254 MRS BULLETIN • VOLUME 37 • MARCH 2012 • www.mrs.org/bulletin © 2012 Materials Research Society 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 tech- nologies used thin-film metallization (evaporation or sputtering) on an elastic polydimethylsiloxane (PDMS) substrate. Since those early days, many more interesting technology develop- ments have been reported worldwide, exploring the use of a wide variety of materials such as composite and liquid con- ductors, 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 equip- ment, 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 technolo- gies, 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 compo- nents and sensors, providing the capability to realize circuits with a high degree of complexity and functionality • Standard solder assembly technology for mounting compo- nents 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 Printed circuit board technology inspired stretchable circuits J. Vanfleteren, M. Gonzalez, F. Bossuyt, Y.-Y. Hsu, T. Vervust, I. De Wolf, and M. Jablonski In the past 15 years, stretchable electronic circuits have emerged as a new technology in the domain of assembly, interconnections, and sensor circuit technologies. In the meantime, a wide variety of processes using many different materials have been explored in this new ield. In the current contribution, we present an approach inspired by conventional rigid and lexible printed circuit board (PCB) technology. Similar to PCBs, standard packaged, rigid components are assembled on copper contact pads using lead-free solder relow processes. Stretchabi lity is obtained by shaping the copper tracks as horseshoe-shaped meanders. Elastic materials, predominantly polydimethylsiloxanes, are used to embed the conductors and the components, thus serving as a circuit carrier. We describe mechanical modeling, aimed at optimizing the build-up toward maximum mechanical reliability of the structures. Details on the production process, reliability assessment, and a number of functional demonstrators are described. J. Vanfleteren, Centre for Microsystems Technology, Ghent University and Interuniversity Microelectronics Centre, Ghent, Belgium; Jan.Vanfleteren@ugent.be M. Gonzalez, Interuniversity Microelectronics Centre, Leuven, Belgium; mario.gonzalez@imec.be F. Bossuyt, Centre for Microsystems Technology, Ghent University and Interuniversity Microelectronics Centre, Ghent, Belgium; Frederick.bossuyt@ugent.be Y.-Y. Hsu, MC10 Inc., Cambridge, MA 02140, USA T. Vervust, Centre for Microsystems Technology, Ghent University and Interuniversity Microelectronics Centre, Ghent, Belgium; thomas.vervust@ugent.be I. De Wolf, Interuniversity Microelectronics Centre and Katholieke Universiteit Leuven, Belgium; Ingrid.dewolf@imec.be M. Jablonski, Centre for Microsystems Technology, Ghent University and Interuniversity Microelectronics Centre, Ghent, Belgium; michal.jablonski@ugent.be DOI: 10.1557/mrs.2012.48