How are LCD displays manufactured?

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(c) BOE, used with permission

One of today&#;s modern technological wonders is the flat-panel liquid crystal display (LCD) screen, which is the key component we find inside televisions, computer monitors, smartphones, and an ever-proliferating range of gadgets that display information electronically.  What most people don&#;t realize is how complex and sophisticated the manufacturing process is.  The entire world&#;s supply is made within two time zones in East Asia.  Unless, of course, the factory proposed by Foxconn for Wisconsin actually gets built.

Last week I had the opportunity to tour BOE Technology Group&#;s Gen 10.5 factory in Hefei, the capital of China&#;s Anhui Province.  This was the third factory, or &#;fab&#; that Beijing-based BOE built in Hefei alone, and in terms of capability, it is now the most advanced in the world.  BOE has a total of 12 fabs in Beijing, Chongqing, and several other major cities across China; this particular factory was named Fab 9.

Liquid crystal display (LCD) screens are manufactured by assembling a sandwich of two thin sheets of glass.  On one of the sheets are transistor &#;cells&#; formed by first depositing a layer of indium tin oxide (ITO), an unusual metal alloy that you can actually see through.  That&#;s how you can get electrical signals to the middle of a screen.  Then you deposit a layer of silicon, followed by a process that builds millions of precisely shaped transistor parts.  This patterning step is repeated to build up tiny little cells, one for each dot (known as a pixel) on the screen.  Each step has to be precisely aligned to the previous one within a few microns. Remember, the average human hair is 40 microns in diameter.

On the other sheet of glass, you make an array of millions of red, green, and blue dots in a black matrix, called a color filter array (CFA). This is how you produce the colors when you shine light through it.  Then you drop tiny amounts of liquid crystal material into the cells on the first sheet and glue the two sheets together.  You have to align the two sheets so the colored dots sit right on top of the cells, and you can&#;t be off by more than a few microns in each direction anywhere on the sheet.  The sandwich is next covered with special sheets of polarizing film, and the sheets are cut into individual &#;panels&#; &#; a term that is used to describe the subassembly that actually goes into a TV. 

For the sake of efficiency, you would like to make as many panels on a sheet as possible, within the practical limitations of how big a sheet you can handle at a time.  The first modern LCD Fabs built in the early s made sheets the size of a single notebook computer screen, and the size grew over time.  A Gen 5 sheet, from around , is x mm, while a Gen 10.5 sheet is x mm (9.6 x 11 ft).  The sheets of glass are only 0.5 - 0.7 mm thick or sometimes even thinner, so as you can imagine they are extremely fragile and can really only be handled by robots.  The Hefei Gen 10.5 fab is designed to produce the panels for either eight 65 inch or six 75 inch TVs on a single mother glass.  If you wanted to make 110 inch TVs, you could make two of them at a time.

(c) BOE, used with permission

The fab is enormous, 1.3 km from one end to the other, divided into three large buildings connected by bridges.  LCD fabs are multi-story affairs.  The main equipment floor is sandwiched between a ground floor that is filled with chemical pipelines, power distribution, and air handling equipment, and a third floor that also has a lot of air handling and other mechanical equipment. The main equipment floor has to provide a very stable environment with no vibrations, so an LCD fab typically uses far more structural steel in its construction than a typical skyscraper. I visited a Gen 5 fab in Taiwan in , and the plant manager there told me they used three times as much structural steel as Taipei 101, which was the world&#;s tallest building from - . Since the equipment floor is usually one or two stories up, there are large loading docks on the outside of the building.  When they bring the manufacturing equipment in, they load it onto a platform and hoist it with a crane on the outside of the building.  That&#;s one way to recognize an LCD fab from the outside &#; loading docks on high floors that just open to the outdoors.

LCD fabs have to maintain strict standards of cleanliness inside.  Any dust particles in the air could cause defects in the finished displays &#; tiny dark spots or uneven intensities on your screen.  That means the air is passed through elaborate filtration systems and pushed downwards from the ceiling constantly.  Workers have to wear special clean room protective clothing and scrub before entering to minimize dust particles or other contamination.   People are the largest source of particles, from shedding dead skin cells, dust from cosmetic powders, or smoke particles exhaled from the lungs of workers who smoke. Clean rooms are rated by the number of particles per cubic meter of air.  A class 100 cleanroom has less than 100 particles less than 0.3 microns in diameter per cubic meter of air, Class 10 has less than 10 particles, and so on.  Fab 9 has hundeds of thousands of square meters of Class 100 cleanroom, and many critical areas like photolithography are Class 10.  In comparison, the air in Harvard Square in Cambridge, MA is roughly Class 8,000,000, and probably gets substantially worse when an MBTA bus passes through. 

Since most display manufacturing has to be done in a cleanroom and handling the glass requires such precision, the factory is heavily automated.  As you watch the glass come in, it is placed into giant cassettes by robot handlers, and the cassettes are moved around throughout the factory.  At each step, robots lift a piece of glass out of the cassette, and position it for the processing machines.  Some of the machines, like the ones that deposit silicon or ITO, orient the glass vertically, and put them inside an enormous vacuum chamber where all the air is first pumped out before they can go to work. And then they somehow manage to deposit micrometer thin layers that are extremely uniform.  It is a miracle that any of this stuff actually works.

(c) BOE, used with permission

It obviously costs a lot to equip and run such a fab.  Including all of the specialized production tools, press reports say BOE spent RMB 46 billion (US$6.95 billion).   Even though you don&#;t see a lot of people on the floor, it takes thousands of engineers to keep the place running. 

The Hefei Gen 10.5 is one of the most sophisticated manufacturing plants in the world.  On opening day for the fab, BOE shipped panels to Sony, Samsung Electronics, LG Electronics, Vizio, and Haier.  So if you have a new 65 or 75-inch TV, there is some chance the LCD panel came from here. 

Important technical improvements of LCD, such as LED backlighting and wide viewing Angle, are directly related to LCD. And account for an LCD display 80% of the cost of the LCD panel, enough to show that the LCD panel is the core part of the entire display, the quality of the LCD panel, can be said to directly determine the quality of an LCD display.

The production of civil LCD displays is just an assembly process. The LCD panel, the main control circuit, shell, and other parts of the main assembly, basically will not have too complex technical problems.

Does this mean that LCDS are low-tech products? In fact, it is not. The production and manufacturing process of the LCD panels is very complicated, requiring at least 300 process processes. The whole process needs to be carried out in a dust-free environment and with precise technology.

The general structure of the LCD panel is not very complex, now the structure of the LCD panel is divided into two parts: the LCD panel and the backlight system.

LCD panel LED backlight system

LCD panel backlight system

These include backlight plates, backlight sources (CCFL or LED), diffuser plates (to distribute light evenly), diffuser plates, and so on.

Due to the LCD does not shine, so you need to use another light source to illuminate, the function of the backlight system is to this, but currently used CCFL lamp or LED backlight, don&#;t have the characteristics of the surface light source, so you need to guide plate, spreadsheet components, such as linear or point sources of light evenly across the surface, in order to make the entire LCD panel on the differences of luminous intensity is the same, but it is very difficult, to achieve the ideal state can be to try to reduce brightness non-uniformity, the backlight system has a lot to the test of design and workmanship.

LCD plate in the case of no current translucent state

The LCD plates are horizontal polarizer, color filter, LCD, TFT glass, and vertical polarizer from outside to inside.

In addition, there is a driving IC and printed circuit board beside the LCD panel, which is mainly used to control the rotation of LCD molecules in the LCD panel and the transmission of display signals. The LCD plate is thin and translucent without electricity. It is roughly shaped like a sandwich, with an LCD sandwiched between a layer of TFT glass and a layer of colored filters.

Micro LCD panel, you will see red, green, and blue for a group of three primary colors, generally a group or two groups for a pixel

LCD with light refraction properties of solid crystals, with fluid flow characteristics at the same time, under the drive of the electrode, can be arranged in a way that, in accordance with the master want to control the strength of the light through, and then on the color filter, through the red, green, blue three colors of each pixel toning, eventually get the full-screen image.

According to the functional division, the LCD panel can be divided into the LCD panel and the backlight system. However, to produce an LCD panel, it needs to go through three complicated processes, namely, the manufacturing process of the front segment Array, the manufacturing process of the middle segment Cell, and the assembly of the rear segment module. Today we will be here, for you in detail to introduce the production of the LCD panel manufacturing process.

(I) Previous Array process: thin-film/yellow light/etching/stripping

The manufacturing process of the LCD panel Array is mainly composed of four parts: film, yellow light, etch and peel film. If we just look at it in this way, many netizens do not understand the specific meaning of these four steps and why they do so.

First of all, the motion and arrangement of LCD molecules need electrons to drive them. Therefore, on the TFT glass, the carrier of LCD, there must be conductive parts to control the motion of LCD. In this case, we use ITO (Indium Tin Oxide) to do this.ITO is transparent and also acts as a thin-film conductive crystal so that it doesn&#;t block the backlight.

The different arrangement of LCD molecules and the rapid motion change can ensure that each pixel displays the corresponding color accurately and the image changes accurately and quickly, which requires the precision of LCD molecule control.ITO film needs special treatment, just like printing the circuit on the PCB board, drawing the conductive circuit on the whole LCD board.

First, the ITO film layer needs to be deposited on the TFT glass, so that there is a smooth and uniform ITO film on the whole TFT glass. Then, using ionized water, the ITO glass is cleaned and ready for the next step.

Next, a photoresist is applied to the glass on which ITO film is deposited, and a uniform photoresist layer is formed on the ITO glass. After baking for a period of time, the solvent of the photoresist was partially volatilized to increase the adhesion of the photoresist material to the ITO glass.

Ultraviolet light (UV) is used to illuminate the surface of the photoresist through a pre-made electrode pattern mask, which causes the photoresist layer to react. The photoresist is selectively exposed under ultraviolet light by covering the photoresist on the glass coated with the photoresist.

 (II)Previous Array process: thin-film/yellow light/etching/stripping

Let&#;s take a pixel unit, like the one above, where the light part is unexposed and the dark part is exposed.

The exposed part of the photoresist is then washed away with the developer, leaving only the unexposed part, and the dissolved photoresist is then washed away with deionized water.

After development, it is heated and baked to make the unexposed photoresist adhere more firmly to the ITO glass.

Then etch off the ITO film without photoresist covering with appropriate acid etching solution, and only retain the ITO film under the photoresist. ITO glass is conductive glass (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.

(III)Previous Array process: thin-film/yellow light/etching/stripping

Stripping: High concentration of alkali solution (NaOH solution) is used as a stripping solution to peel off the remaining photoresist on the glass so that ITO glass can form ITO graphics exactly consistent with the photolithography mask.

Rinse the basic label of glass with an organic solution and remove the photolithographic tape after reaction to keep the glass clean. This completes the first thin-film conductive crystal process, which generally requires at least five identical processes to form a complex and sophisticated pattern of electrodes on the glass.

Use the same method to pull out the other ITO electrode patterns on the glass

The formation of complex and precise electrode patterns can better control the movement of LCD molecules

This completes the previous Array process. It is not difficult to see from the whole process that ITO film is deposited, photoresist coated, exposed, developed, and etched on TFT glass, and finally, ITO electrode pattern designed in the early stage is formed on TFT glass to control the movement of LCD molecules on the glass. The general steps of the whole production process are not complicated, but the technical details and precautions are very complicated, so we will not introduce them here. Interested friends can consult relevant materials by themselves.

The glass that the LCD board uses makes a craft also very exquisite. (The manufacturing process flow of the LCD display screen) At present, the world&#;s largest LCD panel glass, mainly by the United States Corning, Japan Asahi glass manufacturers, located in the upstream of the production of LCD panel, these manufacturers have mastered the glass production technology patents. A few months ago, the earthquake caused a corning glass furnace shutdown incident, which has caused a certain impact on the LCD panel industry, you can see its position in the industry.

(&#;)Middle Cell process: TFT glass and color filter bonding

As mentioned earlier, the LCD panel is structured like a sandwich, with an LCD sandwiched between the lower TFT glass and the upper color filter. The terminal Cell process in LCD panel manufacturing involves the TFT glass being glued to the top and bottom of a colored filter, but this is not a simple bonding process that requires a lot of technical detail.

An LCD layer is sandwiched between the lower TFT glass and the upper color filter

The middle Cell process is divided into TFT and CF (color filter) at first

First, the TFT glass through the previous Array process is rinsed with ionized water

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As you can see from the figure above, the glass is divided into 6 pieces of the same size. In other words, the LCD made from this glass is finally cut into 6 pieces, and the size of each piece is the final size. When the glass is cast, the specifications and sizes of each glass have been designed in advance.

The basic surface of slaughter TFT glass when the directed film is in a solution state

Then, the organic polymer directional material is coated on the surface of the glass, that is, a uniform directional layer is applied to the appropriate position of ITO glass by the method of selective coating. Meanwhile, the directional layer is cured.

Directional friction: Flannelette material is used to rub the surface of the layer in a specific direction so that the LCD molecules can be arranged along the friction direction of the aligned layer in the future to ensure the consistency of the arrangement of LCD molecules. After the alignment friction, there will be some contaminants such as flannelette thread, which need to be washed away through a special cleaning process.

After the TFT glass substrate is cleaned, a sealant coating is applied to allow the TFT glass substrate to be bonded to the color filter and to prevent LCD outflow.

(&#;)Middle Cell process: TFT glass and color filter bonding

The terminal Cell process of the TFT glass substrate has been basically completed. The following is the Cell process of the color filter.

As with TFT glass substrate orientation, color filters also need to be coated with an orientation film

Then the alignment is carried out on the alignment film that has been fixed on the surface of the filter

Spray a pad on the surface of the color filter so that there is a distance between the TFT glass substrate and the color filter

Next, enter the process of TFT glass substrate again

LCD is injected into the sealant frame already coated on the TFT glass substrate

Finally, the conductive adhesive is applied to the frame in the bonding direction of the glass of the color filter to ensure that external electrons can flow into the LCD layer. Then, according to the bonding mark on the TFT glass substrate and the color filter, two pieces of glass are bonded together, and the bonding material is solidified at high temperatures to make the upper and lower glasses fit statically.

Color filters are very important components of LCD panels. Manufacturers of color filters, like glass substrate manufacturers, are upstream of LCD panel manufacturers. Their oversupply or undersupply can directly affect the production schedule of LCD panels and indirectly affect the end market.

(&#;)Middle Cell process: TFT glass and color filter bonding

After fitting, the LCD plate can be cut according to the previously designed cutting size to get the final size

As can be seen from the above figure, each LCD panel is left with two edges after cutting. What is it used for? You can find the answer in the later module process

Finally, a polarizer is placed on both sides of each LCD substrate, with the horizontal polarizer facing outwards and the vertical polarizer facing inwards.

A polarizer is an optical plate that allows only light from a certain direction to pass through. It is an optical element that converts natural light into straight polarized light. The mechanism of action is to make the vertical direction light pass through the straight incident light after passing through the vertical polarizer, and the other horizontal direction light is absorbed, or use reflection and scattering and other effects to make its shade.

When making LCD panel, must up and down each use one, and presents the alternating direction, when has the electric field and does not have the electric field, causes the light to produce the phase difference and to present the light and dark state, uses in the display subtitle or the pattern.

Now, you can enter the final process of LCD panel manufacturing: the assembly of the rear module.

(&#;)Rear module assembly: Drive IC/ PRINTED circuit board press

The rear Module manufacturing process is mainly the integration of the drive IC pressing of the LCD substrate and the printed circuit board. This part can transmit the display signal received from the main control circuit to the drive IC to drive the LCD molecules to rotate and display the image. In addition, the backlight part will be integrated with the LCD substrate at this stage, and the complete LCD panel is completed.

Firstly, the heteroconductive adhesive is pressed on the two edges, which allows external electrons to enter the LCD substrate layer and acts as a bridge for electronic transmission

A drive IC pressed on an LCD substrate

Next is the drive IC press. The main function of the drive IC is to output the required voltage to each pixel and control the degree of torsion of the LCD molecules. The drive IC is divided into two types. The source drive IC located in the X-axis is responsible for the input of data. It is characterized by high frequency and has an image function. The gate drive IC located in the Y-axis is responsible for the degree and speed of torsion of LCD molecules, which directly affects the response time of the LCD display. However, there are already many LCD panels that only have driving IC in the X-axis direction, perhaps because the Y-axis drive IC function has been integrated and simplified.

The press of the flexible circuit board can transmit data signals and act as the bridge between the external printed circuit and LCD. It can be bent and thus becomes a flexible or flexible circuit board

The other end of the flexible circuit board is pasted with heteroconductive adhesive and the printed circuit board is glued together

The flexible circuit board and printed circuit Board (picture from Samsung HM)

The manufacturing process of the LCD substrate still has a lot of details and matters needing attention, for example, rinse with clean, dry, dry, dry, ultrasonic cleaning, exposure, development and so on and so on, all have very strict technical details and requirements, so as to produce qualified eyes panel, interested friends can consult relevant technical information by a search engine.

(&#;) Let LCD panel glow: non-negligible backlight system

LCD (LC) is a kind of LCD, which has the properties of light transmission and refraction of solid Crystal, as well as the flow property of Liquid. It is because of this property that it will be applied to the display field.

However, LCD does not emit light autonomously, so the display equipment using LCD as the display medium needs to be equipped with another backlight system.

First, a backplate is needed as the carrier of the light source. The common light source for LCD display equipment is CCFL cold cathode backlight, but it has started to switch to an LED backlight, but either one needs a backplate as the carrier.

Samsung&#;s 26-inch widescreen HM CCFL backlit tube

CCFL backlight has been with LCD for a long time. Compared with LED backlight, CCFL backlight has many defects. However, it has gradually evolved to save 50% of the lamp and enhance the transmittance of the LCD panel, so as to achieve the purpose of energy-saving.

The Samsung XL&#;s side white LED backlight is in the non-emitting (left) and emitting (right) state

With the rapid development of LED in the field of lighting, the cost has been greatly reduced.LCD panels have also started to use LED as the backlight on a large scale. Currently, in order to control costs, an LED backlight is placed on the side rather than on the backplate, which can reduce the number of LED grains.

The diffuser plate (light guide plate) on the side LED backlighting system has countless dot printing

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

On the transparent diffuser plate, point-like printing can block part of the light. The LED backlight on the side drives the light from the side of the diffuser plate, and the light reflects and refracts back and forth in the diffuser plate, distributing the light evenly to the whole surface. Point-like printing blocks part of the light, screening the light evenly like a sieve.

The diffuser plate above the diffuser plate helps to distribute light evenly over the entire surface

At the top of the diffusion plate, there will be 3~4 diffuser pieces, constantly uniform light to the whole surface, improve the uniformity of light, which is directly related to the LCD panel display effect. Professional LCD in order to better control the brightness uniformity of the screen, panel procurement, the later backlight control circuit, will make great efforts to ensure the quality of the panel.

The backlight system also includes a backlight module laminator, located behind the backplane. In the CCFL backlight era, you can often see the long strip laminator like the one above, with each coil responsible for a set of tubes.

However, it is much simpler to use a side white LED as a backlight. The small circuit board on the far left of the figure above is the backlight of the LED.

This is the general structure of the backlight system. Since I have never seen the backlight mode of R.G.B LED, I cannot tell you what the backlight mode is like. 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 drive IC/ PCB press is completed, the backlight system is completed, and the LCD panel can be manufactured by integration.

Integrate the backlight module with the LCD substrate

Since the LCD substrate and the backlight system are not fixed by bonding, a metal or rubber frame is needed to be added to the outer layer to fix the LCD substrate and the backlight system.

Conduct high-temperature aging test

The boxes are shipped from the factory and can be supplied to LCD manufacturers

Schematic diagram of the LCD panel manufacturing process

After the period of the Module, the process is completed in LCM (LCDModule) factory, the core of this part of the basic does not involve the use of LCD manufacturing technology, mainly is some assembly work, so some machine panel factories such as chi mei, Korea department such as Samsung panel factory, all set with LCM factories in mainland China, Duan Mo group after the LCD panel assembly, so that we can convenient mainland area each big monitor procurement contract with LCD TV manufacturers, can reduce the human in the whole manufacturing and transportation costs.

However, neither Taiwan nor Korea has any intention to set up factories in mainland China for the LCD panel front and middle manufacturing process involving core technologies. Therefore, there is still a long way to go for China to have its own LCD panel industry.

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