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Folding screen phones are so "fragile", why are they selling so expensive?


The existing folding screen mobile phones on the market include at least Samsung Galaxy Fold, Huawei Mate X, Samsung Galaxy Z Flip, Motorola Moto Razr 2019, Royole FlexPai, etc. It seems that there are not many models that can be counted in detail, but the price and Whether it can be bought or not, folding screen products seem to be still far away.


Along with the sci-fi technology of folding screens, there is also the fragile reputation of folding screen phones today. Samsung has invested in flexible screen technology for more than 10 years, but soon after the launch of the Galaxy Fold, it faced problems such as abnormal screen display and film layer separation. The first generation of folding screen devices like Huawei Mate X can leave permanent and irreparable dents just by gently scratching the screen with your fingernails. Even if Samsung's second-generation folding screen device Galaxy Z Flip claims to improve the process, judging from the hardness test, it is quite easy to leave scratches on the screen surface; and the fold may even shatter due to low room temperature.


This fragile property of the folding screen further distances it from ordinary people: when consumers spend 20,000 yuan to buy a folding screen mobile phone, they need to reach out to touch the screen every morning when the mobile phone alarm rings. You have to think about whether you didn't cut your nails first. The experience is still daunting. Therefore, this article will explore from the technical level why folding screen mobile phones are so fragile, and present the current development stage of folding screen mobile phones from the side.

 

Folding screen: the advanced stage of flexible display technology

 

First of all, it is still necessary to delineate the scope of the discussion: what exactly do we mean by flexible screens and folding screens. If it is divided according to different display panel technologies, it is well known that LCD and OLED have their own flexible development routes in the general direction-but LCD flexible screens are relatively special, and they are not the mainstream technology we discuss about mobile phone folding screens.


From the perspective of electro-optic materials, not only LCD and OLED, but also electrophoretic (E-Ink) and Gyricon can also be flexible, which are more common in e-books and e-paper . There are already many such flexible screen products on the market, most of which focus on reading and writing.


Nowadays, the relatively popular flexible screen and folding screen technologies on mobile phones and mobile products refer to flexible OLED panels. When talking about folding screens and flexible screens in this article, unless otherwise specified, it refers specifically to OLEDs. The scope of the discussion is clear. Another problem that needs to be solved is, what is the relationship between the flexible screen and the folding screen?


It is generally believed that the development of flexible display technology can be divided into several different stages. The first stage is a flexible screen with fixed curvature , that is, the screen already exhibits curved surface characteristics, but its curvature is fixed in the form of the final product and cannot be controlled by the user. This has been achieved many years ago. Represented by Samsung Galaxy series mobile phones, Huawei's flagship phones in the past two years have also adopted this so-called "3D curved screen"; many monitors and TV products also have such designs.


The second stage is a bendable and rollable display ; the third stage is a foldable display ; the fourth stage is a fully flexible display that can be folded and stretched arbitrarily . Among them, the bendable and curly screen of the second stage can be seen at many display technology exhibitions. The important difference between it and the third stage is that the "bending radius" is significantly different. The bendable screens often seen at exhibitions have a relatively large bending radius (3-15mm). The foldability of the third stage means a very small bending radius (0.5-3mm), and the technical level is much more difficult to realize than the second stage.


From the perspective of bending radius, the inner folding screen like Samsung Galaxy Fold is more difficult in terms of panel technology than the outer folding screen solution of Huawei Mate X. Because the bending radius of the former is significantly smaller than that of the latter.


It can be seen that the folding screen is an advanced stage of the flexible screen, even if it is not the final form. Regarding the value of folding screens in practical applications, I won’t go into details here: at least as far as mobile products are concerned, folding screens essentially put a device with a larger screen in your pocket to improve portability.

 

How are flexible OLEDs made?

 

To understand why folding screens are so fragile, you first need to understand the structure of the screen and how it is manufactured.


Nowadays, the common OLED displays of mobile phones and TVs are AMOLED (Active Matrix OLED) panels, which structurally include a substrate, a cathode layer (cathode), an organic molecular layer (including an emission layer, a conductive layer), and an anode layer ( anode)—these constitute the OLED frontplane (as shown in Figure 1); of course, the TFT array layer (thin film transistor) is also required—this part is what we often call the backplane, which is essentially the control circuit.


The light emitting principle of OLED is electro-phosphorescence, and this mechanism will not be described in detail here. When it becomes the final form of the screen, it is also necessary to package the panel. The upper cover of the AMOLED screen of the traditional mobile phone is the sealing glass.


To make such a screen into a flexible form, that is to say, each layer is required to be bendable and foldable. This has not yet involved the touch panel, the outermost protective material and other constituent layers, which also need to be bendable and foldable. In the general direction, if the OLED frontplane and TFT backplane are to be made into bendable and foldable forms, the problem may not be too big. However, the substrate of the traditional AMOLED display, the upper cover, and the outermost protective layer of the screen are all made of glass.


Conventional glass can bend very little, so at least the materials of these layers must be replaced with flexible materials—the most intuitive thing for users is that it is impossible to use Corning’s Gorilla Glass for the outer layer. This has become the first major challenge in the manufacture of flexible screens, such as the selection of materials for substrates and cover plates. Due to the manufacturing process of the OLED panel, the material selection of the substrate is actually very limited.


OLED panel manufacturing needs at least various processes such as etching, sputtering, evaporation, and cutting, and materials need to withstand various high-temperature and corrosive environments; in the flexible panel manufacturing process, there is also a process such as UV ultraviolet light stripping. Therefore, there are the most basic requirements in the selection of materials.


Here we can talk about the mentioned TFT layer separately. The material of this layer is divided according to the switching device. The relatively popular ones are LTPS (low temperature polysilicon) and IGZO (indium gallium zinc oxide). LTPS is the mainstream in flexible screen manufacturing, and it is also a solution commonly used by panel manufacturers such as Samsung and BOE when developing flexible screens. LTPS is low-temperature polysilicon, which can be synthesized at a relatively lower temperature than traditional solutions (such as a-Si). But even "relatively lower" may need to reach 600°C (or lower).


Royole uses a technology called ULT-NSSP (ultra-low temperature amorphous silicon semiconductor process) here. According to Royole, this more "low temperature" technology can further reduce costs-this seems to be another path that Royole has taken in the development of flexible panels compared with other panel manufacturers, and the specific results are unknown. In any case, lower temperatures are always more valuable in the production manufacturer.


Relatively speaking, the manufacturing process of the flexible panel is similar to that of the traditional rigid OLED panel in the early stage. The glass support layer (Carrier Glass Panel) is also required in the early stage, as shown in the figure, but there is a laser stripping process in the end: the entire panel is separated from the OLED panel. The glass support layer is separated.


After so many processes, such as the high temperature (or relatively high temperature) during TFT manufacturing mentioned above, there are not many materials that can still stand. Since it is difficult to choose glass as the substrate material, it is still necessary to ensure light transmission, plus bendable and foldable properties. The industry generally uses PI (Polyimide, polyimide)—to put it bluntly, it is some kind of plastic film. It is already shown in the figure. Of course, there are still some technical details that cannot be detailed here. For example, the glass substrate may need to adopt a PI coating solution, and a debonding layer is required between the support layer and the PI substrate.

 

Each layer has to be bent and folded

 

In fact, ultra-thin glass is also an alternative substrate material that can be bent to a certain extent. After all, glass has higher thermal stability and better transparency, but it is still limited by the degree of bendability. In addition to the choice of substrate materials, there are still some issues that need to be considered in flexible panels.


For example, ITO (indium tin oxide, or other conductive polymer materials) of the conductive layer, on the one hand, requires a lower temperature process, on the other hand, ITO is deposited on the plastic substrate, which may cause tensile strain Big question.


As another example, the TFT layer is also affected by bendability, not only because external forces can damage it, but also because of the forces generated by thermal expansion/contraction of other layers, and it is very sensitive to humidity. TFT layer In addition to the LTPS mentioned earlier, OTFT (Organic Thin Film Transistor) is also an option for flexible panels.


Actions like bending, especially when the bending radius is small enough to "fold in half", imagine folding a book in half along the middle of the cover: after folding in half, the deformation state of the pages in the inner circle and the pages in the outer circle will be different. In order to adapt to this kind of bending and folding, all the pages of the book will be deformed to different degrees in different positions of the whole book. The screen is also a multi-layer structure. Of course, the screen panel is not as thick as a book, but the materials and processes of each layer of the panel are different, and the deformation and thermal expansion characteristics are also different. This can cause considerable hindrance to the folding action.


It is not difficult to imagine that when using a folding screen mobile phone, the problem of film layer separation, film layer slippage, and even direct brittleness will easily occur if the number of folds is too high - just like a book folded in half, the positional relationship between different pages and Compared with the flat state, it is quite different. So the problem of creases is not difficult to understand, that is, the deformation that has occurred is difficult to recover-maybe the surface material cannot be recovered, or it may be the material of other layers.

 

When dealing with such problems, different panel manufacturers also have their own different solutions. For example, the passivation structure is added with a buffer layer (BL), an inorganic waterproof layer, an adhesive layer (AIL), etc. The soft buffer layer can largely offset the force generated during the bending process and reduce the bending radius (as shown in the figure). This is a solution that Taiwan Industrial Technology Research Institute has begun to try a few years ago. Of course, this part of the content is actually related to the encapsulation to be mentioned below.


At the Royole press conference in March this year, Royole mentioned the establishment of an "intelligent mechanical simulation model" to form a database of material mechanics parameters - various parameters of different material layers, and simulate the physical properties of materials for comparison with experiments. Through this simulation model, a better stacking scheme and material selection can be found.

 

Packaging Technology in Exploration

 

However, during the use of folding screen mobile phones, in addition to the destructiveness caused by the folding action itself, display and touch failures often come from water and oxygen intrusion into the panel, causing serious problems. Because organic materials are easily oxidized and hydrolyzed. Therefore, the barrier to water and oxygen is particularly important for flexible panels.


This involves encapsulation technology-although the previous paragraph of this article is also related to encapsulation, the focus of the previous part is on the "buffer" for folding, and this part is mainly related to "isolation" and "delay". As mentioned above, there is a big difference in packaging requirements between traditional OLED screens and flexible screens. The form of the former is fixed, and after being applied to terminal products such as mobile phones and TVs, the environment it faces is relatively stable; while the latter is due to Flexible form, packaging needs to achieve multi-directional protection, especially for the barrier of water and oxygen.


This is also the main reason why some display areas of the screens fail during use of many folding screen phones that are already on sale. At least as far as the status quo is concerned, the packaging technology of flexible panels seems to be not so mature yet.


Multilayer film encapsulation is a relatively common solution: multilayer films usually alternately stack inorganic layers and organic layers, and each organic/inorganic layer is stacked to form a pair; more than three pairs of multilayer films, the water and oxygen barrier properties are improved by 3- 4 orders of magnitude, WVTR (water vapor transmission rate) can also be increased accordingly; the thinner the organic layer, the more favorable it is to form a uniform and uniform layer; at the same time, this organic/inorganic pair should not exceed 5 pairs. In general, the actual performance still depends on the material and workmanship.


Samsung adopts a multi-layer thin film packaging technology called Barix-this is a technology commercialized by Vitex in the United States, and it is still widely used in flexible thin film packaging today. Barix multilayer films can largely meet some specifications. Barix-coated plastic films can also be used as transparent substrates.


However, Barix technology also faces some challenges, such as some inherent defects of sputtered AlOx films in the early days. This technology also requires the panel to enter and exit the deposition chamber as many as six times, and the cost is relatively high. Oxide deposition is one of the most speed-limiting steps in the process – and of course technology development for this problem is ongoing. During the flexible OLED manufacturing process, packaging becomes a large proportion of the overall cost.


Even with more than 10 years of research and development investment, the first generation of folding screen mobile phones still faces various problems. After all, the thickness of today's folding screen itself is less than 1mm, and the bending radius is only 1.5mm. Considering the transistors, semiconductor materials, chemical layers, and optical film layers involved, the screen manufacturing challenges are naturally conceivable. Also care about reliability issues: 200,000 times of folding is the basic guarantee of use today.


Coupled with production yield control, the overall cost of folding screens is still relatively high. According to IHS's OLED Display Cost Model data, the cost of the display part of a traditional 7.3-inch QHD OLED screen is US$50-35, and the cost of the touch component is US$15. In comparison, the cost of a foldable 7.3-inch WQHD OLED screen is in the range of US$100-70, and the touch component is US$25. Here we have not discussed the flexible technology related to the touch layer - this is also a lot of ways, Samsung's Y-Octa touch solution is a typical example.


From the perspective of partial terminal products, the cost required for the hinge of the folding screen mobile phone is also huge. In addition, the internal layout design of the mobile phone has made concessions for the folding screen, as well as the system and software level. Out of the UI and function development. Today's folding screen phones are expensive, so it's easy to understand.

step by step to maturity

 

Above we roughly explained why the flexible screen is so fragile. Finally, let’s briefly talk about the hardness of the folding screen that consumers are very concerned about: the hardness here actually refers to the hardness of the outermost covering material of the screen. This question is very related to user experience. After all, no one wants to use a mobile phone for a few days, and there will be a bunch of dents and marks on the screen.


The first-generation folding screen mobile phones that have been on the market generally have such problems. The outer covers of these mobile phones are also made of PI materials-after all, transparency and bendability need to be guaranteed. In the choice of surface covering materials, Samsung’s second-generation folding screen mobile phone Galaxy Z Flip began to use an ultra-thin glass cover—this glass cover named UTG by Samsung is actually produced by Dowoo Insys in South Korea .


Judging from user feedback, the durability performance of this ultra-thin glass is indeed significantly better than that of PI, but that is only a relative term. It is also not very hard and there is a risk of chipping. Korean media previously reported that Samsung expects to develop its own UTG cover because Dowoo Insys' solution is still not strong enough. The latter’s current solution has a glass thickness of 30 μm, and Samsung is preparing to develop a cover plate with a thickness of 60 μm, which will make it more reliable while ensuring foldability.


In any case, from this iteration of the surface glass cover of the folding screen mobile phone, it is not difficult for the audience to find that the folding screen still has a lot of room for improvement and development at the technical level: but it is becoming mature, and it is a kind of technology that has accumulated a lot of technology. reflect.


Regarding the market prospects of flexible screens and folding screens, no analysis agency has ever questioned their potential. Last year, DSCC predicted that by 2022, the shipment of foldable OLEDs would reach 63 million. In 2019, this figure was still 3 million, with a compound annual growth rate of 173%. The market size is expected to be USD 8 billion in 2025. The continuous improvement of technology has led to a further reduction in the cost of folding screens. In another 1-2 years, the folding screen mobile phones on the market will probably exist at a par, and the reliability will be better than today's folding screen mobile phones. Much more, at that time the era of folding screens really arrived.

 

*The content is excerpted from Electronic Engineering Seminar

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