TANAKA Confirms Improvement in Bending Durability of Flexible Touch Panels

Flexible Touch Panels
Enlarged exterior view of the metal mesh film with a single-sided, a dual-layer wiring structure for use in touch sensors

Tanaka Holdings Co., Ltd. announced today that Tanaka Kikinzoku Kogyo K.K., which operates the Tanaka Precious Metals manufacturing business, discovered a single-sided, dual-layer wiring structure for metal meshes used in touch sensors and a method of manufacturing the structure and has commenced development for commercial application. This technology will contribute to higher image quality, thinner devices, increased flexibility, and improved durability of smart phone touch panels and other applications.

Touch panels normally have a structure made from two sensor substrates – an X sensor substrate and a Y sensor substrate. Tanaka Kikinzoku Kogyo discovered a technique for forming both the X sensor and Y sensor wiring on a single layer flexible substrate. The discovery was based on the results of research conducted by Professor Tatsuo Hasegawa, Principal Research Manager at the Flexible Electronics Research Center of the National Institute of Advanced Industrial Science and Technology. Development was consigned to Tanaka Kikinzoku Kogyo under the Joint Industry-Academia Practical Application Development Project (NexTEP). The discovery was made by applying the metal mesh wiring technology developed under consignment from April 2014 to September 2017 and forming overlapping silver nano-ink wire circuity on one side of the film (creating a single-sided, dual-layer metal mesh film). As a result, only one sensor substrate is needed. This will contribute to reducing costs as well as improving touch panel image quality and making panels slimmer. Tanaka Kikinzoku Kogyo also discovered a transparent (conducting) electrode formed by etching[2] indium tin oxide (ITO), which is commonly used in current touch panel sensors, on a glass substrate and created a structure expected to improve bending strength (increased flexibility) that even metal mesh films cannot withstand as well as a method of manufacturing this structure.

Features of the New Technology

  • Single-sided, dual-layer structure contributes to slimness and improved bending strength (improved flexibility).
  • By using a low temperature sintered silver nano-ink and an SuPR-NaP technique for pattern formation instead of etching, fine wires less than 4 um (2 um – 4 um) can be formed.
  • Long films can be produced using a roll-to-roll process.

As a result of these advantages offered by this product, application in high-end smart phone touch panels, which are expected to shift to bendable displays, and uses and applications in the flexible electronic device market, which is projected to grow, are expected.

Flexible Touch Panels
Rendering of a final representative product using a single-sided, dual-layer structure mesh film (a bendable smart device)

Tanaka Kikinzoku Kogyo’s Metal Mesh Film Printing Technology

Tanaka Kikinzoku Kogyo’s metal mesh film printing technology can create fine wires thinner than 4 um. This is accomplished by applying low-temperature sintered silver nano-ink that can be formed into wires on PET film, which is not heat resistant, and fluoropolymer on a PET film or second substrate, causing adsorption and sintering of silver nano-ink on a fluoropolymer surface activated by irradiation with deep ultraviolet light, and using a SuPR-NaP (Surface Photo-Reactive Nanometal Printing) technique. In addition, Tanaka Kikinzoku Kogyo established a manufacturing process for the fine-wire film using a roll-to-roll processing method. This makes possible bulk printing of metal mesh films with mixed patterns ranging from several microns to tens of microns, sensor units, and frame parts. Tanaka Kikinzoku Kogyo is currently offering samples of metal mesh films with standard specifications (4 um, single-sided, single-layer structure) and is conducting research and development with the aim of providing sample shipments of single-sided, dual-layer structure metal mesh films in the future.

Background to Development

High-end smart phones with foldable displays and other components are expected to appear in the marketplace starting 2019 to 2020, and there is a need to create thinner and highly durable flexible touch panels.

Currently, high-transparency, projection-capacitive[6] type touch panels with multi-touch functionality (detection of multiple points) are commonly used in smart phones. Because of the high transparency and for considerations of mass production, the touch sensors in projection-capacitive type touch panels generally use transparent electrodes made from indium tin oxide (ITO) etched into a glass substrate. Research is currently being conducted, however, to find replacements for ITO because of difficulties reducing prices in the future and environmental concerns raised by waste liquids generated during the etching process. In addition, ITO has high electrical resistance and is weak when bent making large and flexible panels difficult, and consequently, it is not suitable for the future smart phone market.

Accordingly, companies are developing sensors for touch panels using metal meshes, and they are already in use in some touch panel displays and PCs. The most common wire width of metal meshes currently in use is 3 um to 7 um, and the wiring portions are within the range of human vision, posing an issue regarding widespread use in smart phones and other devices that are used at close distances.

Tanaka Kikinzoku Kogyo achieved fine wire formation smaller than 4 um, which was thought to be difficult, by adopting a low-temperature sintered silver nano-ink and a SuPR-NaP technique. In addition, by using this technique with a roll-to-roll format, mass production using bulk printing of sensor units and frame parts as well as cost reductions will be possible. By using the wiring technology discussed above, the outlook for single-sided, dual-layer wiring structure metal mesh films for use in touch sensors is now positive, and contributions to further development in the market for next-generation bendable and foldable smart phones that consumers want as well as the market for flexible electronic devices can be expected.

The resistance of the metal wires after the bending test tended to increase overall, but the degree of the increase in the resistance value of the metal wire on the two-sided structure second electrode surface (bottom side) was conspicuous. Also, even when the wire width on the test prototype was changed to 1 um, 2 um, or 5 um, the two-sided structure was weak when bent, and it was determined that the single-sided, dual-layer structure is resistant to bending deformation.

Source : Tanaka Holdings Co., Ltd.