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86 DESIGN007 MAGAZINE I JULY 2020 now demand both transparency and heat resis- tance for high-temperature processing, such as soldering and wire bonding. Material suppliers found a way to satisfy these new requirements, but it was not easy to have both transparency and heat resistance without a trade-off. Some material suppliers were successful in creating clear polyimide films that have transparencies greater than 85% and stable mechanical prop- erties with temperatures over 300°C. One poly- ethylethylketone (PEEK) film manufacturer suc- cessfully created a heat-resistant and transpar- ent film, and a few fluorocarbon resin manu- facturers successfully developed heat-resistant and transparent films. The next hurdle is to build copper-clad lami- nates with transparent and heat-resistant films. A couple of laminate manufacturers—includ- ing DKN Research—already developed a uni- versal metalizing and copper plating process to produce copper-clad laminates. The bond strength is proven, and these laminates are ready for the standard etching processes. The coverlay is currently under development. Laminate manufacturers are not prepared to ship reliable coverlay films with transparent glue. Chemical companies are close to deliver- ing transparent and heat-resistant ink (actually transparent polyimide ink) as a screen-print- able transparent coverlay. There is one last detail to overcome before we complete transparent and heat resistant flex circuits—transparent conductors. We are considering several candidates. ITO film is still useful because of its long history. Low perfor- mance with flex endurance is the major issue of the flex circuits. A relatively high conduc- tor resistance stands in the way of application expansion. Organic conductive ink could be an option, but relatively low conductivity is still a problem. A lower heat resistance restricts applications. A screen-printable silver nano- wire ink could be used, but its relatively high cost eliminates it as an option. Fine copper mesh generated by chemical etching has a very different concept as a transparent conductor. It has almost the same conductivity as copper foils but requires a very high etching capability for the fine-line mesh and is cost-prohibitive. The perfect solution is within reach. Once it comes to fruition, there will be high demand for transparent flex circuits. FLEX007 Dominique K. Numakura is the managing director of DKN Research LLC. To read past columns or contact Numakura, click here. In collaboration with the U.S. Army Research Laboratory, FlexTech will fund half of the total $2.6 million for three projects aimed at maturing the flexible hybrid electronics technology ecosystem. 1. The University of Colorado project is focused on inte- grating soft actuators and flexible electronic control cir- cuits to demonstrate a complete soft robotic system. The system will mimic muscular structures such as an octopus arm or elephant trunk and will use a human-machine inter- face for robotic manipulation. CU will be partnering with PARC on this 18-month project. 2. Researchers at the University of Washington will focus on an 18-month project to improve and optimize sen- sor design with ultra-high-resolution printed structures using the novel piezoelectric material of a TMCM MnCl3 (trimethylchloromethyl ammonium trichloromanganese). Deliverables include demonstration of large area roll-to-roll printed electronics fabrication and integration, with com- pact signal conditioning and wireless data transmission. 3. UCLA will develop and demonstrate a foldable, high- resolution microdisplay on PDMS substrates by developing an ultra-high yield, scalable and low-cost mass transfer process for assembly of high-quality GaN µLEDs at 100um size for a resolution of >200 pixels per inch (PPI). The pro- totype will be made using heterogeneous integration of mass-transferred GaN µLED with Si CMOS driver circuitry on a common flexible organic substrate using fan-out- wafer-level packaging. (Source: SEMI) SEMI FlexTech Launches Three Flexible Hybrid Projects

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