Printed circuit board technology inspired stretchable circuits
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Introduction The first developments on stretchable circuits were reported in the early 2000s in a number of U.S.-based research institutes, including Lawrence Livermore National Laboratory,1 Princeton University,2 and Johns Hopkins University.3 All of these technologies used thin-film metallization (evaporation or sputtering) on an elastic polydimethylsiloxane (PDMS) substrate. Since those early days, many more interesting technology developments have been reported worldwide, exploring the use of a wide variety of materials such as composite and liquid conductors,4 ultrathin bendable silicon,5 and perforated polyimide sheets.6 In the current article, the focus is on a technology that makes use of conventional rigid and flexible printed circuit board (PCB) processing equipment. This reasoned choice was made taking a potential future transfer of the technology to an industrial production environment, including compatibility with existing conventional electronic circuit fabrication equipment, in mind. Therefore, the stretchable circuit technology, which will be described later, shows some similarities with conventional PCB fabrication and assembly technologies.7,8 Like these technologies, it involves the use of: • Standard copper conductor material as interconnections
• Lithography and wet etching technology for the patterning of copper conductors • Conventionally packaged off-the-shelf electronic components and sensors, providing the capability to realize circuits with a high degree of complexity and functionality • Standard solder assembly technology for mounting components on the copper tracks of the stretchable PCB. Since normal off-the-shelf components are rigid, it is clear that the areas in the circuit where the components are present cannot deform. The overall concept of a stretchable circuit is illustrated in Figure 1.8,9 The circuit comprises a number of rigid (or moderately flexible in some cases) component islands, where each island holds a single component or a limited number of components. The total area of each island is small and acceptable for the application in which the stretchable circuit is to be used. The interconnection between the components on a single island can be formed using the same copper conductor foil that provides the stretchable interconnects. Alternatively, these components can first be mounted on a separate (small) standard PCB or FCB interposer board, acting as an interface between components and stretchable interconnects. The interposer board is subsequently assembled on the large-area stretchable circuit. A proper design and technological implementation of the transition between the
J. Vanfleteren, Centre for Microsystems Technology, Ghent University and Interuniversity Microelectronics Centre, Ghent, Belgium; Jan.Vanfl[email protected] M. Gonzalez, Interuniversity Microelectronics Centre, Leuven, Belgium; [email protected] F. Bossuyt, Centre for Microsystems Technology, Ghent University and Interuniversity Microelectronics Centre, Ghent, Belgium; [email protected] Y.-Y. Hs
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