- Thickness and bending radius of foldable AMOLED are keys to improving mobility.
- The challenges in manufacturing foldable displays include the thinness and robustness of the display structure, new materials required for foldable OLEDs and touch cover panels, the manufacturing yield rate and reliability, the high cost, and the supply chain management.
According to the AMOLED & Flexible Display Intelligence Service, the foldable OLED display is a new form factor that will rejuvenate the display industry but also bring disruptive changes to smart handheld devices.
As shown in the following figure, there are currently several types of foldable AMOLED displays that have been developed. Display makers such as Samsung Display, BOE, Visionox, AUO, and LG Display are all working on different generations of the foldable display. Despite the differences in specifications (especially the thickness and bending radius), each mobile brand has its own concept of the foldable smart handheld device.
Most developments are focusing on combining the smartphone display with the tablet PC display. But as analyzed in [Display Dynamics] Will foldable OLED displays arrive in 2018?, display makers are not in a hurry to launch foldable displays into the end market because of production challenges.
The thickness and bending radius of the foldable AMOLED are keys to improve mobility. However, none of the companies are ready to launch the foldable display into the end market, in spite of all the patents they have filed in the past couple of years. There are some basic challenges in manufacturing foldable AMOLED displays.
The challenges of manufacturing foldable displays, especially for the smartphone displays, are attributed to the following:
The ultra-thin but robust display structure
In order to achieve the “foldable” feature, he flexible OLED must be used as the display technology because traditional LCD cannot be applied to the flexible substrate. The display substrate must be polyimide instead of the traditional glass substrate. Furthermore, a foldable touch panel and robust cover window are necessary for smartphone applications.
The entire display structure (including the display and touch module) must be at a thickness of less than 1.0mm to accommodate a bending radius of 1.5mm.
As there are many semiconductors, transistors, chemical layers, and optical films layers on the top of the display substrate, each layer must be very thin to achieve a total thickness of <1.0mm. According to the AMOLED & Flexible Display Intelligence Service, this is a major challenge in display design capability; consumers are not interested in a foldable but thick display.
Meanwhile, as analyzed in [Display Dynamics] Foldable OLED display prototypes from BOE, LG Display, and Samsung Display and [Display Dynamics] Royole announces foldable smartphone FlexPai with self-supplied AMOLED, the thickness of the Royole and Samsung foldable devices are different. (Samsung’s is thinner.) This is because Royole adapts the “out-fold” design/structure while Samsung apply the “in-fold” design/structure. The “in-fold” design/structure needs a thinner display but is harder to manufacture.
New materials for foldable OLED and touch cover panel
In order to be both foldable and reliable, many new materials are needed, not just for the display but also for the foldable touch panel. There are three major new materials:
- Colorless polymide and hard-coats.
- Multibuffer &and cushion via hole for better flexibility.
- Specialized TFT shape in the backplane for better reliability.
The characteristics of these new materials can be shown as below:
The manufacturing yield rate and reliability
Yield rate refers to the manufacturing input and output ratio. Because of the thin structure and complicated (and immature) process, the foldable display yield rate is assumed to be only less than 30% at the current stage—below the standard smartphone OLED display yield rate of 60–70%.
Samsung Display may be superior to other manufacturers in terms of yield rate given its experience in the process and equipment, and other display makers will have a difficult time ramping up their production yield.
The reliability of the device is indicated by the folding times—how often a device can be folded and unfolded before it malfunctions. So far, there is no consensus on what the folding times should be as part of the device’s durability because real products have yet to be launched, but the general assumption is 200,000 times of folding should be the basic requirement of a foldable display. This means that, after bending/folding/unfolding the device 200,000 times, the display and touch panel/cover window would still function. Devices that malfunction after just 100,000 times of folding and unfolding are undesirable.
To guarantee a reliability of 200,000 times is a major challenge, as analyzed in the AMOLED & Foldable/Rollable Display Technology Trend - 2019.
The high cost
The cost is a direct result of the the abovementioned features; the thinner structure, new materials, the complex manufacturing process, the manufacturing yield, and the reliability warranty all contribute to the higher cost of the foldable display. According to the OLED Display Cost Model , a standard 7.3” QHD OLED display costs $50 (display $35 + touch model $15), while a foldable 7.3” WQHD OLED foldable display costs $100 (display $70 + touch module $25). As analyzed in [Display Dynamics] Cost analysis for different foldable touch solution, with the exception of SDC’s Y-OCTA, it appears that metal mesh and silver nanowires can be viable options for a foldable touchscreen solution. Regardless of the foldable display, the Y-OCTA touch solution has a cost advantage because it does not need additional film substrate. The metal mesh and silver nanowires all contribute to the product’s higher costs.
The supply chain
Unlike other smartphone displays, the foldable display will be custom-made. For example, many smartphones are using the 5.8” or 6.0” FHD+ 18:9 aspect ratio LCD or OLED display. This means that display makers can mass produce these displays and sell high-end products to the top-tier smartphone brands, then sell the comparably lower quality ones to the second-tier brands, or whitebox brands for the emerging markets.
However, there are no second-tier brands or whitebox market for foldable display, for now. The brands that will launch foldable displays are the leading brands (including Samsung, LG, Huawei, Xiaomi, and Lenovo) because the development cost is very high and the market prospects are unclear; there is no precedent for this device and it is difficult to gauge how the market will respond. Each brand has its own design and form factor, which means that display makers can only develop a single model at a time for an individual brand. This increases the risk of Return on Development Investment or Return on Design-In (RODI). This is one reason why Royole and Samsung are adopting the roles of both display maker and smartphone device maker at the same time.
If the project fails, the manufacturer will be unable to find other customers to absorb the high development cost, or display inventories will become a challenge for foldable display makers.
This is why we have seen is a kind of “marriage” situation for the development of foldable displays: