Flexible On-Cell AMOLED has been a key differentiating advantage of Samsung's flagship smartphones since 2017. At the A3 Gen 6 plant in Tangjeong, South Korea, Samsung Display began to produce flexible AMOLED panels based on thin-film packaging, using polyimide instead of glass substrates.
In 2017, rigid (glass) and flexible technologies were used for Galaxy S7 and S7 Edge, respectively, but Galaxy S8 and Note 8 only adopted flexible technologies. Similar to rigid technology, flexible AMOLED displays also use On-Cell touch sensors, but in terms of sensor performance, sensor patterning capacity, supply chain, display structure stacking and future integration, the structure of flexible displays is more complex.
AMOLED touch sensor structure
AMOLED is a self-luminous display, so it does not require backlight and color filters. Rigid AMOLED only needs a piece of upper glass to encapsulate the TFT backplane. Since the upper glass is quite simple, this is an ideal location for patterned touch sensors. Samsung Display mainly uses single-sided ITO (SITO) patterning (with insulating bridges and jumpers) on the packaging glass and under the polarizer. Single-layer multi-point patterning is also suitable for entry-level and mid-range models, but with the increase in display size, Samsung Display has rarely adopted it.
Panel makers such as AUO and Hehui have used SITO patterning in a large number of smart phone OLED panels. Samsung Display will not produce rigid On-Cell touch sensors on its own lithography production line. This work was outsourced to Dongwoo, Hexin and Huaying, which allowed them to reserve the production line for a more profitable panel process. On-Cell touch sensor suppliers should perform sensor patterning in the 5.5 or 6 generation factory to match the A2 (5.5 generation) production line of Samsung Display.
In 2015, Samsung released its flagship model (Galaxy S6 Edge), which is based on flexible AMOLED external touch technology. Samsung Display outsourced its SITO film business to ALPS and Dongwoo. This SITO film is laminated between the cover glass and the AMOLED display polarizer (GF SITO).
However, the yield of this SITO film is very poor, because the substrate of the film is less rigid than glass, and it is easy to cause waste in the complex bridging process. In 2016, Samsung Display decided to put the SITO film under the polarizer and tried to use other materials, such as silver nanowires.
Placing the sensor under the polarizer helps to improve the optical performance. Therefore, flexible AMOLED display manufacturers have switched from GF SITO to film-based On-Cell touch sensors (or AP1S, Samsung Display’s marketing term, but it is not a common practice in the industry), while Samsung’s Galaxy S7 Edge, S8 Plus, Note 8 AP1S is used in the. Similar to rigid technology, AP1S does not use Samsung Display’s internal photolithography production line because SITO film can be outsourced.
AP1S is a transitional technology because it is not the best option for thinner flexible AMOLED. In 2016, Y-OCTA (Samsung’s marketing term is YOUM On-Cell touch AMOLED) was proposed as an advanced flexible AMOLED display technology. Y-OCTA removes the film substrate, allowing the touch circuit to be patterned directly on the film encapsulation layer. Y-OCTA makes the AMOLED display thinner, but it uses the lithography production line inside the panel. If Samsung Display abandons its current outsourcing strategy, it will have mixed advantages and disadvantages.
The reason for adopting Y-OCTA
Samsung Display cannot outsource Y-OCTA sensor imaging like GF SITO and AP1S, because it is an intermediate step in flexible AMOLED production. Outsourcing will bring productivity and responsibility issues to Samsung display or sensor manufacturers. Samsung Display has decided to use Y-OCAT instead of AP1S on high-end models, which means that Y-OCTA must be taken into account in the production line.
The current trend of smartphone form factors constitutes one of the reasons for the switch to Y-OCAT. Displays with an aspect ratio of 18:9 require a thin and narrow profile, especially for flexible AMOLEDs. Y-OCTA eliminates the ITO film substrate (approximately 50 µm or more). The photolithography process in the production line can directly pattern the touch sensor (titanium/aluminum/titanium) on the thin film encapsulation (TFE) layer, which is first coated with a thin layer of resin to keep the surface flat. As for the touch sensor leads and the display pixel leads are integrated, they are connected to the chip through the COP (chip on plastic substrate) method.
COP has shortened the bottom edge, but if TDDI is adopted in the future, the system assembly of the entire display module can be simplified. Cost savings include COF's FPC and ITO thin film substrates. Metallic touch sensor circuits are also easier to bend than ITO-based. It can be applied to the future In-Cell structure, where the cathode or anode is used as a touch sensor to pattern. Y-OCTA uses a photolithography production line for patterning, but it also reduces the composite yield.
Therefore, more capacity preparation is needed, and it takes some time to mature. Samsung Display is considering allowing the L7-1 plant to expand its Y-OCT production capacity in 2018. Currently, only the 5.8-inch Galaxy S8 uses Y-OCTA.
