Effect of solder volume on joint shape with variable chip-to-board contact pad ratio

The objective of this paper is to investigate the effect of the pad size ratio between the chip and board end of a solder joint on the shape of that solder joint in combination with the solder volume available. The shape of the solder joint is correlated to its reliability and thus of importance. For low density chip bond pad applications Flip Chip (FC) manufacturing costs can be kept down by using larger size board pads suitable for solder application. By using “Surface Evolver” software package the solder joint shapes associated with different size/shape solder preforms and chip/board pad ratios are predicted. In this case a so called Flip-Chip Over Hole (FCOH) assembly format has been used. Assembly trials involved the deposition of lead-free 99.3Sn0.7Cu solder on the board side, followed by reflow, an underfill process and back die encapsulation. During the assembly work pad off-sets occurred that have been taken into account for the Surface Evolver solder joint shape prediction and accurately matched the real assembly. Overall, good correlation was found between the simulated solder joint shape and the actual fabricated solder joint shapes. Solder preforms were found to exhibit better control over the solder volume. Reflow simulation of commercially available solder preform volumes suggests that for a fixed stand-off height and chip-board pad ratio, the solder volume value and the surface tension determines the shape of the joint.

Magnetic materials and soft-switched topologies for high-current DC-DC converters

The thesis is focused on the magnetic materials comparison and selection for high-power non-isolated dc-dc converters for industrial applications or electric, hybrid and fuel cell vehicles. The application of high-frequency bi-directional soft-switched dc-dc converters is also investigated. The thesis initially outlines the motivation for an energy-efficient transportation system with minimum environmental impact and reduced dependence on exhaustible resources. This is followed by a general overview of the power system architectures for electric, hybrid and fuel cell vehicles. The vehicle power sources and general dc-dc converter topologies are discussed. The dc-dc converter components are discussed with emphasis on recent semiconductor advances. A novel bi-directional soft-switched dc-dc converter with an auxiliary cell is introduced in this thesis. The soft-switching cell allows for the MOSFET's intrinsic body diode to operate in a half-bridge without reduced efficiency. The converter's mode-by-mode operation is analysed and closed-form expressions are presented for the average current gain of the converter. The design issues are presented and circuit limitations are discussed. Magnetic materials for the main dc-dc converter inductor are compared and contrasted. Novel magnetic material comparisons are introduced, which include the material dc bias capability and thermal conductivity. An inductor design algorithm is developed and used to compare the various magnetic materials for the application. The area-product analysis is presented for the minimum inductor size and highlights the optimum magnetic materials. Finally, the high-flux magnetic materials are experimentally compared. The practical effects of frequency, dc-bias, and converters duty-cycle effect for arbitrary shapes of flux density, air gap effects on core and winding, the winding shielding effect, and thermal configuration are investigated. The thesis results have been documented at IEEE EPE conference in 2007 and 2008, IEEE APEC in 2009 and 2010, and IEEE VPPC in 2010. A 2011 journal has been approved by IEEE Transactions on Power Electronics.

An exploration of leadership in Irish post-primary education

The purpose of this study is to explore the nature and how of leadership in Irish post-primary schools. It considers school leadership within the context of contemporary Distributed Leadership theory. Associated concepts such as Distributed Cognition and Activity Theory are used to frame the study. From a distributed perspective, it is now widely accepted that other agents (e.g. teachers) have a leadership role, as part of collaborative, participative and supportive learning communities. Thus, this study considers how principals interact and build leadership capacity throughout the school. The study draws on two main sources of evidence. In analysing the implications of accountability agendas for school leadership, there is an exploration and focus on the conceptualisations of school leadership that are fore-grounded in 21 WSE reports. Elements of Critical Discourse Analysis are employed as an investigative tool to decipher how the construction of leadership practice is produced. The second prong of the study explores leadership in 3 case-study post-primary schools. Leadership is a complex phenomenon and not easy to describe. The findings clarify, however, that school leadership is a construct beyond the scope of the principal alone. While there is widespread support for a distributed model of leadership, the concept does not explicitly form part of the discourse in the case-study schools. It is also evident that any attempt to understand leadership practice must connect local interpretations with broader discourses. The understanding and practice of leadership is best understood in its sociohistorical context. The study reveals that, in the Irish post-primary school, the historical dimension is very influential, while the situational setting, involving a particular set of agents and agendas, strongly shapes thinking and practices. This study is novel as it synthesises two key sources of evidence. It is of great value in that it teases out the various historical and situational aspects to enhance understandings of school leadership in contemporary Ireland. It raises important questions for policy, practice and further research.

Molecular mechanisms underpinning IFN-gamma and TNF-alpha synergy in intestinal epithelial cells

Cytokine-driven signalling shapes immune homeostasis and guides inflammatory responses mainly through induction of specific gene expression programmes both within and outside the immune cell compartment. These transcriptional outputs are often amplified via cytokine synergy, which sets a stimulatory threshold that safeguards from exacerbated inflammation and immunopathology. In this study, we investigated the molecular mechanisms underpinning synergy between two pivotal Th1 cytokines, IFN-γ and TNF-α, in human intestinal epithelial cells. These two proinflammatory mediators induce a unique state of signalling and transcriptional synergy implicated in processes such as antiviral and antitumour immunity, intestinal barrier and pancreatic β-cell dysfunction. Since its discovery more than 30 years ago, this biological phenomenon remains, however, only partially defined. Here, using a functional genomics approach including RNAi perturbation screens and small-molecule inhibitors, we identified two new regulators of IFN-γ/TNF-α-induced chemokine and antiviral gene and protein expression, a Bcl-2 protein BCL-G and a histone demethylase UTX. We also discovered that IFN-γ/TNF-α synergise to trigger a coordinated shutdown of major receptor tyrosine kinases expression in colon cancer cells. Together, these findings extend our current understanding of how IFN-γ/TNF-α synergy elicits qualitatively and quantitatively distinct outputs in the intestinal epithelium. Given the well-documented role of this synergistic state in immunopathology of various disorders, our results may help to inform the identification of high quality and biologically relevant druggable targets for diseases characterised by an IFN-γ/TNF-α high immune signature