Mechanical behaviour and reliability of Sn3.8AgO.7Cu solder for a surface mount assembly

The demands for compact, light weight and Iow cost electronic products have resulted in the miniaturisation of solder interconnects to a sub-millimetre scale. With such a reduction in size, the solder joints cannot be assumed to behave in the same way as bulk solder in terms of reliability due to the fact that their material behaviours are influenced by the joint size and microstructure. The complexity of their reliability assessment is furthermore compounded by the demand for the replacement of traditional SnPb solder alloys with lead-free alloys, due to the presence of the toxic and health hazardous element (Pb) in the former alloy. However, these new lead-free alloys have much less history of industrial applications, and their material and reliability data is not as well developed as traditional lead-based alloys. In addition, most previous reliability assessments using finite element analysis have assumed a uniform distribution of temperature within the electronic assembly, which conflicts the actual temperature conditions during circuit operation. Therefore, this research was undertaken to analyse the effect of solder joint size on solder material properties from which material models were developed, and to determine the effect of an actual (nonuniform) temperature distribution in an electronic assembly on the reliability of its solder joints. Following a review of lead-free solders and potential lead-free alloys, lead-free solder microstructures, and the reliability issues and factors affecting the reliability of solder joints, the practical aspects of this research were carried out in two main parts. The first part consisted of substantial work on the experimental determination of the temperature distribution in a typical surface mount chip resistor assembly for power cycling conditions, and the stress-strain and creep behaviour for both Sn3.8AgO.7Cu solder joints and reflowed bulk solder. This also included building material models based on the experimental data for the solder joints tested and comparison with that for bulk solder. Based on the comparison of the material properties, two extreme material models were selected for the reliability study. Size and microstructure effects on the solder material properties were also discussed in this part. The second part comprised of extensive finite element analysis of a surface mount chip resistor assembly and reliability assessment of its solder joints. The simulation began with elasto-plastic analysis for 2D and 3D chip resistor assemblies to decide upon the kind of formulation to be used when the full complexity of both plasticity and creep is considered. The simulation was carried out considering the determined non-uniform temperature distribution and idealized or traditional uniform temperature condition. The solder joint's material properties were modelled using the two material models determined from the experimental results. The effect of temperature distribution during thermal cycling and of the selected material models on the solder joint reliability was demonstrated using finite element analysis and subsequent fatigue life estimation. In summary, this research has concluded that the material behaviour of the solder joint is different from that of bulk solder due to the effect of its size and microstructure. The anisotropic behaviour of the solder joint cannot be ignored in reliability studies, since it has a significant effect on the solder joint's fatigue life. The research also showed the significant effect of an actual (non-uniform) temperature distribution in the electronic assembly on the solder joint fatigue life

A Finite Element approach to understanding constitutive elasto-plastic, visco-plastic behaviour in lead free micro-electronic BGA structures

This work investigates the non-linear elasto-plastic and visco-plastic behaviour of lead free solder material and soldered joints. Specifically, Finite Element (FE) tools were used to better understand the deformations within Ball Grid Array solder joints (BGA), and numerical and analytical methods were developed to quantify the identified constituent deformations. FE material models were based on the same empirical constitutive models (elastic, plastic and creep) used in analytical calculations. The current work recognises the large number of factors influencing material behaviour which has led to a wide range of published material properties for near eutectic SnAgCu alloys. The work discovered that the deformation within the BGA was more complex than is generally assumed in the literature. It was shown that shear deformation of the solder ball could account for less than 5% of total measured displacement in BGA samples. Shear displacement and rotation of the solder balls relative to the substrate are sensitive to the substrate orthotropic properties and substrate geometry (relative to solder volume and array pattern). The FE modelling was used to derive orthotropic FR4 properties independently using published data. An elastic modulus for Sn3.8Ag0.7Cu was measured using homologous temperatures below 0.3. Suggested values of Abaqus-specific creep parameters m and f (not found in literature) for Sn3.8Ag0.7Cu have been validated with published data. Basic verification against simple analytical calculations has given a better understanding of the components of overall specimen displacement that is normally missing from empirical validation alone. A combined approach of numerical and analytical modelling of BGAs, and mechanical tests, is recommended to harmonise published work, exploit new material data and for more informed analysis of new configurationsEPSRC-funded PhD studentshi

Analysis of the thermo-mechanical reliability of an SMT attachment

Microsystems and microelectronics are new technologies that are being increasingly integrated into everyday life. Some of the main concerns in the reliability of micro-electronics are the solders, leads and packages within the systems. Our MQP involved the observation, testing, and analysis of the different failures that occur in surface mount technology (SMT). We have conducted extensive literary research on the history of SMT, what failures and issues commonly occur, and how failures and issues are being addressed. Lastly, we used finite element analysis (FEA) software to test failures due to, but not limited to, vibrations, thermal expansion mismatch, and material properties. We correlated the results of our modeling with laboratory testing and supportive detailed uncertainty analysis

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

Vibration Fatigue of Leaded Solder Joint Interconnects for PCB Electronics

With the increasing prevalence of electronic equipment worldwide, there is also a decrease in the size of the components on their printed circuit boards (PCBs), leading to an increase in the density of these components. A significant amount of failure in electronic equipment is vibration fatigue of solder joints and their attachments. However, the complexity of these PCBs and their components has made finite element modeling (FEM) more complex, adding considerable time to create and analyze a model. This paper aims to provide a literature review for the vibration fatigue of leaded solder components, create a test setup, and validate an analytical solder joint stress model. The literature review provides a walkthrough on modeling PCBs and their components using FEMs and analytical models, fatigue modeling methodology, and fatigue testing data and highlights gaps in the literature. This review was important to compile due to the limited data and the rigor required to find it all when searching. With this literature review collected, testing was to be completed using an analytical model highlighted. Therefore, a setup and procedure have been developed to test the vibration fatigue of leaded solder attachments. The setup combines a test specimen, specimen mounting head, and preliminary model correlation between the test specimen and FEM. Using initial model correlations, an analytical solder joints stress model, and fatigue curves from literature, a vibration fatigue life prediction was made for the test specimen, and tests were run. However, the results were inconclusive and further testing is deemed necessary. Suggestions have been made, such as picking other analytical models to test, modifying the test setup, and increasing the fidelity of local areas in the FEM

Characterisation of the cyclic softening properties of solder

Master'sMASTER OF ENGINEERIN

Experimental Characterization Of Cu Free-Air Ball And Simulations Of Dielectric Fracture During Wire Bonding

Wire bonding is the process of forming electrical connection between the integrated circuit (IC) and its structural package. ICs made of material with low dielectric constant (low-k) and ultra low-k are porous in nature, and are prone to fracture induced failure during packaging process. In recent years, there is increasing interest in copper wire bond technology as an alternative to gold wire bond in microelectronic devices due to its superior electrical performance and low cost. Copper wires are also approximately 25% more conductive than Au wires aiding in better heat dissipation. At present, validated constitutive models for the strain rate and temperature dependent behavior of Cu free-air ball (FAB) appear to be largely missing in the literature. The lack of reliable constitutive models for the Cu FAB has hampered the modeling of the wire bonding process and the ability to assess risk of fracture in ultra low-k dielectric stacks. The challenge to FAB characterization is primarily due to the difficulty in performing mechanical tests on spherical FAB of micrometers in size. To address this challenge, compression tests are performed on FAB using custom-built microscale tester in the current study. Specifically, the tester has three closed-loop controlled linear stages with submicron resolution, a manual tilt stage, a six-axis load cell with sub-Newton load resolution for eliminating misalignment, a milliNewton resolution load cell for compression load measurement, a capacitance sensor to estimate sample deformation and to control the vertical stage in closed loop, a high working depth camera for viewing the sample deformation, and controllers for the stages implemented in the LabVIEW environment. FAB is compressed between tungsten carbide punches and a constitutive model is developed for Cu FAB through an inverse modeling procedure. In the inverse procedure, appropriate constitutive model parameter values are iterated through an automated optimization workflow, until the load-displacement response matches the experimentally observed response. Using the material properties obtained from the experiment, a macroscale finite element model for the impact and ulatrasonic vibration stages of wire bonding process is constructed to simulate (a) Plastic deformation of the Cu FAB at different time steps (b) Evolution of contact pressure (c) Phenomenon such as pad splash and lift-off. The deformations from the macroscale model are provided as input to a microscale model of the dielectric with copper vias as well as line-type heterogeneities. The microscale model is used to identify potential crack nucleation sites as well as the crack path within the ILD stack during wire bonding. The modeling provides insight into the relative amounts of damage accumulated during the impact and the ultrasonic excitation stages. In general, Bonding over Active Circuit (BOAC) has made wire bonding a considerable challenge due to the brittleness of the dielectric. Identifying and locating microscale fractures beneath the bond pads during wire bonding require extensive sample preparation and investigation for microscopic characterization. While simulations of fracture are an attractive alternative to trial and error microscopic characterization, the length scale of components involved in wire bonding varies from millimeters to nanometers. Therefore, constructing a finite element mesh across the model is computationally costly. Also, a multi-scale simulation framework is necessary. Such a modeling framework is also developed in this work to predict crack nucleation and propagation in wire bond induced failure

Thermo-mechanical reliability studies of lead-free solder interconnects

N/ASolder interconnections, also known as solder joints, are the weakest link in electronics packaging. Reliability of these miniature joints is of utmost interest - especially in safety-critical applications in the automotive, medical, aerospace, power grid and oil and drilling sectors. Studies have shown that these joints' critical thermal and mechanical loading culminate in accelerated creep, fatigue, and a combination of these joints' induced failures. The ball grid array (BGA) components being an integral part of many electronic modules functioning in mission-critical systems. This study investigates the response of solder joints in BGA to crucial reliability influencing parameters derived from creep, visco-plastic and fatigue damage of the joints. These are the plastic strain, shear strain, plastic shear strain, creep energy density, strain energy density, deformation, equivalent (Von-Mises) stress etc. The parameters' obtained magnitudes are inputted into established life prediction models – Coffin-Manson, Engelmaier, Solomon (Low cycle fatigue) and Syed (Accumulated creep energy density) – to determine several BGA assemblies' fatigue lives. The joints are subjected to thermal, mechanical and random vibration loadings. The finite element analysis (FEA) is employed in a commercial software package to model and simulate the responses of the solder joints of the representative assemblies' finite element models. As the magnitude and rate of degradation of solder joints in the BGA significantly depend on the composition of the solder alloys used to assembly the BGA on the printed circuit board, this research studies the response of various mainstream lead-free Sn-Ag-Cu (SAC) solders (SAC305, SAC387, SAC396 and SAC405) and benchmarked those with lead-based eutectic solder (Sn63Pb37). In the creep response study, the effects of thermal ageing and temperature cycling on these solder alloys' behaviours are explored. The results show superior creep properties for SAC405 and SAC396 lead-free solder alloys. The lead-free SAC405 solder joint is the most effective solder under thermal cycling condition, and the SAC396 solder joint is the most effective solder under isothermal ageing operation. The finding shows that SAC405 and SAC396 solders accumulated the minimum magnitudes of stress, strain rate, deformation rate and strain energy density than any other solder considered in this study. The hysteresis loops show that lead-free SAC405 has the lowest dissipated energy per cycle. Thus the highest fatigue life, followed by eutectic lead-based Sn63Pb37 solder. The solder with the highest dissipated energy per cycle was lead-free SAC305, SAC387 and SAC396 solder alloys. In the thermal fatigue life prediction research, four different lead-free (SAC305, SAC387, SAC396 and SAC405) and one eutectic lead-based (Sn63Pb37) solder alloys are defined against their thermal fatigue lives (TFLs) to predict their mean-time-to-failure for preventive maintenance advice. Five finite elements (FE) models of the assemblies of the BGAs with the different solder alloy compositions and properties are created with SolidWorks. The models are subjected to standard IEC 60749-25 temperature cycling in ANSYS 19.0 mechanical package environment. SAC405 joints have the highest predicted TFL of circa 13.2 years, while SAC387 joints have the least life of circa 1.4 years. The predicted lives are inversely proportional to the magnitude of the areas of stress-strain hysteresis loops of the solder joints. The prediction models are significantly consistent in predicted magnitudes across the solder joints irrespective of the damage parameters used. Several failure modes drive solder joints and damage mechanics from the research and understand an essential variation in the models' predicted values. This investigation presents a method of managing preventive maintenance time of BGA electronic components in mission-critical systems. It recommends developing a novel life prediction model based on a combination of the damage parameters for enhanced prediction. The FEA random vibration simulation test results showed that different solder alloys have a comparable performance during random vibration testing. The fatigue life result shows that SAC405 and SAC396 have the highest fatigue lives before being prone to failure. As a result of the FEA simulation outcomes with the application of Coffin-Manson's empirical formula, the author can predict the fatigue life of solder joint alloys to a higher degree of accuracy of average ~93% in an actual service environment such as the one experienced under-the-hood of an automobile and aerospace. Therefore, it is concluded that the combination of FEA simulation and empirical formulas employed in this study could be used in the computation and prediction of the fatigue life of solder joint alloys when subjected to random vibration. Based on the thermal and mechanical responses of lead-free SAC405 and SAC396 solder alloys, they are recommended as a suitable replacement of lead-based eutectic Sn63Pb37 solder alloy for improved device thermo-mechanical operations when subjected to random vibration (non-deterministic vibration). The FEA simulation studies' outcomes are validated using experimental and analytical-based reviews in published and peer-reviewed literature.N/

TEMPERATURE AND RATE DEPENDENT PARTITIONED CONSTITUTIVE RELATIONSHIPS FOR 95.5PB2SN2.5AG SOLDER ALLOY

One of the biggest challenges for power electronic devices is to be reliable in harsh environments. The operating temperatures in typical applications can go as high as 200ºC. The die attachment material of a power electronic device is one of the weak links in the system. The eutectic Sn-Pb solder alloy, which is the most commonly used permanent interconnect in electronics packaging cannot fulfill these service requirements, hence there is a need to find suitable replacements. Durability characterization is essential in order to accurately predict the reliability of the solder alloy chosen for the die attach material under life cycle loads. A large number of models are available, which can be used to determine the life of die attach in small signal and power modules, however the shortfall of these models is the lack of test data for all but the most common (e.g. eutectic Sn-Pb solder) die attach materials. Hence, relevant constitutive properties must be measured, as they are essential for quantitative characterization of damage accumulated in the die attach, the knowledge of which is essential for accurate durability assessment. The aim of this study is to determine the relevant constitutive properties for high temperature high lead 95.5Pb2Sn2.5Ag solder alloy (Indalloy 163) by implementing the direct local measurement technique. Temperature and loading rate dependent mechanical and constitutive properties of the afore mentioned solder alloy have been obtained by modeling the experimental data gathered by conducting monotonic, isothermal, constant strain rate tests at a range of temperatures and strain rates utilizing miniature single-lap shear specimens, with a partitioned form of the general constitutive equation