Thermo-mechanical simulation of a multichip press-packed IGBT

The reliability of press-packed integrated gate bipolar transistors (IGBT) depends on satisfactory contact conditions applied at assembly stage and mantained throughout the service life. The objective of this work is the simulation of the thermo-structural behavior of a multichip IGBT during initial assembly and subsequent uniform thermal cycling using the finite element method. A detailed axisymmetric FE model of the 3D-device is developed to assess multi-zone contact conditions. Elastic-plastic material behavior and Coulombian friction on contact surfaces are prescribed. The role of dimensional tolerances on contact conditions is discussed. The thermal cycling associated to accelerated testing is then introduced to determine the contact pressure evolution as well as local stick/slip conditions. The device sensitivity to potential damage initiation due to thermo-mechanical fatigue and/or fretting is addressed.     

THE EFFECT OF MICROSTRUCTURE ON FRACTURE TOUGHNESS OF POLYCRYSTALLINE ALUMINA

Abstract— A study of the dependence of the fracture toughness on the microstructural morphology of polycrystalline materials is presented. The microstructure of several aluminas was characterized by thermal etching and image analysis. The fracture toughness of these materials was then determined by bridge-indentation pre-cracking followed by a four-point bending test procedure. A SEM investigation of the mechanisms of crack propagation was also carried out. The observed microstructure-related fracture toughness responses are discussed.     

The ultrasound brain helmet: new transducers and volume registration for in vivo simultaneous multi-transducer 3-D transcranial imaging

Because stroke remains an important and time-sensitive health concern in developed nations, we present a system capable of fusing 3-D transcranial ultrasound volumes acquired from two sides of the head. This system uses custom sparse array transducers built on flexible multilayer circuits that can be positioned for simultaneous imaging through both temporal acoustic windows, allowing for potential registration of multiple real-time 3-D scans of cerebral vasculature. We examine hardware considerations for new matrix arrays-transducer design and interconnects-in this application. Specifically, it is proposed that SNR may be increased by reducing the length of probe cables. This claim is evaluated as part of the presented system through simulation, experimental data, and in vivo imaging. Ultimately, gains in SNR of 7 dB are realized by replacing a standard probe cable with a much shorter flex interconnect; higher gains may be possible using ribbon-based probe cables. In vivo images are presented, showing cerebral arteries with and without the use of microbubble contrast agent; they have been registered and fused using a simple algorithm which maximizes normalized cross-correlation.