#1932 / Transparent Flexible Circuits

Five years ago, a material supplier in Japan commercialized a clear polyimide film to use as the base material for heat resistant flexible circuits. Etching companies did not have the capability to produce circuits without copper foils, so they could not use the new polyimide films without another step in the process. Laminate manufacturers provided that next step by developing copper laminates using transparent polyimide film. This was the only option at the time, and there were problems. The copper laminate properties were worse than the plain film because of its transparency. The glue layer in the laminates reduced the transparency considerably because glue is not very transparent.
My team faced this issue and developed a series of flexible copper laminates without using a glue layer; instead we used a direct plating process. Flexible circuit manufacturers were able to produce these circuits using the new laminate with their etching process, but they were hesitant to expand this part of their business. One of their concerns was a lack of coverlay material to finish the flexible circuits. We collaborated with several material manufacturers and developed a film base / screen printable coverlay ink.
Our next challenge was presented to us from device manufacturers. They required transparent conductors for use in optical applications. This was a difficult request for traditional flex circuit manufactures and required a lot of conversation and critical selling skills to convince them to explore this potential new avenue for business.
There were several choices to reach this goal. The first option was Indium Tin Oxide (ITO) film, but it was quickly ruled out due to its lack of flexibility. Next was transparent conductive ink created from organic molecules, but this too was ruled out due to its instability during high temperature processes (soldering). The third option was silver nano wire ink, but it too was ruled out for many reasons – difficulty in finding appropriate process conditions, high cost, greater conductor resistance( > 100 ohm cm), and the conductivity restricts applicable areas. The last option was fine etched mesh pattern made of a thin copper layer. In this case, the conductivities have a trade-off relation with transparency, and the transparency depends on the fine pattern etching capabilities from the circuit manufacturers. To summarize, there is no perfect solution to create transparent conductors.
The constantly evolving consumer electronics market continues to push demands from our customers. Wearable device manufacturers are pursuing the possibility of elastic, transparent and heat resistant flexible circuits. At face value, this looks almost impossible to satisfy all of the requirements. However, one of our affiliated companies developed silicone rubber based flexible circuits using screen-printable silver inks. The base materials are elastic, heat resistant and transparent – this satisfied 80% or our customer’s requirements.
There is no limit in flexible circuit design. Medical device manufacturers are on the cutting edge due to advances from the medical field, and are pushing the envelope with properties required from the flexible circuits.
My team and other circuit manufacturers continue to develop a broad range of materials using various manufacturing processes. We can satisfy 80 – 90 percent of the requirements from the new demands in flexible circuits. Advances from every segment of the manufacturing process will satisfy all the circuits’ requirements in the near future. DKN Research will share these innovative ideas.
Dominique K. Numakura, dnumakura@dknresearch.com
DKN Research, www.dknresearch.com

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Headlines of the week
(Please contact haverhill@dknreseach.com for further information and news.)

  1. Sharp (Major electronics company in Japan) 10/15
    Has demonstrated the new secondary battery system “JH-WBPB150” for home use. Capacity: 6.5kWh Price: 2.6 million yens
  2. JPEA (Organization of solar generators in Japan) 10/16
    Reported that 80% of home solar generator holders utilized self operation systems during the power outage caused by Typhoon #1915 in September.
  3. Toyota (Major automobile manufacturer in Japan) 11/6
    Will introduce flexible organic EL display for the next generation concept car “LQ”
  4. Panasonic (Major electronics company in Japan) 11/6
    Has co-developed a volume production process to produce chemical chips on glass substrate with Institute of Microchemical Technology.
  5. Tokyo University (Japan) 11/8
    Has succeeded to produce wafers of organic semiconductor by simple printing process. It could be valuable step-up to make organic transistors.
  6. Sharp (Major electronics company in Japan) 11/8
    Has co-developed a new 30” foldable flexible OLED display with NHK. It can be put in 4 cm diameter pipe. Weight: 100 grams,
  7. SK Hynix (Major semiconductor manufacturer in Korea) 11/8
    Has opened a new R&D center of C-MOS Sensor devices in Hamamatsucho, Tokyo to catch up the growing market.
  8. AIST (Major R&D organization in Japan) 11/12
    Has succeeded to improve the resolution of EDS element analysis in SEM system more than two orders to determine the 3D structure of carbon nano tube molecules.
  9. Hokkaido University & Bsan University (Japan & Korea) 11/12
    Have succeeded to make the chemical reaction process of SrCoOx, the basic material of the next generation memory devices.
  10. Touchence (Device manufacturer in Japan) 11/14
    Has agreed with Kagawa University to co-develop finger type touch sensors using MEMS manufacturing process.

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