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72 The PCB Design Magazine • October 2017 How important is ease of processing with my choice of thermal management material? The design of your LED lighting unit will de- termine the type of thermal management ma- terial you choose, ensuring that it is correctly applied to the critical areas of the assembly. Processing requirements depend on production capabilities, volumes of manufacture and the prevailing conditions during application. For example, a thermal interface material should be applied in an even film, in as thin a layer as pos- sible. If it is not possible to apply a thin layer, it might be better to use a pre-cut thermal pad, for example. This will, however, be thicker than a thermal paste or phase change material, which will result in higher thermal resistance. So, in common with all engineering problem-solving exercises, a compromise must be made; in this case, the compromise is between your process- ing requirements and your expectations of per- formance in the end application. Choosing an appropriate solution for your LED application is not exactly straightforward. I strongly recommend you get some expert ad- vice before you settle on any particular mate- rial or processing method. Look out for more thermal management tips in my column next month. PCBDESIGN Jade Bridges is the European technical support specialist for Electrolube. THERMAL MANAGEMENT FOR LED LIGHTING MANUFACTURERS Physicists at the University of California, Riv- erside have developed a photodetector, a device that senses light, by combining two distinct inor- ganic materials and producing quantum mechani- cal processes that could revolutionize the way so- lar energy is collected. Photodetectors are almost ubiquitous, found in cameras, cell phones, remote controls, solar cells, and even the panels of space shuttles. Measur- ing just microns across, these tiny devices convert light into electrons, whose subsequent movement generates an electronic signal. Increasing the ef- ficiency of light-to-electricity conversion has been one of the primary aims in photodetector con- struction since their invention. Lab researchers stacked two atomic layers of tungsten diselenide (WSe2) on a single atomic layer of molybdenum diselenide (MoSe2). Such stacking results in properties vastly different from those of the parent layers, allow- ing for customized electronic engi- neering at the tiniest possible scale. Within atoms, electrons live in states that determine their energy level. When electrons move from one state to an- other, they either acquire or lose energy. Above a certain energy level, electrons can move freely. An electron moving into a lower energy state can transfer enough energy to knock loose another electron. UC Riverside physicists observed that when a photon strikes the WSe2 layer, it knocks loose an electron, freeing it to conduct through the WSe2. At the junction between WSe2 and MoSe2, the electron drops down into MoSe2. The energy giv- en off then catapults a second electron from the WSe2 into the MoSe2, where both electrons be- come free to move and generate electricity. "We are seeing a new phenomenon occurring," said Nathaniel M. Gabor, an assistant professor of physics, who led the research team. "It's like a wave stuck between walls closing in," Gabor said. "Quantum mechanically, this changes all the scales. The combi- nation of two different ultra-small materials gives rise to an entirely new multiplication process. Two plus two equals five." Prototype Shows How Tiny Photodetectors Can Double Their Efficiency

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