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26 SMT007 MAGAZINE I JULY 2019 the paste volume and the reflow temperature have to be set so that the volume of the ball alloy that will have to dissolve to bring the mixed alloy to its solidus composition at that temperature is no greater than the loss of orig- inal alloy volume that can be tolerated. SMT007 References 1. S. Mokler, R. Aspandiar, K. Byrd, O. Chen, S. Walwad- kar, K.K. Tang, M. Renavikar, & S. Sane, "The Application of Bi-Based Solders for Low-Temperature Reflow to Reduce Cost While Improving SMT Yields in Client Computing Sys- tems," Proceedings of the SMTA International Conference, September 2016. 2. H. Fu, R. Aspandiar, J. Chen, S. Cheng, Q. Chen, R. Coyle, S. Feng, B. Hardin, M. Krmpotich, S. Mokler, J. Radhakrish- nan, M. Ribas, B. Sandy-Smith, K.K. Tang, G. Wu, A. Zhang, & W. Zhen, "iNEMI Project on Process Development of BiSn- Based Low-Temperature Solder Pastes—Part II: Character- ization of Mixed Alloy BGA Solder Joints," Proceedings of Pan Pacific Microelectronics Conference, 2018. This paper was first presented at the IPC APEX EXPO 2019 Technical Conference and is published in the 2019 Technical Conference Proceedings. Keith Sweatman is senior techni- cal advisor at Nihon Superior Co. Ltd. and is an Adjunct Senior Fellow supporting the Nihon Supe- rior Centre for the Manufacture of Electronic Materials at the Univer- sity of Queensland, Australia. Wearable devices that harvest energy from movement are not a new idea, but a material created at Rice Univer- sity may make them more practical. The Rice lab of chemist James Tour has adapted laser- induced graphene (LIG) into small, metal-free devices that generate electricity. Like rubbing a balloon on hair, putting LIG composites in contact with other surfaces produces static electricity that can be used to power devices. For that, thank the triboelectric effect, by which materi- als gather a charge through contact. When they are put together and then pulled apart, surface charges build up that can be channeled toward power generation. "This could be a way to recharge small devices just by using the excess energy of heel strikes during walking, or swinging arm movements against the torso," Tour said. In experiments, the researchers connected a folded strip of LIG to a string of light-emit- ting diodes and found that tapping the strip produced enough energy to make them flash. A larger piece of LIG embedded within a flip-flop let a wearer generate energy with every step as the graphene composite's repeated contact with skin produced a current to charge a small capacitor. "The nanogenerator embedded within a flip-flop was able to store 0.22 millijoules of electrical energy on a capacitor after a 1-kilometer walk," said Rice postdoctoral researcher Michael Stanford, lead author of the paper. "This rate of energy storage is enough to power wearable sensors and electronics with human movement." (Source: Rice University) Flexible Generators Turn Movement Into Energy

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