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94 DESIGN007 MAGAZINE I DECEMBER 2019 consume the new polyimide films directly be- cause they could not make the circuits with- out copper foils. Therefore, laminate manufac- turers developed copper laminates using the transparent polyimide film. Unfortunately, the customers were not very happy about the properties of the copper laminates because the glue layer reduced the transparency remark- ably, and the glue materials were not very transparent. Next, it was our turn. We developed a se- ries of flexible copper laminates without a glue layer using plating processes. Several flexible circuit manufacturers could make the circuits with the new laminate with their existing etch- ing process, but they were not very positive about expanding that business. They needed coverlay material to finish the flexible circuits. I asked material manufacturers to develop film base coverlay or screen-printable coverlay ink, and they did. On the other hand, device manufacturers have been demanding transparent conduc- tors for optical applications. It was a hard re- quirement for traditional flex circuit manu- facturers, so we had to change their minds. We had several choices to reach the goal. The first candidate was an indium tin oxide (ITO) film, but it did not have good flexibility. The second candidate was transparent conduc- tive ink made of an organic molecule, but it was not stable at high-temperature processes, such as soldering. The third candidate was a silver nanowire ink, which was difficult to find appropriate process conditions, and its high cost was another headache. These can- didates have larger conductor resistance than 100 ohm-cm. This conductivity restricts ap- plicable areas. The fourth candidate was a finely etched mesh pattern made of thin copper layer. In this case, the conductivities have a trade-off rela- tionship against transparency. And the trans- parency depends on the fine pattern etching capabilities of the circuit manufacturers. Cur- rently, we do not have a perfect solution for transparent conductors. We have to make ap- propriate balances between the materials and performances. Our new customers sometimes request crazy properties because they do not know standard design rules. Recently, some wearable device manufacturers have asked for elastic, transpar- ent, and heat-resistant flexible circuits, and it's almost impossible to satisfy all of the require- ments. However, one of our colleague com- panies developed silicone rubber base flex- ible circuits with screen-printable silver inks. The base materials are elastic, transparent, and heat-resistant, and the customer's require- ments were 80% satisfied. There is no limit to customers' require- ments, especially the medical device manu- facturers, who have been generating ultimate properties for the flexible circuits. Circuit manufacturers have been developing a broad range of materials with various manufactur- ing processes, and they do not satisfy merely 10% of the requirements, but 80–90%. Addi- tional design ideas will help the requirement satisfaction reach 100%. We will be very hap- py to share our experiences regarding these new demands. Headlines 1. Sharp (electronics company in Japan) Demonstrated the secondary battery system "JH-WBPB150" for home use (capacity: 6.5 kWh; price: 2.6 million yen). 2. JPEA (organization of solar generators in Japan) Reported that 80% of home solar generator holders utilized self-operation systems during the power outage caused by Typhoon #1915 in September. 3. Toyota (automobile manufacturer in Japan) Will introduce a flexible organic EL display for the next-generation concept car "LQ." 4. Panasonic (electronics company in Japan) Co-developed a volume production process to produce chemical chips on a glass sub- strate with the Institute of Microchemical Tech- nology.

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