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84 The PCB Magazine • November 2016 dustry that doesn't readily embrace technologi- cal change, there were nevertheless innovations in materials and processes: Digital imaging be- came a reality after overcoming many hurdles. New base materials were introduced to accom- modate the needs of lead-free and high-speed signal processing. Coreless structures, parallel processing, and the use of conductive pastes gained acceptance. Technologies crossed the borders between wafer fabrication, wafer-lev- el processing, substrate and PCB fabrication. These trends are likely to continue, but I dare not predict new game-changers. I am looking forward to following these events in the coming years. My plan is not to write Tech Talk columns regularly anymore, as I have done for the past 21 years, but to write occasionally as a guest columnist about topics that capture my attention and that I deem of in- terest to the readers of The PCB Magazine. Many thanks to all of you who have let me know over the years that you enjoyed my column! Feel free to contact me if you have questions that I may be able to help you with! PCB Karl Dietz is President of Karl Dietz Consulting LLC. He is offering consulting services and tutori- als in the field of circuit board and substrate fabrication technology. Karl can be reached by e-mail by clicking here, or by phone at (001) 919 870 6230. To view past columns, click here. 225 TECH TALKS—1995 TO 2016 Almost all solid materials, from rubber and glass to gran- ite and steel, inevitably expand when heated. Only in very rare instances do certain materi- als buck this thermodynamic trend and shrink with heat. For instance, cold water will con- tract when heated between 0 and 4 degrees Celsius, before expanding. Engineers from MIT, the Uni- versity of Southern California, and elsewhere are now adding to this curious class of heat-shrinking materials. The team, led by Nicholas X. Fang, an associate professor of mechanical engineering at MIT, has manufactured tiny, star-shaped structures out of interconnected beams, or trusses. The structures, each about the size of a sugar cube, quickly shrink when heated to about 540°F (282 C). Each structure's trusses are made from typi- cal materials that expand with heat. Fang and his colleagues realized that these trusses, when ar- ranged in certain architectures, can pull the struc- ture inward, causing it to shrink like a Hoberman sphere—a collapsible toy ball made from interconnecting lat- tices and joints. The researchers consider the structures to be "metama- terials"—composite materials whose configurations exhibit strange, often counterintuitive properties that are not normal- ly found in nature. In some cases, these struc- tures' resistance to expand- ing when heated—rather than their shrinking response per se—may be especially useful. Such materials could find applications in computer chips, for example, which can warp and deform when heated for long periods of time. The researchers have published their results in the journal Physical Review Letters. Fang's co- authors include former MIT postdoc Qi Ge, along with lead author Qiming Wang of the University of Southern California, Jonathan Hopkins of the University of California at Los Angeles, and Julie Jackson and Christopher Spadaccini of Lawrence Livermore National Laboratory (LLNL). Counterintuitive 'Metamaterial' May Enable Heat-resistant Circuit Boards

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