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EPTE Newsletter: One Step Closer to Transparent Electronics

Transparent electronics is an emerging technology for printed circuit design. This technology was considered an unattainable or fanciful goal for electronic devices because the substrate materials (glass/epoxy boards and polyimide films) were not transparent. PET film-based flexible circuits with ITO conductors were developed for commercial use in touch screen devices; however, it is unreliable in industrial applications due to low heat resistance. A tradeoff between heat resistance and transparency stymied advancement for many years.

A few recent advancements place us closer to realizing transparent electronics as a practical platform. Several material companies developed transparent and heat-resistant films over the last few years using polyimide and PEEK films. Their dimensional stabilities stand up to temperatures over 200°C, and their transparency characteristics are between 85 and 90% for wave lengths longer than 400 nanometers. This is suitable for substrate and cover materials in transparent flexible circuits.

Transparent conductors are the next hurdle for transparent electronics. There are a few hopefuls currently in use. Indium tin oxide (ITO) coated film using a sputtering process is used in display devices. Their limitation is low conductivity and brittleness when used as a conductor for flexible circuits. Polyethylenedioxythiophene (PEDOT) is a transparent and conductive organic polymer mixture. PEDOT:PSS films are flexible and screen-printable, and their conductivity is equivalent to ITO film. Their issues are low heat resistance, and high-temperature termination processes such as soldering or wire bonding are not available. Silver nanowire ink is also considered. It is screen-printable and flexible, and circuit density can be down to 70-micron line and space. Its conductivity is slightly better than ITO or PEDOT, but traditional termination processes such as soldering or wire bonding are not available, and high material cost is another limitation.

Thin copper foil is a suitable conductor for transparent flexible circuits. The metallic copper has no transparency; however, it is possible to attain transparencies greater than 80% if the conductor’s area ratios are less than 10%. The entire conductivities for the circuits can be one to two orders higher compared to ITO or PEDOT. One issue is how to create the copper laminates. The traditional lamination process with an adhesive layer is not preferred because the transparency is significantly reduced from the adhesive layer. A direct metallization process was developed on the transparent films to make thin copper laminates. The process can generate fine circuits using a semi-additive process because it is the same construction as traditional adhesiveless flex circuit. The transparent circuits remain intact using standard termination technologies such as soldering and wire bonding. Fine-line capabilities remain a hurdle for circuit manufacturers.

There is still no perfect technology to generate transparent flex circuits. However, recent technical progresses put us one step closer to practical transparent circuits. DKN Research will be able to provide a design and manufacturing guide detailing methods and materials list. Send your requests to: dnumakura@dknresearch.com or visit our website at www.dknresearch.com. 

Headlines of the week 

1. SII (Major electronic device manufacturer in Japan) 3/2

Has commercialized a new ultra compact size mobile printer “MP-820” for business use. Size: 79 x 110 x 44 mm, 180 g, printing width: 58 mm.

2. KYOCERA (Major electronics company in Japan) 3/7

Has rolled out the industry’s smallest energy storage system (3.2 kWh) using lithium ion batteries for apartment use.

3. Furukawa Electric (Major cable manufacturer in Japan) 3/3

Has developed a new copper-tin alloy plating material for the connecting terminals of the automobile wire harness.

4. Murata (Major component supplier in Japan) 3/7

Has commercialized the world smallest aluminum electrolytic capacitors for mobile equipment. 3.5 mm x 2.8 mm for 33 micro Farads.

5. NEC (Major electronics company in Japan) 3/7

Has developed a new reliable LSI for aerospace use. The new devices are stable under high radiation circumstances.

6. JX Nippon Mining & Metals (Major mining company in Japan) 3/13

Will expand the manufacturing capacities of rolled annealed copper foil. It will re-start the operation of the surface treatment. The demands of flexible circuits are still growing.

7. NOK (Major material supplier in Japan) 3/9

Has developed a large-scale hydrogen separation system with thin carbon layer for fuel cells of the EV automobiles.

8. Hitachi (Major electric & electronics company in Japan) 3/9

Has developed a new conversion process to change bamboo to equivalent material as biomass fuels for power generators. By products are also valuable.

9. NEDO (Major R&D organization in Japan) 3/13

Has co-developed a new compact heat storage system for temperature sources lower than 100°C. It is valuable for transportation of the heat.

10. AIST (Major R&D organization in Japan) 3/14

Has developed a new organic thermo-electric conversion material based on carbon nanotube. It is printable, and has achieved 600 micro W/mK2.

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