ceramic crucible) is used for microstructural analysis and also as joining material between Cu substrate to investigate the mechanical response in tension and shear. The current research demonstrates a new processing method developed for producing composite lead-free solders utilizing micro and nanometer scale metallic particles. Generally the preparation of nano-composite solders requires several process parameters that restrict their industrial use. In literature even the exact process route is still not disclosed due to technical secrecy. This processing issue could be overcome by adopting the mechanical mixing method reported in this research. A significant improvement has been achieved in reducing porosity within the newly developed composite solders. This reduced porosity improves the consistency of mechanical properties in many folds. Till date no standardized mechanical test method for solder alloys has been reported. In this regard, the design of a shear test specimen proposed in this PhD thesis is novel. Extensive finite element (FE) simulations accompanied by series of experimental tests is performed to optimize the specimen geometry. This proposed specimen geometry minimizes the effect of plastic strain localization which ensures almost uniform plastic strain at the center of the joining layer. This design leads to "cleaner" shear tests minimally affected by stress concentration and with a low risk of premature failure at the substrate/solder interface. As a consequence, reliable mechanical properties can be identified from experimental results. In case of tensile specimens, the gap width is chosen such that effects of plastic constraints from rigid substrates (Cu) are kept at minimum. In-situ optical extensometry combined with Digital Image Correlation (DIC) is used to determine the stress-strain response of the solder joint. Finally, the constitutive properties of solder materials are numerically identified from the global stress-strain response of the solder joint by inverse FE modeling of the shear test. For the first time, a 3D FE homogenization model for particle reinforced composites is employed to predict the solder elastoplastic behavior as a function of the particle volume fraction. This objective has been demonstrated on the example of composite solder prepared with micrometer sized Ni particles. The homogenization results are then compared with the constitutive model of the composite solder identified from the experimental data. This comparison serves as a basis for the discussion of the effects of particle reinforcement in the composite solder. Another objective of the current research is to investigate the effect of adding reactive particles to the solder alloy on the subsequent growth of the interfacial intermetallic layer and the ensuing tensile strength of the joint. To address this objective, two composite solders (solder alloy with additional particles) were fabricated for comparison against the un-reinforced alloy through aging at 150 °C for up to one week. Either micrometer scale or nanometer scale Cu particles are employed as the compositing particles to subsequently produce additional Cu6Sn5 intermetallic reinforcing phases. The expectation is that the large difference in initial particle size of this additional source of Cu will help to elucidate the rate limiting mechanism growth of the interfacial intermetallics. It is demonstrated that while a similarly reduced steady-state of intermetallic growth is obtained for the two composite samples, it is achieved sooner in the one fabricated with the nanometer scale particles. Mechanical tests indicate that the composite specimen prepared with nanometer scale particles also provides the highest tensile strength both before and after aging. As a final objective, creep tests were conducted at room temperature to assess the steady state creep strain rate of composite solders. Though many publications report creep tests conducted at various temperatures, the current study is unique with respect to the attempt made to clarify the effect of applied stress on the creep strain rate of the composite solders. Subsequent effects of particle reinforcement on the creep strain rate are discussed. It is demonstrated that composite solder prepared with Cu nanoparticles exhibit the best creep properties (lowest creep strain rate) among the solders considered in this study. The mechanisms responsible for strength increase, ductility loss and creep properties observed for various composite solders are rationalized based on the current understanding of the microstructure-mechanical response relationship. Differential Scanning Calorimetric (DSC) studies were conducted to investigate the solidification temperature of the particle reinforced lead-free solders. The distinct microstructural features obtained for composite solder are shown to be significantly influenced by disparity in solidification kinetics caused by particle addition. In this context a key result achieved during this research was the solidification mechanism proposed for the composite solders. Overall the results of this research lead to: (i) development of a viable processing method for nano composite solders with excellent creep properties and (ii) demonstration of a novel approach (experimental and modeling) to better understand the effect of particle volume fraction on the elasto-plastic response of the composite solder. This approach will provide some valuable guidelines for tailoring the properties of composite lead-free solder based on the target application.

EPFL2009

Assessment of mechanical properties in unirradiated and irradiated zircaloys and steels with non-standard tests and finite element calculations

Emiliano Campitelli

Nuclear materials have been successfully employed in commercial fission reactors for many decades. Research and material development for advanced fission system and future thermo-nuclear fusion reactors have acquired maturity while great challenges remain to be taken up. The goal of the material scientist is to understand the way neutron irradiation and aggressive reactor environment alter the original material properties and to safely manage the operation conditions for each particular reactor component. In the framework of surveillance programme, the need has arisen to design experimental methods to be used with a limited amount of material for Post Irradiation Examination (PIE) of reactor components to test the integrity of those on a regular time basis. Having to deal with radioactive material, it is particularly important to devise experimental testing procedures as simple as possible but from which reliable information can be drawn. Within materials development programme, irradiations have continuously been performed either in reactors or with particle accelerators. In both cases, the irradiation volumes are usually limited. The concern was to optimize the volume of the irradiated samples in order to increase their number to obtain a good statistical representation of their mechanical behavior after irradiation, and to have a very definite and homogeneous damage dose and irradiation temperature. The concerns explained above originated the development of small-scale specimen testing methods. Advantage is taken of the fact that, within size and geometry limitations, some mechanical properties of a sample are independent of its absolute size. In this way it is possible to perform mechanical tests on samples of reduced volume that represents big advantages to face the aforementioned challenges. Non-standard test on very small specimen have also been proposed to extract mechanical properties of the bulk. If these non-standard testing methods are to be employed for PIE, it becomes necessary to develop modeling tool to understand under what circumstances they can provide useful information and specially how to obtain it. Finite element modeling (FEM) of the non-standard tests has played a key role in this sense, providing a link between standard test analysis and that of the non-standard ones. The goal of this work was to implement finite element models for non-standard tests to ultimately study the mechanical properties of Zircaloy cladding tube and tempered martensitic steels in the unirradiated and irradiated condition. One model was developed for ring tensile tests. It was first validated with a quite ductile iron-chromium ferritic alloy, for which ring and uniaxial tensile test were performed at room temperature. Using the constitutive behavior determined from the uniaxial tensile tests, the load – displacement curves of the ring tensile tests were very well reproduced. Another model was developed for the so-called small ball punch test, which consists in deforming a 3 mm diameter disk clamped between two dies with a ball puncher and to record the load – displacement curve. In order to validate the model, a series of ball punch tests and of tensile tests were carried out on the austenitic steel 316L at room temperature. In this last case, the ball punch test curves were also well reconstructed with the model. Based upon a simple analysis and simulation results, we showed that it is possible to obtain, by ring tensile tests, a quick and accurate assessment of the constitutive behavior of an unknown material, simply by processing the experimental force-elongation data in the same way as for uniaxial tests, taking half the ring perimeter as initial length and twice the ring cross section as the sample cross section. The ring tensile test method was used to determine the hoop mechanical properties of Zircaloy tubes used in fission reactors. Tensile tests were also performed in the axial direction of the Zircaloy tubes. Differences between the yield stress as well as the post-yield behaviors in the axial direction and those in the circumferential direction of the tube could be measured reflecting some component of the anisotropy of the tube. Ball punch specimens were cut also out of the tube in the radial direction and tested. Using the FE model for punch test, in which the anisotropy of the mechanical properties are taken into account through the Hill's theory, the values of the Hill's yield function parameters were estimated. The irradiation hardening of neutron irradiated Zircaloy tube was finally determined. The ball punch test model was extensively used for numerical investigations, to determine how the constitutive behavior of the materials (uniaxial true stress – true strain) employed as input acts in combination with other experimental factors (specimen thickness, friction coefficient) in mediating the shape of the experimental curves. In the light of the results, we derived calibrations between parameters of the non-standard and standard tests. For instance we proposed a new approach to determine the yield stress from the ball punch test. We also showed how the slope of the punch tests scales with the average flow stress of the material. The calibrations proposed in the past by authors and published in the open literature have been revisited and critically reviewed. Based upon the findings of the numerical studies, we propose a given methodology to unambiguously evaluate the material constitutive behavior of a material in a case where the only available data comes from various non-standard tests. Experimental ball punch test results obtained on proton-irradiated specimen (0.5 dpa and Tirr = 523 K), of the tempered martensitic steel, EUROFER97, have been obtained. The irradiation hardening was found equal to about 150 MPa, consistently with previous data on similar steels and irradiation conditions. From the simulations of the ball punch test curves of the irradiated specimens, it was concluded that the irradiation does not affect the strain hardening at relatively low fluence.