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56 SMT007 MAGAZINE I AUGUST 2019 there is a need to balance the amount of flex- ible materials on the sides of the conductors where flexing is to occur. As in many PCB decisions that need to be made, the optimum solution may be a hybrid or blend of multiple materials. These material choices include materials that are laminated to the copper circuits using heat and pressure; materials that can be laminated and then pho- toimaged, like solder mask, to define points of connection; and materials that are simply screen printed on to seal traces while leaving open features of interest for further processing or for making interconnections. Conclusion: Due Diligence The best advice I can offer is to talk with your PCB supplier to discuss the proper cov- erlayer to use for your specific application. When preparing for this discussion, the fol- lowing questions will help arrive at the proper coverlayer choice: • What is the final product application? • What are the bend requirements (how many, how often)? • Are there any cost constraints? • Are there any lead-time constraints? • How many holes need to be machined per board? • What is the smallest size needed in the coverlayer? • Does the fabricator have laser machining capability? • What is the copper weight on the surface layers? This early engagement will eliminate costly mistakes and position your design for the best possible result the first time. SMT007 Dave Lackey is VP of business development at American Standard Circuits. Lackey is also a co-author of The Printed Circuit Designer's Guide to… Flex and Rigid-Flex Fundamentals. Visit I-007eBooks.com to download your free copies of American Standard's books, including The Printed Circuit Designer's Guide to… Fundamentals of RF/Microwave PCBs, as well as other educational titles. Some of the vast amount of wasted energy that machines and devices emit as heat could be recaptured using an inexpensive nanomaterial developed at King Abdullah University of Science and Technology (KAUST), and turned back into useful electricity. Waste heat emitted by machines and devices could be recaptured by thermoelectric materials. Until now, though, thermoelectric materials have been made using expensive and energy-intensive processes. Researchers from KAUST have developed a new thermoelectric material made by spin-coating a liquid solution of quantum dots. The team spin-coated a thin layer of lead-sulfide quantum dots on a surface and then added a solution of short linker ligands that crosslink the quantum dots together to enhance the material's electronic properties. After repeating the spin-coating process layer by layer to form a 200-nanometer film, gentle thermal anneal- ing dried the film and completed fabrication. The quan- tum-dot-based thermoelectric material is only heated up to 175°C. This lower processing temperature could cut production costs and means that thermoelectric devices could be formed on a broad range of surfaces, including cheap, flexible plastics. The team's material showed promising thermoelectric properties. One important parameter of a good thermoelectric is the Seebeck coefficient. The team was able to show that an effect called the quantum confinement, which alters a material's electronic properties when it is shrunk to the nanoscale and was important for enhancing the Seebeck coefficient. The discovery is a step toward practical high-performance, low-temperature, solution- processed thermoelectric generators. (Source: KAUST) Energy-harvesting Nanomaterials Created Using Low-temp Process