The initial flexible electronic concepts were to use conventional PCB manufacturing techniques to interconnect functional hard-PCB islands with off-the-shelf electronic components onto a stretchable matrix made of wavy metallic interconnects. The interconnects can be etched or laser ablated from a fully plated temporary adhesive substrate. The full assembly is then encapsulated on both sides into any suitable stretchable rubber or silicone resin, depending on the final application requirements. The metal meander shapes can be optimized as a function of the maximum elongation that the final circuit will have to undergo during its use.
So far Vanfleteren has demonstrated the mechanical reliability of such assemblies encapsulated into different polymers, with over 60,000 cycles reached for interconnects made of 17um thick copper traces cladded by 50um thin plastic materials and elongated by 20% (the test was actually stopped before reaching failure). 20% elongation is probably much more than what would be needed in most flexible applications, he explained. The tricky part is to ensure that the transition from the hard PCB islands to the fully flexible polymer support are progressive enough in cladding thickness to prevent hard bends that could disrupt the PCB contacts. The labs have developed several prototypes in a number of European projects, including a platform for low-level blue light therapy, for the treatment of repetitive strain injuries (in cooperation with Philips in the Place-It project).
In the future, imec could be looking at integrating flexible electronic component blocks into this scheme (such as flexible OLEDs and thin-film organic circuits) instead of relying on hard islands.
From an industrial point of view, stretchable and flexible circuits may have more applications as a way to build fully conformable circuits, using thermo-formable plastics as an encapsulation material to freely set the deformable circuit into specific but solid shapes.
Applications could range from fancy 2.5D light sources to automotive interiors, free form keyboard and other consumer electronics. In fact, to further explore the manufacturability of these concepts, Imec and its project partners have just launched TERASEL (Thermo-plastically deformable circuits for embedded randomly shaped electronics), a project under the European Union’s Seventh Framework Programme for Information and Communication Technologies (FP7).
Running for the next three years under Vanfleteren’s direction, the project will look at the development, industrial implementation and application of large-area, cost-effective, randomly shaped electronics and sensor circuit technologies. It also aims to set up a complete multi-competence industrial production chain, capable to achieve mature, near-to-production industrial processes for manufacturing these randomly shaped circuits.
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