Have you seen this? Exclusive explanation of the entire process of LCD panel manufacturing

The panel scandal that broke out before is enough to explain the importance that users attach to the type of LCD panel used in LCD monitors. In addition, important technical improvements of LCD monitors, such as LED backlight and ultra-wide viewing angle, are directly related to the panel. The LCD panel, which accounts for 80% of the cost of an LCD monitor, is enough to show that it is the core part of the entire monitor. Its quality can directly determine whether an LCD monitor is excellent.

The overall structure of the LCD panel is actually not very complicated. The author divides it into two parts: the LCD panel and the backlight system.

LED backlight system for LCD panel

The backlight system includes a backlight panel, a backlight source (CCFL or LED), a diffuser (used to evenly distribute light), a diffuser, etc. Since liquid crystals do not emit light, they need to be illuminated by other light sources, and this is the role of the backlight system. However, the CCFL lamps or LED backlights currently used do not have the characteristics of surface light sources, so components such as light guides and diffusers are needed to make the light of linear or point light sources evenly distributed across the entire surface. The purpose is to make the luminous intensity of different points on the entire surface of the liquid crystal panel the same. However, it is actually very difficult to achieve the ideal state, and the only way is to minimize the unevenness of brightness, which is a great test for the design and workmanship of the backlight system.

The LCD panel is semi-transparent when not powered on

The bendable flexible printed circuit board plays the role of signal transmission, and is pressed together with the printed circuit board (the blue PCB board) through anisotropic conductive adhesive to make the two connected.

From the outside to the inside, the LCD panel consists of a horizontal polarizer, a color filter, a liquid crystal, a TFT glass, and a vertical polarizer. In addition, there are driver ICs and printed circuit boards next to the LCD panel, which are mainly used to control the rotation of liquid crystal molecules in the LCD panel and the transmission of display signals. The LCD panel is very thin and semi-transparent when not powered. Its general structure is like a sandwich, with the liquid crystal sandwiched between the lower TFT glass and the upper color filter.

When looking at a microscopic LCD panel, you will see that red, green and blue are a group of three primary colors, and generally one or two groups form one pixel.

Liquid crystal has the light refraction properties of solid crystals and the flow characteristics of liquids. Driven by electrodes, it can be arranged in the way the master controller wants, controlling the intensity of light passing through. Then, on the color filter, the three primary colors of red, green, and blue are used to adjust the color of each pixel to finally obtain a complete image.

According to the functional division, the LCD panel can be divided into the LCD panel and the backlight system. To produce an LCD panel, it needs to go through three complex processes: "front-end array process, middle-end cell process, and back-end module assembly". Today, we will introduce the production process of LCD panels in detail.

● Front-end Array process: thin film/photolithography/etching/stripping (I)

The front-end array process of LCD panel manufacturing mainly consists of four parts: "thin film, yellow light, etching, and film stripping". If you only look at it this way, many netizens don't understand the specific meaning of these four steps, and why they are done this way.

First, the movement and arrangement of liquid crystal molecules require electrons to drive them, so there must be a conductive part on the carrier of liquid crystal, the TFT glass, to control the movement of liquid crystal. ITO (Indium Tin Oxide, transparent conductive metal) will be used here to do this. ITO is transparent and also forms a thin film conductive crystal, so it will not block the backlight.

The different arrangement of liquid crystal molecules and their rapid movement changes can ensure that each pixel accurately displays the corresponding color, and the image changes accurately and quickly, which requires precise control of liquid crystal molecules. ITO film needs special processing, just like printing circuits on PCB boards, drawing conductive lines on the entire LCD panel.

First, an ITO film layer needs to be deposited on the TFT glass, so that the entire TFT glass has a smooth and uniform ITO film. Then, the ITO glass is cleaned with ionized water and prepared for the next step.

Next, photoresist is applied on the glass with the ITO film deposited to form a uniform photoresist layer on the ITO glass. Then it is baked for a period of time to partially volatilize the solvent of the photoresist and increase the adhesion between the photoresist material and the ITO glass.

Ultraviolet light (UV) is used to irradiate the photoresist surface through a pre-made electrode pattern mask to cause the irradiated photoresist layer to react. A photoresist mask is covered on the glass coated with photoresist and the photoresist is selectively exposed under ultraviolet light.

● Front-end Array process: thin film/photolithography/etching/stripping (II)

Let’s take a pixel as an example. As shown in the figure above, in this pixel, the light-colored part is not exposed, while the dark-colored part is the exposed part.

Next, the exposed portion of the photoresist is washed away with a developer, leaving only the unexposed portion of the photoresist, and then the dissolved photoresist is washed away with deionized water.

After development, it needs to be heated and baked to make the unexposed photoresist more firmly attached to the ITO glass.

Then use appropriate acid etching solution to etch away the ITO film not covered by photoresist, leaving only the ITO film under the photoresist. ITO glass is a conductive glass of (In2O3 and SnO2). The ITO film not covered by photoresist is easy to react with acid, while the ITO film covered by photoresist can be retained to obtain the corresponding wire electrode.

● Front-end Array process: thin film/light irradiation/etching/stripping (III)

Stripping: Use a high concentration of alkaline solution (NaOH solution) as a stripping solution to strip off the remaining photoresist on the glass, so that the ITO glass forms an ITO pattern that is completely consistent with the photolithography mask.

The glass base label is washed with an organic solution to take away the reacted photoresist and keep the glass clean. This completes the first thin film conductive crystal process, which generally requires at least 5 identical processes to form complex and precise electrode patterns on the glass.

Use the same method to draw other ITO electrode patterns on the glass

Forming complex and precise electrode patterns can better control the movement of liquid crystal molecules

In this way, the front-end Array process is over. It is not difficult to see from the whole process that the previous deposition of ITO film on TFT glass, coating of photoresist, exposure, development, and etching are ultimately to form the pre-designed ITO electrode pattern on the TFT glass in order to control the movement of liquid crystal molecules on the glass. The general steps of the entire production process are not complicated, but the technical details and precautions are very cumbersome. We will not introduce them here. Interested friends can check the relevant information by themselves.

The manufacturing process of the glass used in LCD panels is also very particular. At present, the world's largest LCD panel glass is mainly provided by manufacturers such as Corning in the United States and Asahi Glass in Japan. They are at the upstream of LCD panel production and manufacturing. These manufacturers have technical patents for glass production processes. A few months ago, the Corning glass furnace shutdown caused by the earthquake had a certain impact on the LCD panel industry, which shows its position in the industry.

● Mid-cell process: TFT glass and color filter bonding (I)

As we mentioned before, the structure of the LCD panel is like a sandwich, with the liquid crystal sandwiched between the lower TFT glass and the upper color filter. In the terminal cell process of LCD panel manufacturing, the TFT glass and the color filter are bonded together, but this is not a simple bonding process, and a lot of detailed technical work is required.

The liquid crystal layer is sandwiched between the lower TFT glass and the upper color filter

The middle cell process is first divided into two parts: TFT and CF (color filter).

First, clean the TFT glass that has gone through the previous Array process with deionized water.

From the picture above, you can see that the glass is divided into 6 pieces of the same size, which means that the LCD panel made of this piece of glass will be cut into 6 pieces in the end, and the size of each piece is the final size. When the glass is cast, the specifications and sizes of each piece of glass have been designed in advance.

When the alignment film is in solution state, the surface of the TFT glass is basically

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Then, the organic polymer alignment material is coated on the surface of the glass, that is, a uniform alignment layer is coated on a proper position on the ITO glass by a selective coating method, and the alignment layer is cured at the same time.

Alignment rubbing: Use flannel-like materials to rub the surface of the alignment layer in a specific direction, so that the liquid crystal molecules can be arranged along the rubbing direction of the alignment layer in the future, ensuring the consistency of the arrangement of the liquid crystal molecules. After alignment rubbing, there will be some contaminants such as flannel lines, which need to be washed away through a special cleaning process.

After the TFT glass substrate is cleaned, sealant is applied to allow the TFT glass substrate to be bonded and fixed to the color filter, while also preventing the liquid crystal from flowing out.

● Mid-cell process: TFT glass and color filter bonding (Part 2)

The terminal Cell process of the TFT glass substrate has been basically completed, and the next step is to carry out the Cell process of the color filter.

Similar to the TFT glass substrate alignment, the color filter also needs to be coated with an alignment film.

Then, the alignment is performed on the alignment film that has been fixed on the surface of the filter.

Spray the spacer on the color filter surface to ensure a certain distance between the TFT glass substrate and the color filter.

Next, we enter the TFT glass substrate manufacturing process again.

Inject liquid crystal into the sealant frame already applied on the TFT glass substrate

Finally, apply conductive glue to the border of the color filter glass in the bonding direction to ensure that external electrons can flow into the liquid crystal layer. Then, bond the two pieces of glass according to the bonding marks on the TFT glass substrate and color filter, and solidify the bonding material at high temperature to make the upper and lower glasses fit stably.

Color filters are very important components of LCD panels. Color filter manufacturers, like glass substrate manufacturers, are upstream of LCD panel manufacturers. Their oversupply or shortage can directly affect the production progress of LCD panels and indirectly affect the terminal market.

● Mid-cell process: TFT glass and color filter bonding (Part 3)

The bonded LCD panel can be cut according to the previously designed cutting size to get the final size.

From the above picture, we can see that each LCD panel has two borders after cutting. What are they used for? You can find the answer in the following module manufacturing process.

Finally, polarizers are attached to both sides of each liquid crystal substrate, with the horizontal polarizer facing outwards and the vertical polarizer facing inwards.

Polarizer is an optical plate that only allows light in a certain direction to pass through. It is an optical element that can convert natural light into linear polarized light. Its mechanism of action is that after the orthogonal incident light passes through the vertical polarizer, the vertical light is allowed to pass through, while the other horizontal light is absorbed or blocked by reflection and scattering.

When making a liquid crystal panel, one piece must be used on the top and one on the bottom, and they must be in an alternating direction. When there is an electric field or no electric field, the light will have a phase difference and appear bright and dark, which is used to display subtitles or patterns.

At this point, the middle cell process is complete. Next, we can enter the last process of LCD panel manufacturing: back-end module assembly.

● Back-end module assembly: driver IC/printed circuit board lamination

The back-end module process mainly involves the integration of the driver IC of the liquid crystal substrate and the printed circuit board. This part can transmit the display signal received from the main control circuit to the driver IC, drive the liquid crystal molecules to rotate, and display the image. In addition, the backlight part will be integrated with the liquid crystal substrate in this link, and the complete liquid crystal panel is completed.

First, anisotropic conductive glue is pressed on the two frames, so that external electrons can enter the liquid crystal substrate layer, which is a bridge for electron transmission.

Driver IC laminated on the LCD substrate

Next is the lamination of the driver IC. The main function of the driver IC is to output the required voltage to each pixel and control the degree of twisting of the liquid crystal molecules. There are two types of driver ICs. The source driver IC located on the X-axis is responsible for data input, and its characteristics are high frequency and imaging function; the gate driver IC located on the Y-axis is responsible for the degree and speed of twisting of the liquid crystal molecules, which directly affects the response time of the liquid crystal display. However, there are already many LCD panels that only have driver ICs in the X-axis direction, perhaps because the Y-axis driver IC function has been integrated and simplified.

The lamination of flexible circuit boards can transmit data signals and act as a bridge for electronic transmission between external printed circuits and LCD panels. It can be bent, so it becomes a flexible or soft circuit board.

Apply anisotropic conductive adhesive to the other end of the flexible circuit board and laminate the printed circuit board.

Flexible circuit board and printed circuit board (photo taken from Samsung 2693HM)

There are many details and precautions in the manufacturing process of liquid crystal substrates, such as ion water cleaning, drying, blowing, air drying, ultrasonic cleaning, exposure, development, etc. There are very strict technical details and requirements, so that qualified quality eye panels can be produced. Interested friends can check the relevant technical information through search engines.

● Make LCD panels glow: The backlight system cannot be ignored

Liquid crystal (LC) is a liquid crystal that has the light transmission and refraction properties of solid crystals and the flow properties of liquids. It is precisely because of these properties that it is used in the display field.

However, liquid crystal does not emit light on its own, so display devices that use liquid crystal as the display medium require a separate backlight system.

First, a backplane is needed as a carrier of the light source. The light source commonly used in LCD display devices is CCFL cold cathode backlight tubes, but it has begun to shift to LED backlights. However, no matter which one is used, a backplane is needed as a carrier.

Samsung 26-inch widescreen 2693HM CCFL backlight tube

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CCFL backlight has been with LCD for some time. Compared with LED backlight, its quality has many defects. However, it has gradually evolved to save 50% of lamp tubes and enhance the transmittance of LCD panels to achieve energy saving.

Samsung XL2370's side-mounted white LED backlight in the off (left) and on (right) states

The rapid development of LED in the lighting field has greatly reduced costs. LCD panels have also begun to use LED as backlight sources on a large scale. Currently, in order to control costs, LED backlights are placed on the side rather than on the back panel, which can reduce the number of LED chips used.

There are numerous dot prints on the diffuser (light guide) of the side-mounted LED backlight system.

However, no matter whether CCFL backlight or LED backlight is placed in various ways, the light source of the backlight cannot be a surface light source, but a line light source or a point light source. Therefore, other components are needed to evenly distribute the light to the entire surface. This task is accomplished by the diffuser plate and diffuser sheet.

On the transparent diffuser plate, dot printing can block part of the light, and the side-mounted LED backlight injects light into the diffuser plate from the side. The light is reflected and refracted back and forth in the diffuser plate, and the light is evenly dispersed over the entire surface. The dot printing blocks part of the light, and the light is evenly filtered out like a sieve.

The diffuser above the diffuser helps to even out the light across the entire surface.

There are 3~4 diffusers above the diffuser to evenly distribute the light over the entire surface, improving the uniformity of the light, which is directly related to the display effect of the LCD panel. In order to better control the brightness uniformity of the screen, professional LCD monitors will put a lot of effort into panel procurement and later backlight control circuits to ensure the quality of the panel.

The backlight system also includes a backlight module igniter, which is located behind the back panel. In the CCFL backlight era, you can often see long strip igniters as shown above, with each coil responsible for a group of lamps.

The lighting device that uses side-mounted white light LED as the backlight source is much simpler. The small circuit board on the far left of the picture above is the lighting device of the LED backlight.

This is the general structure of the backlight system. Since I have never seen what an LCD monitor backlight module with RGB LED backlight looks like, I cannot tell you here. I will share it with you when I see it in the future.

● Back-end module assembly: LCD substrate and backlight integration

The LCD substrate driver IC/printed circuit board lamination is completed, the backlight system is also completed, and the final step is integration to complete the manufacturing of the LCD panel.

Integrate the already completed backlight module with the LCD substrate up and down

Since the liquid crystal substrate and the backlight system are not fixed by bonding, a metal or plastic frame needs to be added to the outer layer to fix the liquid crystal substrate and the backlight system.

Conduct high temperature aging test

After packing, it can be supplied to LCD manufacturers.

Schematic diagram of LCD panel manufacturing process

The back-end module process is completed in the LCM (LCD Module) factory. This part basically does not involve the core technology of LCD panel manufacturing, and is mainly some assembly work. Therefore, some Taiwanese panel manufacturers such as Chimei and Korean panel manufacturers such as Samsung have set up LCM factories in mainland China to assemble the back-end modules of LCD panels. This can facilitate procurement by major display OEMs and LCD TV manufacturers in mainland China, and can reduce labor and transportation costs in the entire manufacturing process.

However, when it comes to the core technology of the front-end and middle-end processes of LCD panels, neither Taiwanese nor Korean companies have any intention of setting up factories in mainland China for the time being. Therefore, mainland China still has a long way to go if it wants to have its own LCD panel industry.

● The development of domestic LCD panel industry has a long way to go

The LCD panel industry is not just about owning a few panel factories. It requires capital, technology, and industrial support to support the operation of LCD panel factories while attracting upstream and downstream manufacturers to join the entire industry chain, invigorating the industry, strengthening it, and ensuring its sustainable development.

The higher the generation of the panel production line, the larger the size of the single glass substrate that can be produced, and the larger LCD panels can be cut at a lower cost.

First of all, it is about capital. The LCD panel industry is a money-intensive industry. Take the smallest 6th generation line that can undertake the production of LCD TV panels, for example. The total investment requires hundreds of billions of RMB, and it also involves the asset-liability ratio, high bank interest, and subsequent capital increase. Not to mention the 8th, 10th, and 11th generation lines. From this perspective, panel factories in mainland China are already "playing with what others have left", not to mention the investment layout related to the next-generation display technology OLED.

Secondly, technology. Mainland China does not have its own LCD panel manufacturing technology, except for "copying, modifying and renaming". In the past, there were cases where the mainland purchased Korean panel factories but did not obtain technical patents, and finally had to spend money to buy technology again, which again required money.

Finally, there is the industrial support. Even if they have their own panel factories, downstream manufacturers do not buy the panels, and they do not cooperate well with upstream component manufacturers. In the market competition, they have no advantages in management, marketing, publicity, and products. They are beaten badly by Korean and Taiwanese companies, and then they go to their parents to complain and use administrative intervention in the market to achieve short-term goals. In the end, they still suffer losses every year, and no one closes the stalls. Taxpayers have no choice but to pay the bill. How to establish an industrial chain centered on the LCD panel industry and with complete upstream and downstream manufacturers is the foundation for the healthy development of the industry.

The merger of the three largest panel manufacturers in China has also caused a lot of controversy. When the industry was at a low point, they wanted to work together to tide over the difficulties. Before they could work together, the industry became prosperous and they forgot the pain. The lack of long-term sustainable development vision has made people worry about the development of the domestic LCD panel industry.

Recently, the construction of Rainbow 6th generation line and Hefei LCD industry is in full swing. It is hoped that the bitter experiences of domestic panel manufacturers in the early stage can serve as a warning to later generations, so that they can develop the domestic LCD panel industry responsibly with a long-term vision.