Black technology in everyone's eyes: How is the flexible touch screen formed?

From the hot sales of Samsung Galaxy Edge series flagships to the launch of various curved screen bracelets and TVs, there is no doubt that curved screen technology has been widely valued and sought after by consumers and the industry in recent years. In the future, with the continuous development and improvement of fixed curvature curved screen technology, flexible touch screens that can bend arbitrarily will become an important development direction.

On the 25th, at the 13th International Touch Screen and Display Technology Development Forum, Huang Hanfeng, deputy general manager of touch R&D from Shenzhen OFILM Technology, gave a detailed introduction to flexible touch screens from the perspective of the industrial chain, focusing on the four key elements of flexible display, flexible touch, flexible cover and flexible bonding.

Flexible Display

For LCD displays, the liquid crystal is usually responsible for controlling the grayscale of the display, and the three primary color filters control the display color, which requires them to correspond one to one in order to display normally. If an LCD panel is used to make a curved screen, the inner perimeter and the outer perimeter will be unequal, and the original one-to-one corresponding liquid crystal and filter will have position deviations, resulting in color distortion, in addition to a series of problems such as light leakage, dark screen ghosting, and snow. Therefore, for curved screens, OLED display technology represented by AMOLED has almost become the only choice.

Huang Hanfeng said that Samsung is the undisputed leader in AMOLED in terms of patents, production capacity, yield rate and technology accumulation. Its sixth-generation AMOLED panels, which began mass production at the end of 2014, can now produce 50,000 pieces per month, and the yield rate can reach 60%. The second largest company is LG, but LG has previously focused on large-screen curved TVs. As the trend of small-screen AMOLED becomes increasingly obvious, LG is also accelerating the layout of small and medium-sized flexible screens. Although its production is not as good as Samsung's, its yield rate can reach more than 60%. The newly released Xiaomi Note2 uses LG's flexible AMOLED screen. In addition, Japan's JDI and Sharp (already acquired by Foxconn), Taiwan's AUO, and domestic screen manufacturers such as BOE and Royole are also accelerating the layout of their own AMOLED technology. AMOLED replacing LCD is an inevitable trend.

Flexible touch

In terms of touch control, ITO (indium tin oxide) transparent conductive film has become the most important touch layer material for non-curved LCD and OLED display screens due to its good light transmittance, low thickness, excellent hardness and conductivity, mature manufacturing process and many other reasons. However, since ITO itself is a brittle material, it is not suitable for making flexible touch layers with large curvature or even arbitrary bends, and it is expensive and currently accounts for about 30%-40% of the cost of the upstream materials of the entire touch screen industry. It also uses "indium", a rare metal with limited reserves. Therefore, with the advent of the curved and flexible era, it is likely to be replaced.

Huang Hanfeng said that the main substitutes for ITO materials are: graphene, carbon nanotubes, nanosilver and metal mesh. Among them, graphene and carbon nanotubes are very good substitutes for ITO from the characteristics of the materials themselves. However, graphene is still in the research and development stage and is still a long way from mass production. The industrial mass production technology of nano-carbon tubes has not yet been perfected, and the conductivity of the film products made from them is not as good as ITO. Therefore, from the perspective of technology and marketization, metal mesh and nano-silver technology will be the protagonists of development in recent years. Among them, metal mesh technology has been applied in some PC display markets.

The metal mesh technology mentioned here is a conductive metal mesh formed by pressing metal conductive materials such as silver, copper or oxides on a thin film substrate such as PET. Its main advantages are low raw material cost and good foldability, but due to the yield, output and the problem of Murray interference ripples caused by high line width and high pixels, it is more suitable for use in desktop all-in-one computers, laptops and TVs with low resolution and relatively long distance use.

Nanosilver technology refers to applying nanosilver ink material on PET or glass substrates, and then using laser lithography technology to carve a nano-level silver wire conductive network. Its main advantages are high yield, small line width, good conductivity and resistance to folding, and its disadvantage is high cost. Moreover, compared with metal grids, nanosilver material has a smaller radius of curvature and a small rate of change of resistance when bent. In addition, due to the line width, it is more suitable for use in high-resolution close-range scenes such as mobile phones, smart watches and bracelets.

Relatively speaking, the industry currently seems to be more optimistic about the prospects of nanosilver.

Flexible cover

In order to achieve the ability to bend at will, in addition to solving the flexible display and flexible touch, the last and most critical part is the flexible cover and bonding. However, due to its inherent hardness and brittleness, the glass cover may not be the best choice in the flexible field.

Huang Hanfeng said that compared with glass, PET and PI (polyimide) materials with high surface hardness and high light transmittance may be more suitable for flexible screens, but PET and PI also have many problems to be solved. On the one hand, they are not resistant to high temperatures and have poor light transmittance: PET and PI will partially melt and change their properties at not too high ambient temperatures, and they do not have good light transmittance compared to glass.

Secondly, it is difficult to strike a balance between hardness and foldability. Huang Hanfeng said that if a certain hardness requirement is to be achieved, a hardening layer must be applied. However, after the hardening layer is applied, it will crack and the hardness will decrease after repeated folding. Therefore, how to strike a balance between the two is currently the biggest bottleneck.

Compared with the flexible cover plate that needs further development, the development prospect of 3D glass cover plate, which is currently a must for curved screen mobile phones, is very good.

Data shows that 2018 may become the explosion year of 3D glass cover plates. It is estimated that by around 2020, the penetration rate of 3D glass cover plates in mobile phones will exceed 50%. As the processing technology of 3D glass cover plates matures and the unit price decreases, the penetration rate will further increase. It is estimated that the future market size will reach more than 19 billion yuan.

Huang Hanfeng said that the common 1R (single arc bending) and 2R cover manufacturing processes are basically mature, and basically use hot bending technology to achieve a bending effect. However, the yield rate is a problem to be solved. At present, it is basically only 30%, and it is difficult to improve the yield rate in the short term. If 4R is to be made in the future, the yield rate will be even lower. Therefore, the yield rate is one of the problems that need to be solved in the manufacturing of cover plates.

Flexible fit

After the cover is selected, the next step is lamination, which is to integrate the cover with the touch and display layers below to form a complete module. Currently, there are two common methods: roller lamination and vacuum lamination. The roller method has high efficiency and requires rolling the object to be laminated. The vacuum method has relatively low efficiency, but does not require rolling the object to be laminated, so it is not easy to cause damage to the object to be laminated.

Huang Hanfeng said that if the curvature is large, vacuum bonding is the only option. However, for the 4R bonding that may appear in the future, due to its special 4-side bending characteristics, it is difficult to avoid wrinkles and bubbles in places with large curvatures even if vacuum bonding is used. At present, the industry uses customized extruded airbags or plastic parts to alleviate this problem, but it has not been completely solved. Therefore, the industry still has room for improvement in the bonding process of multiple curved surfaces and high curvatures.

Huang Hanfeng said frankly: "From the perspective of the supply chain, it will take several years for flexible screens that can be folded at will to achieve mass production. At the same time, it also requires the upstream and downstream supply chains to work together to overcome related technical difficulties."

Regarding future development, Huang Hanfeng believes that the integration of various modules will be the general trend. He said: "In the Incell technology of LCD liquid crystal display, the ITO touch film and the liquid crystal layer are integrated together, which is a good integration. Now the fingerprint module is independent, and it can also be integrated with the touch in the future. I think there will be a process of mutual integration and progress between functional modules in the future."