There is an increasing demand for electronics that can operate at temperatures in excess of 200 degrees C, well above the maximum operating temperature of traditional silicon microelectronics. Key application areas are in the power, automotive, aerospace and defence industries. Electronic devices capable of operating at such high temperatures are now available. However, new methods are also needed for integrating these devices into circuits and systems, and in particular for attaching them, both mechanically and electrically, to circuit boards and heatsinks. At present high-temperature devices are typically attached by soldering using high-melting-point, lead-rich solders. However, there is a strong environmental imperative to reduce the use of lead in all electronics, so this cannot be accepted as a long-term solution. Alternative solutions employing gold-rich solders or sintered nano-silver pastes can be used, but these are expensive and can suffer from reliability issues. Low-cost, lead-free high-temperature solder alloys are also available; however, these tend to require significantly higher soldering temperatures and longer processing times, leading to slower production and higher thermal load on the devices during soldering. This project will explore the use of quasi-ambient bonding (QAB) with reactive nanofoils as a route to lowering the process time and thermal load during packaging of high-temperature electronic devices. Reactive nanofoils are multilayer materials comprising alternating layers of two elements (typically nickel and aluminium) that react exothermically i.e. with the release of heat. Once the reaction is triggered, it is self-propagating and spreads throughout the foil. If the foil is sandwiched between two parts that are pre-coated with solder, the heat generated can be used to melt the adjacent solder layers momentarily and form a permanent bond. The heating is intense, but occurs over a short timescale, so that while the local temperature can reach up to 1500 degrees C, heating is confined to a narrow region around the foil, with negligible temperature rise occurring elsewhere. Up to now, quasi-ambient bonding applications have used traditional lower-temperature solders. In this project we will extend the application of QAB to a range of low-cost, lead-free high-temperature alloys. The primary aim will be to develop bonding processes tailored for applications in high-temperature power electronics and optoelectronics. We will also explore the use of QAB for sealing of hermetic packages which is another key area where low cost and low thermal load can be an advantage. The processes developed will be evaluated in terms of bonding strength and in-service reliability, and benchmarked against alternative processes based on lead- and gold-based solders. Alongside the process development and evaluation, we will carry out extensive modelling and characterisation aimed at gaining an improved understanding of the QAB process. Developments to date have been mainly empirical, and fundamental aspects of the process remain poorly understood. QAB is fundamentally different from traditional soldering because of the very short timescale over which the process takes place. In order for it to become established in mainstream electronics manufacturing, the potential detrimental effects of residual stresses and microstructural defects incorporated into QAB bonds need to be fully understood. The proposed research has the potential to provide a low-cost, sustainable joining technology for electronics manufacturing that can continue to meet the operating temperature requirements of high-temperature electronics for many years to come. At the same time it will yield new fundamental insights into processes involving rapid solidification of complex alloys that will be of wide interest to the materials science and manufacturing research communities.

Over the next twenty years, the automotive and aerospace sector will undergo a fundamental revolution in propulsion technology. The automotive sector will rapidly move away from petrol and diesel engine powered cars towards fully electric propelled vehicles whilst planes will move away from pure kerosene powered jet engines to hybrid-electric propulsion. The automotive and aerospace industry has worked for the last two decades on developing electric propulsion research but development investment from industry and governments was low until recently, due to lag of legislation to significantly reduce greenhouse gases. Since the ratification of the 2016 Paris Agreement, which aims to keep global temperature rise this century well below 2 degrees Celsius, governments of industrial developed nations have now legislated to ban new combustion powered vehicles (by 2040 in the UK and France, by 2030 in Germany and similar legislation is expected soon in China). The implementation of this ban will see a sharp rise of the global electric vehicle market to 7.5 million by 2020 with exponential growth. In the aerospace sector, Airbus, Siemens and Rolls-Royce have announced a 100-seater hybrid-electric aircraft to be launched by 2030 following successful tests of 2 seater electric powered planes. Other American and European aerospace industries such as Boeing and General Electric must also prepare for this fundamental shift in propulsion technology. Every electric car and every hybrid-electric plane needs an electric drive (propulsion) system, which typically comprises a motor and the electronics that controls the flow of energy to the motor. In order to make this a cost-effective reality, the cost of electric drives must be halved and their size and weight must be reduced by up to 500% compared to today's drive systems. These targets can only be achieved by radical integration of these two sub-systems that form an electric drive: the electric motor and the power electronics (capacitors, inductors and semiconductor switches). These are currently built as two independent systems and the fusion of both creates new interactions and physical phenomena between power electronics components and the electric motor. For example, all power electronics components would experience lots of mechanical vibrations and heat from the electric motor. Other challenges are in the assembly of connecting millimetre thin power electronics semiconductors onto a large hundred times bigger aluminium block that houses the electric motor for mechanical strength. To achieve this type of integration, industry recognises that future professional engineers need skills beyond the classical multi-disciplinary approach where individual experts work together in a team. Future propulsion engineers must adopt cross-disciplinary and creative thinking in order to understand the requirements of other disciplines. In addition, they will need an understanding of non-traditional engineering subjects such as business thinking, use of big data, environmental issues and ethical impact. Future propulsion engineers will need to experience a training environment that emphasises both deep subject knowledge and cross-disciplinary thinking. This EPSRC CDT in Power Electronics for Sustainable Electric Propulsion is formed by two of UK's largest and most forward thinking research groups in this field (at Newcastle and Nottingham Universities) and includes 16 leading industrial partners (Cummins, Dyson, CRRC, Protean, to name a few). All of them sharing one vision: To create a new generation of UK power electronics specialists, needed to meet the societal and industrial demand for clean, electric propulsion systems in future automotive and aerospace transport infrastructures.