Solder pastes are complex materials whose properties are governed by many factors. Variations exhibited in solder paste characteristics make it increasingly difficult to understand the correlations between solder paste properties and their printing process performance. The recent EU directives on RoHS (Restriction of Hazardous Substances – enacted by UK regulations) and WEEE (Waste from Electrical and Electronic Equipment) has led to the use of lead-free soldering in the SMA (surface mount assembly) process, and an urgent need for better understanding of the characteristics and printing performance of new solder paste formulations. Equally, as the miniaturisation of hand-held and consumer electronic products continues apace, the solder paste printing process remains a real challenge to the electronics assembly industry. This is because the successful assembly of electronic devices at the ultra-fine pitch and flip-chip geometry requires the deposition of small and consistent paste deposits from pad to pad and from board to board. The paste printing process at this chip-scale geometry depends on conditions such as good paste roll, complete aperture filling and paste release from the apertures onto the substrate pads. This means that the paste flow and deformation behaviour, i.e. the paste rheology, is very important in defining the printing performance of any solder paste. Rheological measurements can be used as a tool to study the deformation or flow experienced by the pastes during the stencil printing process. In addition, the rheological measurements can also be used as a quality control tool in the paste production process for identifying batch-to-batch variation, and to reduce the associated printing defects in the paste printing process. The work reported here on the characterisation of lead-free solder pastes and their correlation with the stencil printing process is divided into five main parts. The first part concerns the study of the effect of variations in flux and particle size distribution (PSD) on the creep recovery performance of lead-free solder pastes used for flip-chip assembly. For this study, a novel technique was calculating the extent of paste recovery and hence characterising the slumping tendency in solder pastes. The second part of the study concerns the influence of long-term ageing on the rheology and print quality of lead-free solder pastes used for flip-chip assembly, and the main focus of the work was to develop methodologies for benchmarking new formulations in terms of shelf life, rheological deterioration and print performance. The third part of the work deals with a rheological simulation study of the effect of variation in applied temperature on the slumping behaviour of lead-free solder pastes, and the fourth part considers the rheological correlation between print performance and abandon time for lead-free solder paste used for flip-chip assembly. The final part of the study concerns the influence of applied stress, application time and recurrence on the rheological creep recovery behaviour of lead-free solder pastes. The research work was funded through the PRIME Faraday EPSRC CASE Studentship grant, and was carried out in collaboration with Henkel Technologies, Hemel Hempstead, UK. The extensive set of results from the experimental programme, in particular relating to the aspect of key paste performance indicators, has been adapted by the industrial partner for implementation as part of a quality assurance (QA) tool in its production plant, and the results have also been disseminated widely through journal publications and presentations at international conferences.

In conventional cellular wireless communication system, interference modelling has focused on the six primary co-channel interfering cells (first tier co-channel cells). In the current accepted interference model, co-channel interfering cells beyond the first tier (subsequent tier co-channel cells) are neglected. This currently accepted interference models is suitable for cellular wireless communication systems operating at carrier frequencies, f c = 0.9 and 1.8 GHz, cell size radii R > 1 km and basic path loss exponent α ≥ 2. The future and emerging wireless communication systems are expected to be operating at frequencies f c > 2 GHz (3.35 - 15.75 GHz), cell size radii R≤ 1 km and basic path loss exponent α ≤ 2. This, makes the current acceptable co-channel interference model unsuitable for information capacity analysis of the future cellular systems. Therefore, a co-channel interference model suitable for future and emerging wireless communication system becomes necessary. In this thesis a new and modified interference model is proposed. The proposed interference model includes the first and subsequent tier co-channel interfering cells. The proposed interference model will be suitable for cellular wireless communication systems operating at carrier frequencies f c > 2 GHz, cell size radii R≤ 1 km and basic path loss exponent α ≤ 2. A mathematical analysis, supported by computer simulation is used, to study the uplink information capacity performance for the conventional and proposed interference model. The analysis and simulation results of the proposed interference model show that at carrier frequencies f c > 2 GHz, co-channel interfering cells beyond the first tier become active as cell size radius R, reduces. As an example for a carrier frequency f c = 15.75 GHz, cell size radius R = 100 m at a normalized reuse distance Ru = 4, there was a 15.32 % decrease in the information capacity between the conventional and proposed interference model. An information capacity - cost analysis is used to find a minimum cell size for information capacity increase in cellular wireless network, thus a theoretical limit to cell size reduction. The results show that as the cell size radius R reduces to 300 m and less, the proposed interference model show a 5.76 - 18.89 % decrease in the information capacity per unit cost (£, $, etc) at microwave carrier frequencies f c > 3.35 GHz. This result illustrates that there is a theoretical limit to cell size reduction in relation to information capacity performance and cost. An inductive approach is used to generate a formula for calculating the number of co-channel interfering cells Nn in a cellular wireless site layout. Such a formula allows one to calculate the number of co-channel interfering cells in subsequent tiers of a cellular wireless site layout. The geometric derivation shows that the number of co-channel interfering cell Nn in a subsequent tier is the product of the number of co-channel interfering cells in the first tier NI and the tier number n. Thus, the number of co-channel interfering cell in a subsequent tier Nn = NI × n. This formula enables subsequent tier co-channel interference to be included in the information capacity analysis of future and emerging, and finding the minimum cell size for information capacity increase in a cellular wireless communication system.

The need to maximise thermal performance of electronic devices coupled with the continuing trends on miniaturization of electronic packages require innovative package designs for power devices and modules such as Electronic Control Unit (ECU). Chip scale packaging (CSP) technology offer promising solution for packaging power electronics. This is as a result of the technology’s relatively improved thermal performance and inherent size advantage. In CSP technology, heat removal from the device could be enhanced through the backside of the chip. Heat dissipating units such as heat spreader and/or heat sink can be attached to the backside (reverse side) of the heat generating silicon die (via TIM) in an effort to improve the surface area available for heat dissipation. TIMs are used to mechanically couple the heat generating chip to a heat sinking device and more crucially to enhance thermal transfer across the interface. Extensive review shows that solder thermal interface materials (STIMs) apparently offer better thermal performance than comparable state-of-the-art commercial polymer-based TIMs and thus a preferable choice in packaging power devices. Nonetheless, voiding remains a major reliability concern of STIMs. This is coupled with the fact that solder joints are generally prone to fatigue failures under thermal cyclic loading. Unfortunately, the occurrence of solder voids is almost unavoidable during manufacturing process and is even predominant in lead (Pb)-free solder joints. The impacts of these voids on the thermal and mechanical performance of solder joints are not clearly understood and scarcely available in literature especially with regards to STIMs (large area solder joints). Hence, this work aims to investigate STIM and the influence of voids on the thermo-mechanical and thermal performance of STIM. As previous results suggest that factors such as the location, configuration (spatial arrangement) and size of voids play vital roles on the exact effect of voids, extensive three dimensional (3D) finite element modelling is employed to elucidate the precise effect of these void features on a Pb-free STIM selected after thermo-mechanical fatigue test of standard Pb-free solder alloys. Finite element analysis (FEA) results show that solder voids configuration, size and location are all vital parameters in evaluating the mechanical and thermal impacts of voids. Depending on the location, configuration and size of voids; solder voids can either influence the initiation or propagation of damage in the STIM layer or the location of hot spot on the heat generating chip. Experimental techniques are further employed to compare and correlate levels of voiding and shear strength for representative Pb-free solders. Experimental results also suggest that void size, location and configuration may have an influence on the mechanical durability of solder joints. The findings of this research work would be of interest to electronic packaging engineers especially in the automotive sector and have been disseminated through publications in peer reviewed journals and presentations in international conferences.

Turbo coding, a forward error correcting coding (FEC) technique, has made near Shannon Limit performance possible when Iterative decoding algorithms are used. Intersymbol interference (ISI) is a major problem in communication systems when information is transmitted through a wireless channel. Conventional approaches implement an equalizer to remove the ISI, but significant performance gain can be achieved through joint equalization and decoding. In this thesis, the suitability of turbo equalization as a means of achieving low bit error rate for high data communication systems over channels with intersymbol interference was investigated. A modified decision feedback equalizer algorithm (DFE) that provides significant improvement when compared with the conventional DFE is proposed. It estimates the data using the a priori information from the SISO channel decoder and also a priori detected data from previous iteration to minimize error propagation. Investigation was also carried out with Iterative decoding with imperfect minimum mean square error (MMSE) decision feedback equalizer, assuming soft outputs from the channel decoder that are independent identically distributed Gaussian random variables. The prefiltering method is considered in this thesis, where an all-pass filter is employed at the receiver before equalization to create a minimum phase overall impulse response. The band limited channel suffers performance degradation due to impulsive noise generated by electrical appliances. This thesis analysed a set of filter design criteria based on minimizing the bit error probability of impulse noise using digital smear filter.

The use of Permanent Magnet Synchronous Machines (PMSMs) has become widespread across numerous applications and industries. Their high power density, efficiency and accuracy of control make them excellent choices, leading them to become the industrial standard. Two issues concerning PMSMs use in recent years have been associated with the elevated cost of rare earth materials required for the Permanent Magnet (PM) rotor poles and the reliance on a direct rotor position sensor such as an encoder. PMSMs require an accurate rotor position feedback within the control scheme, traditionally provided by an encoder or resolver. These devices are excellent at providing the realtime rotor position accurately but have a negative impact on the machine as a whole. Their use increases the size, weight and cost of the electrical machine, while reducing reliability and often limiting use in extreme environments. This has created motivation for sensorless control of PMSMs, which removes the need for a position sensor. Sensorless control can be categorized into two distinctive aspects. The first is the control scheme and focuses on how position dependent properties can be used to estimate rotor position. The second, which has had less focus, is the machine design. This is focused on the ability of a machine to act as a position sensor with clear position dependent properties. Self-sensing machine design is the common term applied to this field since in essence the machine acts as its own position sensor. This thesis is concerned with self-sensing oriented design. The work presented is focused on PMSMs with inset rotor topologies. A methodology was developed to assess the position tracking capability of a machine and incorporated within a traditional machine design optimization routine. The conceptual design of the machine emphasized a generic geometrical topology, accounting for practical material selections and construction techniques. This ensured the design outcome had widespread implications, as opposed to a novel machine design with limited commercial relevance.

Fossil fuels, which are the main energy sources of the current world, are running low and alternative ways of generating and storing different types of energies are becoming daunting missions. Renewable energy is very attractive because the source of the energy is often free and also environmentally friendly. The key issue to utilise the renewable energies in sustainable manner is how effectively store the energies and provide them on demand. Therefore, the significance of the energy storage devices has been widely recognised in recent time. Electrochemical capacitors (ECs), which are also commonly known as supercapacitors, are a type of the energy storage devices and the ECs are widely used as a back-up power boosting device to the batteries. Due to EC's low energy characteristics (typically, lower than 5 Wh/kg), their applications are limited. Therefore, main aim of this study is to enhance the energy characteristics of the ECs. In particular, aqueous ECs were examined due to a number of advantages offered by the water-based system. It was found that neutral aqueous electrolytes can expand the operating voltage close to 2 V (aqueous ECs commonly operate within 1 V) with advanced designs and hence result in remarkably higher energy. Moreover, the optimum condition of the neutral aqueous electrolytes was investigated by examining the effect of ion size, concentration and temperature. One of the main disadvantages of using the aqueous electrolytes in low temperatures was successfully resolved by using concentrated electrolytes (freezing point depressed up to -20°C). Mn02 with carbon materials were also used to construct asymmetrical ECs. The highest specific energy (Wh/kg) of the neutral aqueous ECs was achieved at 20 Wh/kg using unequal electrode design. This result is very much comparable to the Pb-acid battery. Based on the obtained data, larger scale (thicker and bigger electrode films) of the prototype ECs were constructed and their results correspond well with the results obtained from the smaller ECs. The data of the neutral aqueous ECs, which is contained in this thesis, suggest that the water-based ECs have a remarkable market potential.

The main aim of this project was to research, develop and test an induction motor drive not requiring a speed encoder, but which could be considered commercially viable by motor drives manufacturers, and which should aim to meet the follow requirements: • Dynamic torque performance and steady state speed-holding accuracy to be comparable with encodered vector controlled drives • Extensive and highly accurate knowledge of electrical and mechanical parameters of the motor and load not to be required • Extensive commissioning from an expert engineer not to be necessary • Algorithm not to rely on excessive computational capability being available The drive was to operate, in a stable manner, over speed and load ranges at least comparable with commercially available sensorless induction motor drives. The above requirements were set such that the developed sensorless technique may be considered for synchronised multi-motor process applications, where the advantages of a sensorless system could be exploited for hazardous, damp and hot conditions. The solution developed consists of a leading model-based sensorless method augmented with a speed estimator that tracks harmonics, seen in the stator terminal quantities, due to rotor slotting. The model-based scheme facilitates field-orientated control for dynamic performance. The slot harmonic speed estimator tunes the model for speed accuracy. Slot harmonics are identified using a recursive signal processing method termed the Recursive Maximum Likelihood - Adaptive Tracking Filter. This work is the first example of the method being developed into a practical sensorless drive system and the complete speed identifier is described, including set-up, pre-filtering and the minimal parameter considerations. Being recursive the method is computationally efficient, yet has accuracy comparable with that of FFT identifiers used in other work. The developed sensorless strategy was implemented practically on two motor drive systems. The performance of the scheme is shown to give encoder like speed holding accuracy and field-orientated dynamic performance. The two drives were also configured and tested as a speed synchronised pair, using applicable multi-motor control techniques, themselves compared and contrasted. The sensorless performance is demonstrated, alongside an encodered version acting as a benchmark, and the performance of the two schemes is shown to be highly comparable. The author has found no other example of sensorless techniques considered for use in multi-motor applications. The use of such a technique brings established advantages associated with encoder removal and allows multi-axis electronic synchronisation to be considered for parts of a process where an encoder may not be appropriate.

This thesis describes an investigation of three and five level diode clamped inverters for motor drive applications. The work was completed as a PhD project at the University of Nottingham with funding from EPSRC and Heenan Drives Ltd. The investigation of the three level converter describes the design, development, control and operation of an 11kW prototype. Included in the design is a review of typical switching strategies employed for control of the output voltage. New improvements to the sub-harmonic pulse width modulation method are presented which allow an improved output waveform to be obtained. The problem of DC link capacitor voltage balancing (Neutral Point Control) is addressed and a novel balancing control method is presented based on the addition of a DC offset to the modulation pattern. This method is verified through mathematical analysis and experimental operation. The operational limits of the control are analysed. Improvements to the technique are presented to expand its operating limits. The development of a prototype five level converter is then described. The design again features improvements to the sub-harmonic modulation strategy to provide enhanced output waveform generation, particularly for transient operation. The current demands on the DC link capacitors for the five level arrangement are analysed and it is concluded that the capacitors cannot be regulated by simple modifications to the output switching pattern. A novel circuit is presented to achieve capacitor balancing within the DC link. The circuit behaviour is described and analysed. Operation is confirmed through simulation and experimental implementation. High dynamic performance is demonstrated via the use of a vector controlled induction motor. Neutral point control is successfully achieved through a similar method to that used for the three level inverter. Having demonstrated the principle of operation of the three and five level inverters on low voltage prototypes, the thesis concludes with a review of the main considerations required to implement the configurations as medium voltage drives.

Recently, cooperative diversity system has become an effective way to mitigate multipath fading. This thesis concentrates on the performance analysis of cooperative diversity relaying system by considering several realistic limitations. These limitations are likely to exist in the real network and may cause the system performance degradation significantly. Consequently, it is valuable to produce the more accurate system evaluation for the development of network. Our research work is immersed in amplify-and-forward (AF) relaying protocol due to its simplicity. The effect of limitations such as correlated channels, signal-to-noise ratio (SNR) estimation errors and feedback delay are further studied. At first, we analyze the performance of a single antenna dualhop AF relaying system with correlated links in Nakagami-m fading channels by taking account of blind fixed-gain relaying (BFG), semiblind fixed-gain relaying (SFG), and variable-gain relaying (VG) schemes. Numerical results show that the link correlation can have a considerable effect on these schemes for specific ways of correlation, especially when the correlation coefficient is larger than 0.5. Then we investigate the effect of SNR estimation error which assumes relay selection performed under imperfect channel gain as well as imperfect noise power in Rayleigh fading channels. The numerical derivations of outage probability together with PDF and CDF of the estimated end-to-end SNR are verified and analyzed by simulation results. Finally, regarding the impact of outdated channel state information (CSI) due to feedback delay, we evaluate the performances of several generalized relay selection schemes based on multihop cooperative diversity over Rayleigh fading channels. Our results show that increasing the hop index may lead different peformance degradations for different relay selection schemes. Furthermore, it is also shown that selecting more relays in one hop does not necessarily lead to better performance. Again, simulation results verify the correctness of numerical derivation of outage probability.

Ever since the first mobile telephone call was made by a Motorola researcher in 1973, mobile technology has been growing rapidly to facilitate diverse ubiquitous services. Although cell based radio technology has proved beneficial for reliable long distance transmission, it is faced with challenges when deployed in an indoor environment. With the introduction of white LED - based communication entities at the start of the 21st century, radio and other technologies have become more interdependent, with the possibility for further expansion. Bearing in mind the sophistication offered, with the rapid developments witnessed in mobile telecommunications, this thesis presents an innovative alternative for cell phone connectivity in RF restricted or poor signal connectivity region. To address the research objective of offering uninterrupted signal connectivity in an indoor environment, the term Radio on Visible Light (RoVL) was introduced in this research. Furthermore, five major attributes influencing mobile signal transmission through visible light channels were identified and addressed. The bandwidth constraint offered by a white LED was tackled with an intelligent technique of frequency translation and mapping. In addition, an electronic predistorter was designed and implemented to address the nonlinearity in an LED. For efficient data manipulation, subcarrier continuous phase technique of modulation was introduced and hardware implemented for visible light communication, with a discussion on the LED number and arrangement for efficient illumination and data transmission. Despite the presence of significant research contributions in visible light technology, the study here is specifically focussed on integrating the GSM network with that of the optical wireless systems. The existing visible light entities provide service users with a one way broadband data transmission hub, unlike the dual channel support rendered by the RoVL device as presented in this research. The techniques proposed, backed up with interesting research observations and experimental results, clearly demonstrate the capability of supporting mobile communication through optical wireless channels. The methods proposed and techniques discussed shall lay foundation in supplementing the radio based mobile technology with optical wireless entities, opening new avenues for further research & development in this direction.