Free-form 2.5D thermoplastic circuits using one-time stretchable interconnections
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Free-form 2.5D thermoplastic circuits using one-time stretchable interconnections Jan Vanfleteren, Bart Plovie, Yang Yang, Jelle De Smet, Rik Verplancke, Frederick Bossuyt, Herbert De Smet imec - Ghent University, Gent-Zwijnaarde, Belgium ABSTRACT A technology is presented for the production of soft and rigid circuits with an arbitrary 2.5D fixed shape. The base of this technology is our proprietary technology for elastic circuits with a random shape, in which the elastic thermoset (mostly PDMS) polymer is now replaced by soft or rigid thermoplastic variants. An additional thermoforming step is required to transform the circuit from its initial flat to its final fixed 2.5D shape, but for rigid fixed shape circuits only onetime stretchability of the extensible interconnects is required, relieving the reliability requirements. INTRODUCTION In recent years a lot of research has been spent on the development of dynamically deformable electronics and sensor circuits [1-4]. Such circuits are based on the use of extensible or compressible electrical interconnections, embedded in an elastic polymer like PDMS (silicone rubber) or PU (polyurethane). The circuits, shown in literature, also very often have the property, that they can take different shapes, even when no force other than gravity is executed on it. The same circuit can e.g. be in a flat or a folded state. These circuits are soft random shaped under no external stress. There is however also a lot of interest in electronic circuits which are still soft and dynamically deformable, but return to a fixed shape when external forces are removed. An example of such a circuit is a smart lens with embedded electronic circuits. Such a smart lens should be deformable and soft, but should always have a spherical shape, when no forces are executed on it. Such a circuit can hence be categorized as a soft fixed-shape circuit. Even further, such circuits with a fixed shape can even be completely rigid and not meant to be deformed in any way. Hence such circuits have a rigid fixed (but arbitrary) shape. Applications for such 2.5D free-form rigid circuits include new types of light sources, automotive interior parts (e.g. ceilings with LED illumination), non-flat man-machine interfaces with touch sensors in e.g. household appliances, etc., as they are under development in the frame of the EU funded FP7-TERASEL project [5]. In this contribution a technology will be presented for the fabrication of such 2.5D free form rigid or soft fixed-shape type of circuits, based on the technology, which we have developed earlier for soft random-shape circuits. TECHNOLOGY Common features for the three technologies mentioned above are the following :
Although the circuits finally will have an arbitrary shape, they are produced entirely on flat temporary carriers. This is compatible with practices in PCB (printed circuit board) or semiconductor industry.
Interconnection materials used are those found in PCB industry (Cu, typ. 17 or 35µm thick) or in thin-film circuitry (sputtered metals
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