Browsing Engineering & Technology by Title
Now showing items 32-35 of 35
Thermo-mechanical reliability studies of lead-free solder interconnectsSolder 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.
Towards an efficient indexing and searching model for service discovery in a decentralised environment.Given the growth and outreach of new information, communication, computing and electronic technologies in various dimensions, the amount of data has explosively increased in the recent years. Centralised systems suffer some limitations to dealing with this issue due to all data is stored in central data centres. Thus, decentralised systems are getting more attention and increasing in popularity. Moreover, efficient service discovery mechanisms have naturally become an essential component in both large-scale and small-scale decentralised systems and. This research study is aimed at modelling a novel efficient indexing and searching model for service discovery in decentralised environments comprising numerous repositories with massive stored services. The main contributions of this research study can be summarised in three components: a novel distributed multilevel indexing model, an optimised searching algorithm and a new simulation environment. Indexing model has been widely used for efficient service discovery. For instance; the inverted index is one of the popular indexing models used for service retrieval in consistent repositories. However, redundancies are inevitable in the inverted index which is significantly time-consuming in the service discovery and retrieval process. This theeis proposes a novel distributed multilevel indexing model (DM-index), which offers an efficient solution for service discovery and retrieval in distributed service repositories comprising massive stored services. The architecture of the proposed indexing model encompasses four hierarchical levels to eliminate redundancy information in service repositories, to narrow the searching space and to reduce the number of traversed services whilst discovering services. Distributed Hash Tables have been widely used to provide data lookup services with logarithmic message costs which only require maintenance of limited amounts of routing states. This thesis develops an optimised searching algorithm, named Double-layer No-redundancy Enhanced Bi-direction Chord (DNEB-Chord), to handle retrieval requests in distributed destination repositories efficiently. This DNEB-Chord algorithm achieves faster routing performances with the double-layer routing mechanism and optimal routing index. The efficiency of the developed indexing and searching model is evaluated through theoretical analysis and experimental evaluation in a newly developed simulation environment, named Distributed Multilevel Bi-direction Simulator (DMBSim), which can be used as cost efficient tool for exploring various service configurations, user retrieval requirements and other parameter settings. Both the theoretical validation and experimental evaluations demonstrate that the service discovery efficiency of the DM-index outperforms the sequential index and inverted index configurations. Furthermore, the experimental evaluation results demostrate that the DNEB-Chord algorithm performs better than the Chord in terms of reducing the incurred hop counts. Finally, simulation results demonstrate that the proposed indexing and searching model can achieve better service discovery performances in large-scale decentralised environments comprising numerous repositories with massive stored services.
A Trust Evaluation Framework in Vehicular Ad-Hoc NetworksVehicular Ad-Hoc Networks (VANET) is a novel cutting-edge technology which provides connectivity to millions of vehicles around the world. It is the future of Intelligent Transportation System (ITS) and plays a significant role in the success of emerging smart cities and Internet of Things (IoT). VANET provides a unique platform for vehicles to intelligently exchange critical information, such as collision avoidance or steep-curve warnings. It is, therefore, paramount that this information remains reliable and authentic, i.e., originated from a legitimate and trusted vehicle. Due to sensitive nature of the messages in VANET, a secure, attack-free and trusted network is imperative for the propagation of reliable, accurate and authentic information. In case of VANET, ensuring such network is extremely difficult due to its large-scale and open nature, making it susceptible to diverse range of attacks including man-in-the-middle (MITM), replay, jamming and eavesdropping. Trust establishment among vehicles can increase network security by identifying dishonest vehicles and revoking messages with malicious content. For this purpose, several trust models (TMs) have been proposed but, currently, there is no effective way to compare how they would behave in practice under adversary conditions. Further, the proposed TMs are mostly context-dependent. Due to randomly distributed and highly mobile vehicles, context changes very frequently in VANET. Ideally the TMs should perform in every context of VANET. Therefore, it is important to have a common framework for the validation and evaluation of TMs. In this thesis, we proposed a novel Trust Evaluation And Management (TEAM) framework, which serves as a unique paradigm for the design, management and evaluation of TMs in various contexts and in presence of malicious vehicles. Our framework incorporates an asset-based threat model and ISO-based risk assessment for the identification of attacks against critical risks. TEAM has been built using VEINS, an open source simulation environment which incorporates SUMO traffic simulator and OMNET++ discrete event simulator. The framework created has been tested with the implementation of three types of TM (data-oriented, entity-oriented and hybrid) under four different contexts of VANET based on the mobility of both honest and malicious vehicles. Results indicate that TEAM is effective to simulate a wide range of TMs, where the efficiency is evaluated against different Quality of Service (QoS) and security-related criteria. Such framework may be instrumental for planning smart cities and for car manufacturers.
Vulnerability and adaptive capacity of rural coastal fishing communities in Ghana to climatic and socio-economic stressorsThe global fishing industry is a source of livelihood for about 820 million people. About 90% of this number are small-scale fisherfolk and traders, living in rural fishing-dependent communities in tropical, developing, and least developed countries. Although the industry generates about $362 billion annually, fishing-dependent communities are generally characterised by chronic poverty and deprivation. Decrease in fish productivity and availability in tropical regions, as well as, increase in the frequency and intensity of extreme weather events due to climate change processes have exacerbated the plight of fishing-dependent communities. In 1970, an agenda for research and development of small-scale fishing was set out. However, rural fishing communities are still considered the poorest of the poor today. They are also considered the most vulnerable as future climate change predictions indicate more extreme events and further reductions in maximum fish catch and revenue potentials. Therefore, there are continued calls for research efforts to understand the impacts of multiple climatic and socioeconomic stressors on small-scale fishing livelihoods, in order to identify viable, context-specific management and policy interventions that can reduce their vulnerability. Using two rural coastal fishing communities in Ghana as a case study, the purpose of this study was to explicate how rural coastal fishing-dependent communities in a tropical context are impacted by the interaction of climatic and socio-economic factors and identify viable policy and management options to enhance their adaptive capacity. Three key research questions guided the study: (i) what are the various factors that impact small-scale fishing livelihoods/households, and how do they interact to shape vulnerability? (ii) how are the fishing communities adapting to current livelihood stressors? and, (iii) What context-specific policy and management interventions are needed to enhance their adaptive capacity and safeguard their wellbeing. The Intergovernmental Panel on Climate Change’s (IPCC) vulnerability framework and the Sustainable Livelihoods Approach (SLA) were integrated as the theoretical underpinnings of the study. A mixed-methods approach was adopted. A total of 120 fishing households were selected and surveyed through a stratified-snowball sampling technique. Several gender and age-group disaggregated focus groups with participatory activities, semi-structured interviews, and key informant discussions were also conducted to collect primary data. These were combined with climatic data to assess each household’s vulnerability, and through triangulated analyses, explicate how it is mediated by socio-cultural, institutional, and policy structures.