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82 PCB007 MAGAZINE I FEBRUARY 2019 ic copper. Thus, the actual electroless copper step is eliminated. These processes have been presented and thoroughly discussed elsewhere [3] . While di- rect metalization processes may reach certain limitations for use with very high AR rigid cir- cuit boards, these processes are very efficient and effective for HDI. Direct metalization sys- tems primarily function by coating the sub- strate as opposed to a true chemical reaction such as in electroless copper. Contrarians of direct metalization point to sheet resistance measurements of the direct metalization coat- ings versus electroless copper. Yet, while the DM films are somewhat slightly more resistive than conventional electroless copper, these processes are well established in the industry. For the fabricator, there are many options to enhance the fabrication process for HDI. I will provide more insight into this in a future column. PCB007 References 1. Happy Holden. "How To Get Started In HDI With Microvias," November 2003. 2. HDPUG. 3. Happy Holden, et al. The HDI Handbook, I-Connect007, 2009. Michael Carano is VP of technology and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. Trisha Andrew, materials chemist at the University of Massachusetts Amherst, and Linden Allison, her Ph.D. student, have developed a fabric that can harvest body heat to power small wearable microelectronics such as activity trackers. Writing in Advanced Materials Technologies, Andrew and Allison explain that in theory, body heat can produce power by taking advantage of the difference between Materials Chemists Tap Body Heat to Power Smart Garments body temperature and ambient cooler air—a thermoelec- tric effect. Materials with high electrical conductivity and low thermal conductivity can move electrical charge from a warm region toward a cooler one in this way. "What we have developed is a way to inexpensively va- por-print biocompatible, flexible, and lightweight poly- mer films made of everyday, abundant materials onto cot- ton fabrics that have high enough thermoelectric proper- ties to yield fairly high thermal voltage—enough to power a small device," says Andrew. The researchers took advantage of the naturally low- heat transport properties of wool and cotton to create thermoelectric garments that can maintain a temperature gradient across an electronic device known as a thermo- pile, which converts heat to electrical energy even over long periods of continuous wear. This is a practical con- sideration to ensure that the conductive material is go- ing to be electrically, mechanically, and thermally stable over time. The researchers believe this work will be interesting for device engineers who seek to explore new energy sourc- es for wearable electronics and designers interested in creating smart garments. The research was supported by the National Sci- ence Foundation and by the David and Lucille Packard Foundation. (Source: University of Massachusetts Amherst)

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