The effect of static and dynamic aging on fatigue behavior of Sn3.0Ag0.5Cu solder alloy

In microelectronic assemblies, solder joints serve as interconnection between different packaging levels and are an important cause for the failure of microelectronic products. Sn-Ag-Cu solder alloys became important after lead-based solder alloys were caused to be discarded by regulations in European Union and Japan. However, the constitutive behavior of Sn-Ag-Cu alloys is not as well understood as lead-based solder alloys, and many studies confirm the aging of these alloys with time. The aging of Sn-Ag-Cu alloys and its effect on mechanical behavior challenges the reliability prediction of microelectronic assemblies. In this study, the effect of pretest isothermal aging and in-test aging on the fatigue behavior of Sn3.0Ag0.5Cu alloy are examined using the microstructurally adaptive creep model (MACM) and the maximum entropy fracture model (MEFM). In this thesis, first, the development of microstructurally adaptive creep model is reviewed. Compared to traditional constitutive models, this model considers the effect of thermal history. Two microstructural parameters, the average Ag3Sn particles size and the average primary-Sn cell size are identified as critical parameters and incorporated into a modified Dorn creep form, which can describe both climb-controlled and glide-controlled dislocation motions. Next, the maximum entropy fracture model is discussed and compared to traditional fatigue fracture model. The MEFM utilizes the damage accumulation parameter, which connects the accumulated damage to the accumulated inelastic dissipation. This parameter is independent of sample geometry, test temperature and strain rate. Later, using MACM and MEFM, the extraction of the damage accumulation parameters is presented. The creep models for different aging conditions are constructed first based on microstructural characterization. The damage accumulation parameters of 25 celsius and 100 celsius tests are fit using MEFM. The parameters are presumed different for the two conditions because of the different aging states of the material. The concepts of static aging and dynamic aging are introduced and utilized to describe pretest aging and in-test aging. In 25 celsius test, with longer static aging, the damage accumulation parameter is smaller, indicating a faster fatigue damage accumulation. Through the relationship between damage accumulation parameter and the average primary-Sn cell size, the influence of microstructural evolution introduced by static aging on fatigue behavior is confirmed. In 100 celsius tests, the effect of dynamic aging is captured by the change of damage accumulation parameter in experiments. Comparing the damage accumulation parameters from 25 celsius and 100 celsius tests, during test, further aging of Sn3.0Ag0.5Cu microstructure occurs, degrading fatigue behavior until microstructural evolution is completed. Finally, the thesis is summarized and future work to better characterize the relationship between fatigue behavior and microstructure is put forward. The proposed work includes building a dynamic aging model and microstructural evolution model

A novel entropy production based full-chip TSV fatigue analysis

Through-silicon vias (TSVs) are subject to thermal fatigue due to stress over time, no matter how small the stress is. Existing works on TSV fatigue all rely on measurement-based parameters to estimate the lifetime, and cannot consider detailed thermal profiles. In this paper, we propose a new method for TSV fatigue prediction using entropy production during thermal cycles. By combining thermodynamics and mechanics laws, the fatigue process can be quantitatively evaluated with detailed thermal profiles. Experimental results show that interestingly, the landing pad possesses the most easy-to-fail region, which generates up to 50% more entropy compared with the TSV body. The impact of landing pad dimension and TSV geometries are also studied, providing guidance for reliability enhancement. Finally, full-chip fatigue analysis is performed based on stress superposition. To the best of the authors\u27 knowledge, this is the first TSV fatigue model that is free of measurement data fitting, the first that is capable of considering detailed thermal profiles, and the first framework for efficient full-chip TSV fatigue analysis. --Abstract, page iii

Health Condition Assessment of Multi-Chip IGBT Module with Magnetic Flux Density

To achieve efficient conversion and flexible control of electronic energy, insulated gate bipolar transistor (IGBT) power modules as the dominant power semiconductor devices are increasingly applied in many areas such as electric drives, hybrid electric vehicles, railways, and renewable energy systems. It is known that IGBTs are the most vulnerable components in power converter systems. To achieve high power density and high current capability, several IGBT chips are connected in parallel as a multi-chip IGBT module, which makes the power modules less reliable due to a more complex structure. The lowered reliability of IGBT modules will not only cause safety problems but also increase operation costs due to the failure of IGBT modules. Therefore, the reliability of IGBTs is important for the overall system, especially in high power applications. To improve the reliability of IGBT modules, this thesis proposes a new health state assessment model with a more sensitive precursor parameter for multi-chip IGBT module that allows for condition-based maintenance and replacement prior to complete failure. Accurate health condition monitoring depends on the knowledge of failure mechanism and the selection of highly sensitive failure precursor. IGBT modules normally wear out and fail due to thermal cycling and operating environment. To enhance the understanding of the failure mechanism and the external characteristic performance of multi-chip IGBT modules, an electro-thermal finite element model (FEM) of a multi-chip IGBT module used in wind turbine converter systems was established with considerations for temperature dependence of material property, the thermal coupling effect between components, and the heat transfer process. The electro-thermal FEM accurately performed temperature distribution and the distribution electrical characteristic parameters during chip solder degradation. This study found an increased junction temperature, large change of temperature distribution, and more serious imbalanced current sharing during a single chip solder aging, thereby accelerating the aging of the whole IGBT module. According to the change of thermal and electrical parameters with chip solder fatigue, the sensitivity of fatigue sensitive parameters (FSPs) was analyzed. The collector current of the aging chip showed the highest sensitivity with the chip solder degradation compared with the junction temperature, case temperature, and collector-emitter voltage. However, the current distribution of internal components remains inaccessible through direct measurements or visual inspection due to the package. As the relationship between the current and magnetic field has been studied and gradually applied in sensor technologies, magnetic flux density was proposed instead of collector current as a new precursor for health condition monitoring. Magnetic flux density distribution was extracted by an electro-thermal-magnetic FEM of the multi-chip IGBT module based on electromagnetic theory. Simulation results showed that magnetic flux density had even higher sensitivity than collector current with chip solder degradation. In addition, the magnetic flux density was only related with the current and was not influenced by temperature, which suggested good selectivity. Therefore, the magnetic flux density was selected as the precursor due to its better sensitivity, selectivity, and generality. Finally, a health state assessment model based on backpropagation neural network (BPNN) was established according to the selected precursor. To localize and evaluate chip solder degradation, the health state of the IGBT module was determined by the magnetic flux density for each chip and the corresponding operating conduction current. BPNN featured good self-learning, self-adapting, robustness and generalization ability to deal with the nonlinear relationship between the four inputs and health state. Experimental results showed that the proposed model was accurate and effective. The health status of the IGBT modules was effectively recognized with an overall recognition rate of 99.8%. Therefore, the health state assessment model built in this thesis can accurately evaluate current health state of the IGBT module and support condition-based maintenance of the IGBT module

A Life Prediction Model of Multilayered PTH Based on Fatigue Mechanism.

Plated through hole (PTH) plays a critical role in printed circuit board (PCB) reliability. Thermal fatigue deformation of the PTH material is regarded as the primary factor affecting the lifetime of electrical devices. Numerous research efforts have focused on the failure mechanism model of PTH. However, most of the existing models were based on the one-dimensional structure hypothesis without taking the multilayered structure and external pad into consideration. In this paper, the constitutive relation of multilayered PTH is developed to establish the stress equation, and finite element analysis (FEA) is performed to locate the maximum stress and simulate the influence of the material properties. Finally, thermal cycle tests are conducted to verify the accuracy of the life prediction results. This model could be used in fatigue failure portable diagnosis and for life prediction of multilayered PCB

Modelling of the reliability of flip chip lead-free solder joints at high-temperature excursions

At high-temperature operations of electronic control devices, Tin-Silver-Copper (SnAgCu) alloy solder joints used to assemble the component of the devices are functioning at homologous temperature above 0.8. In such ambient temperatures, solder alloys have limited mechanical strength and will be sensitive to strain rate. The sensitivity of solder properties to creep/visco-plastic deformation increases the rate of accumulation of plastic damage in the alloy and decreases the number of cycles to failure (Nf) of the joints. Most untimely rupture of solder joints in high-temperature electronics (HTE) system usually culminates in colossal loss of resources and lives. Typical incidences are reported in recent automotive and aircraft crashes as well as the collapse of oil-well logging equipment. To increase the mean time to failure (MTTF) of solder joints in HTE, an in-depth understanding and accurate prediction of the response of solder joints to thermally induced plastic strain damage is crucial. This study concerns the prediction of the reliability of lead-free solder joints in a flip chip (FC) model FC48D6.3C457 which is mounted on a substrate and the assembly subjected to high-temperature excursions. The research investigates the effect of the high-temperature operations on reliability of the joints. In addition, the investigation examines the impact of control factors (component stand-off height (CSH), inter-metallic compound (IMC) thickness, number of thermal cycle and solder volume) on Nf of the joints. A model developed in the course of this investigation was employed to create the assembly solder joints architecture. The development of the model and creation of the bump profile involve a combination of both analytical and construction methods. The assembled package on a printed circuit board (PCB) was subjected to accelerated temperature cycle (ATC) employing IEC standard 60749-25 in parts. The cycled temperature range is between -38 oC and 157 oC. Deformation behaviour of SnAgCu alloy solder in the joints is captured using Anand’s visco-plasticity model and the response of other materials in the assembly were simulated with appropriate model. The results demonstrate that the reliability of solder joints operating at elevated temperatures is dependent on CSH, thickness of IMC and solder volume. It also shows that incorporating the IMC layer in the geometric models significantly improves the level of accuracy of fatigue life prediction to ± 22.5% (from the ± 25% which is currently generally accepted). The findings also illustrate that the magnitude of the predicted damage and fatigue life are functions of the number of ATC employed. The extensive set of results from the modelling study has demonstrated the need for incorporating the IMC layer in the geometric model to ensure greater accuracy in the prediction of solder joint service life. The technique developed for incorporating the IMC layer will be of value to R&D engineers and scientists engaged in high-temperature applications in the automotive, aerospace and oil-well logging sectors. The results have been disseminated through peer reviewed journals and also presentations at international conferences

Multiple linear regression parameters for determining fatigue-based entropy characterisation of magnesium alloy

This paper presents the development of the multiple linear regression approach based on the stress ratio and applied load that was assessed using entropy generation. The energy dissipation is associated with material degradation to determine the fatigue life with consideration to the irreversible thermodynamic framework. This relationship was developed by predicting a complete entropy generation using a statistical approach, where a constant amplitude loading was applied to evaluate the fatigue life. By conducting compact tension tests, different stress ratios were applied to the specimen. During the tests, the temperature change was observed. The lowest entropy generation was 2.536 MJm-3 K-1 when 3,000N load with a stress ratio of 0.7 was applied to the specimen. The assumptions of the models were considered through graphical residual analysis. As a result, the predicted regression model based on the applied load and stress ratio was found to agree with the results of the experiment, with only 9.3% from the actual experiment. Therefore, the entropy generation can be predicted to access the dissipated energy as an irreversible degradation of a metallic material, subjected to cyclic elastic-plastic loading. Thermodynamic entropy is shown to play an important role in the fatigue process to trace the fatigue life

AN ENTROPIC THEORY OF DAMAGE WITH APPLICATIONS TO CORROSION-FATIGUE STRUCTURAL INTEGRITY ASSESSMENT

This dissertation demonstrates an explanation of damage and reliability of critical components and structures within the second law of thermodynamics. The approach relies on the fundamentals of irreversible thermodynamics, specifically the concept of entropy generation due to materials degradation as an index of damage. All failure mechanisms that cause degradation, damage accumulation and ultimate failure share a common feature, namely energy dissipation. Energy dissipation, as a fundamental measure for irreversibility in a thermodynamic treatment of non-equilibrium processes, leads to and can be expressed in terms of entropy generation. The dissertation proposes a theory of damage by relating entropy generation to energy dissipation via generalized thermodynamic forces and thermodynamic fluxes that formally describes the resulting damage. Following the proposed theory of entropic damage, an approach to reliability and integrity characterization based on thermodynamic entropy is discussed. It is shown that the variability in the amount of the thermodynamic-based damage and uncertainties about the parameters of a distribution model describing the variability, leads to a more consistent and broader definition of the well know time-to-failure distribution in reliability engineering. As such it has been shown that the reliability function can be derived from the thermodynamic laws rather than estimated from the observed failure histories. Furthermore, using the superior advantages of the use of entropy generation and accumulation as a damage index in comparison to common observable markers of damage such as crack size, a method is proposed to explain the prognostics and health management (PHM) in terms of the entropic damage. The proposed entropic-based damage theory to reliability and integrity is then demonstrated through experimental validation. Using this theorem, the corrosion-fatigue entropy generation function is derived, evaluated and employed for structural integrity, reliability assessment and remaining useful life (RUL) prediction of Aluminum 7075-T651 specimens tested

Modelling to predict the reliability of solder joints

The work in this thesis investigates modelling methods to predict the reliability of solder joints under thermo-mechanical cycling. A literature review is presented covering analytical methods, creep laws and fatigue laws, and advanced damage mechanics methods. The use of FEA (Finite Element Analysis) to model creep along with a fatigue law to predict lifetime appears to be the most widely used and validated technique at present. The FEA discretisation of elasticity problems is derived using the principle of minimum potential energy and implemented in the code FATMAN (Finite-element Analysis Tool, Multi-physics And Nonlinear). A novel implicit solution scheme called LENI is proposed to allow modelling of creep in solder. The sinh law for steady-state creep and the Armstrong-Frederick kinematic hardening law to capture primary creep have been implemented in FATMAN using the LENI scheme. The advantage over an explicit discretisation is investigated. An inverse analysis method for determining material properties is used to determine constants for the kinematic hardening law from experimental creep curves. A damage law is presented which allows the prediction of crack propagation through a solder joint. A failure criteria based on the increase in electrical resistance is used, which removes the need for an empirical fatigue law. The steady state creep law, the kinematic hardening law and the damage law are all applied to modelling of tests developed at the NPL (National Physical Laboratory) including novel crack detection tests, an isothermal fatigue test, and accelerated thermal cycling of resistors