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PCB007-Apr2019

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20 PCB007 MAGAZINE I APRIL 2019 Sameeksha Katoch: I'm a Ph.D. student at Ari- zona State University, working with Emma Pedersen, who is an undergraduate student. And she is working in the NSF Research Ex- perience for Undergraduates (REU) program at the SenSIP center funded by the National Sci- ence Foundation I/UCRC program and industry members (i.e., Raytheon, NXP, Intel, Sprint, etc.). We do a couple of projects where a lot of undergraduate students can gain research experience. The NSF cyber-physical systems (CPS) project that Emma is currently working on is related to fault protection in utility-scale solar arrays. Emma Pedersen: I'm pursuing a degree is in aerospace engineering at Arizona State Univer- sity. Goutham Ezhilarasu: I'm a Ph.D. student at the University of California, Los Angeles (UCLA). And I work on flexible hybrid electronics. I'm demonstrating flexible hybrid electronics using a process called fan-out wafer-level packaging (FOWLP), which is a technique popularized by TSMC (Taiwan Semiconductor Manufacturing Company). You demonstrate a full system by reconstituting a wafer around the dies. What I mean by that is you first assemble the dies on a wafer with an adhesive on top, and then you pour the molding compound and let it cure so that the dies are later embedded in the mold- ing compound. _____________________________ Then, we had separate conversations about their respective research. First up was Tony and his work on thermoelectric generators. Tony describes the current industry situation to give his work some context. Tony Varghese: Most of the devices you see in the market are bulk; they are pretty solid and rigid. This project focuses on transforming them into flexible devices so that you can con- nect them into flexible sensors, and eventually have self-powered sensors and devices. The main property we are looking for in thermoelec- tric material is called the figure of merit (ZT), which is dependent on the electrical conductivi- ty, thermal conductivity, and Seebeck coefficient of the material. In the first process, we devel- op this thermoelectric material using a screen- printing technique. We develop the material to get a ZT of one, which is very close to bulk devices. Our first goal was to replicate the value in the bulk devices using the flexible additive manufacturing technique. We did that with the screen printing thermo- electric legs and were able to develop using a four-leg, 54 microwatts of power and with a temperature difference of 80°C and a power density of 18.8 milliwatts per square centi- meter. That's a very high power density, and we can further improve the power density by stacking more legs on this smaller radius. So, that's where we came up with this aerosol jet printing because it has very fine features and they can print more than legs on a smaller area and improve the power density. Another technique we use is photonic sin- tering. It takes eight to 10 hours for the nor- mal thermal sintering of this material. With the photonic sintering, we were able to reduce it to milliseconds, so it is a very fast process. Now, you can print and sinter in a conveyor belt. You print it, upload to the photonic sinter, and it's done. Nolan Johnson: It's fast, conveyorized, replica- ble, and reliable. Varghese: And it reduces the cost considerably. The cost of manufacturing is almost 80–90% production. In a conventional process, all of this takes place in a semi-additive process. You have to press it and heat it, and again, it could take up to 10 hours to finish the process, and time is money. To do this, we make our own nanocrystals with the right material and tune the nanocrystal size and dimensions; then, we make it into an ink. First, we tune the ink prop- erties to be compatible with aerosol printing. Tony Varghese

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