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22 The PCB Magazine • March 2017 needs to be taken into consideration is the need to bake the material before subjecting the flex circuits to high temperatures. Hand assembly is especially prone to defects with flexible materi- als. This process requires special consideration as to temperature and time and is yet another area that operator training is critical. Flex circuits (and thin core boards in gen- eral) require some support mechanism to run through either wave solder or a reflow process. There are several options to accomplish this. A design with many FR stiffeners may move for- ward by using an FR-4 carrier panel designed to provide stability to the array until assembly is complete and then have the excess FR-4 re- moved, leaving the intended stiffeners. While that is one approach, it is more common to build the flex circuit as a single, individual piece and have a carrier fixture made to transport the circuit through the assembly line. This allows the fabricator to maximize the panel real estate and provide a lower cost unit price for the flex circuit. Carrier tooling is relatively inexpensive and is generally more than off-set by the lower cost flex circuit. Flexible circuits are certainly a growing seg- ment of the market and require not only special design and material consideration, but special handling throughout the manufacturing pro- cess. With material handling cited as the largest cause of yield loss during manufacturing, this is an area with (and for) continuous improve- ment. We all agree that employee training and on-going education is the key to success. Many facilities specialize in just flex and rigid-flex processing and others have teams dedicated to this product subset, but the common theme is knowledge and specialization. Flexible circuits often have a slightly longer manufacturing lead- time than their rigid counterparts and this is of- ten related to the special handling and process- ing required for flex. Whether that is extra time in tooling and process planning, extra time dur- ing wet process, extra care needed to properly register the layers prone to material movement, or extra care needed during assembly, all special handling is done to maximize yield and provide a robust product to the end user. PCB Tara Dunn is the president of Omni PCB. To read past columns or to contact her, click here. Engineers and biologists at MIT have teamed up to design a new "living material"—a tough, stretchy, biocompatible sheet of hydrogel injected with live cells that are genetically programmed to light up in the presence of certain chemicals. In a paper published this week in the Proceedings of the National Academy of Sciences, the researchers demonstrate the new material's potential for sensing chemicals, both in the environment and in the human body. The team fabricated various wearable sensors from the cell-infused hydrogel, including a rubber glove with fingertips that glow after touching a chemically contaminated surface, and bandages that light up when pressed against chemicals on a person's skin. Xuanhe Zhao, the Robert N. Noyce Career Development associate professor of mechanical engineering at MIT, says the group's living material design may be adapted to sense other chemicals and contaminants, for uses ranging from crime scene investigation and forensic science, to pollution monitoring and medical diagnostics. "With this design, people can put different types of bacteria in these devices to indicate toxins in the environment, or disease on the skin," says Timothy Lu, associate professor of biological engineering and of electrical engineering and computer science. "We're demonstrating the potential for living materials and devices." The paper's co-authors are graduate students Xinyue Liu, Tzu-Chieh Tang, Eleonore Tham, Hyunwoo Yuk, and Shaoting Lin. Living Sensors at Your Fingertips FLEX MATERIAL HANDLING: AN INSIDE PEEK

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