The Ultimate Guide of Processes for Producing LCD

Aug 16, 2021 | Articles

Outline:

  1. Introduction of LCD
  2. Production processes of LCD
  3. Putting materials
  4. Pre-cleaning
  5. Gluing
  6. Exposure
  7. Development
  8. Acid Carving
  9. Demolding
  10. Medium Cleaning
  11. PI Silk Screen Printing
  12. High-temperature baking
  13. Friction
  14. Punctuation silkscreen
  15. Powder Coating
  16. Lamination
  17. Press Baking
  18. Glass cutting
  19. Crystal Infusion
  20. Sealing
  21. Post-cleaning
  22. Visual Inspection
  23. Electrical Inspection
  24. Silk Screen Printing
  25. Mounting
  26. Spraying code
  27. Mounting pins
  28. Mounting pin inspection
  29. Packaging
  30. Delivery

 

Do you want to know the whole production process of monochrome LCDs?

In this article, we are going to explain the complete steps for the production of monochrome LCDs.

What is a Liquid Crystal Display?

Liquid crystal displays are commonly known as LCD, consist of liquid crystals that are activated by electric current, and they are used to display lines of alpha-numeric information (one or more lines) in a variety of devices, for example, fax machines, answering machines call counters, laptop computer screens, scientific instruments, portable compact disc players, clocks, and so forth. The basis of LCD technology is the liquid crystal, a substance made of complicated molecules. 

LCD uses a liquid crystal to produce a visible image. Liquid crystal displays are super-thin technology display screens that are generally used in laptop computer screens, TVs, cell phones, and portable video games. Liquid crystal displays (LCDs) have become the dominant technology in televisions and monitors in our homes and offices.

In an LCD, an electric current is used to switch segments of liquid crystals from a transparent phase to a cloudy phase, each segment forming part of a number or letter. The segments can also be in the shape of tiny dots or pixels, and they can be arranged in rows and columns. They are turned on and off individually to either block or allow polarized light to pass through. When the light is blocked, a dark spot is created on the reflecting screen.

Simple facts that should be considered while making an LCD:

  • The basic structure of the LCD should be controlled by changing the applied current.
  • We must use polarized light.
  • The liquid crystal should be able to control both of the operations to transmit or is also able to change the polarized light.

Production Processes Of LCD

Here we have described Raw materials and all steps of production of LCD with images and details for your better understanding.

Raw materials used in LCD manufacturing 

The raw materials used in LCD manufacturing refer to the raw materials retained in the product after LCD production, mainly including:

  • ITO (indium tin oxide) glass, 
  • liquid crystal, 
  • polarizer, 
  • PI liquid, 
  • silkscreen glue (printing frame glue, conductive glue), 
  • sealing glue, 
  • conductive gold ball, 
  • intermediate powder, etc. 

Generally speaking, the three main raw materials of LCD are:

  • liquid crystal, 
  • ITO glass 
  • polarizer. 

The auxiliary materials for LCD manufacturing refer to the raw materials used in the production process but not in the final product, such as: 

  • photoresist and thinner, 
  • NMP, 
  • PI thinner, 
  • BC liquid, 
  • SiO2 thinner, 
  • abrasive cloth, 
  • isopropanol, 
  • ethanol, 
  • Acetone, 
  • cleaning agent, 
  • acid, alkali, etc.

1. Putting materials

Putting materials

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.

The input of ITO glass: According to the requirements of the product, appropriate ITO glass is selected and put into the transfer basket. Specifications and models of ITO glass are required to meet the requirements of the product. Remember that the ITO layer must be inserted into the basket upward.

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 them.  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 the 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.

2. Pre-cleaning

Pre-cleaning

Clean ITO glass with the cleaning agent and deionized water (DI water), wash impurities and oil stains on the ITO surface with physical or chemical methods and then remove and dry the water to ensure the processing quality of the next process.

According to product requirements, prepare ITO glass (thickness, surface resistance, substrate type) suitable for cleaning. The front of the glass is placed in the same direction, that is, the identification angles of all the glass to be cleaned are placed in the same direction. Work in the same direction.

3. Gluing

Gluing

Photoresist Glue coating is the first process of photolithography. It is to coat a layer of photoresist evenly on the ITO surface of ITO glass. The effect of coating directly affects the quality of photolithography. Its main control content is photoresist Preparation, coating thickness, and uniformity, coating surface state, etc. 

Evenly coat a layer of photoresist on the conductive surface of ITO glass. The glass coated with a photoresist must be pretreated at a certain temperature.

4. Exposure

Exposure

During exposure, the mask and photoresist will not damage the photoresist film and contaminate the mask film even if they are in contact with each other. At the same time, only when the photoresist is clean, can the photoresist fully react with light during exposure. 

Ultraviolet light (UV) is used to illuminate the surface of the photoresist through the premade electrode pattern mask, which causes the reaction of the photoresist layer. The photoresist is selectively exposed under an ultraviolet lamp by covering the photoresist on the glass coated with the photoresist (as shown in the figure).

Process control: temperature, time (pulling speed) exposure Use ultraviolet light to irradiate the photoresist surface through the premade film (selective irradiation) to make the photoresist layer of the part irradiated by the ultraviolet light react.

5. Development

Development

The surface of the glass is treated with developer solution, the photoresist layer decomposed by light is removed, the photoresist layer of the unexposed part is retained, the photoresist of the part exposed by UV light is dissolved in the developer solution by chemical method, the developed glass has to undergo a certain temperature hardening film treatment.

The photoresist is selectively exposed under the ultraviolet light development Treat the surface of the glass with a developer solution, remove the photoresist layer (positive glue) that has been decomposed by light, leave the unexposed part of the photoresist layer, and use the photolithography of the part irradiated with UV light to dissolve in the developer solution. 

The latter glass must be treated with a certain temperature of the hard film.

Hard film 

Since the photoresist film softens and swells during development, which affects the anti-corrosion ability of the film, the glass must be baked at an appropriate temperature after development to remove moisture and enhance the adhesion of the film to the glass. This process is called hard film. 

There are two types of hard film: oven and infrared. 

Hard film conditions: one is temperature, and the other is time (drawing speed). Generally, hard film conditions are higher than pre-baking conditions.

6. Acid Carving

Acid Carving

Use a certain ratio of acid to remove the ITO film on the glass that is not protected by the photoresist, and leave the ITO protected by the photoresist, and finally form the desired pattern. 

As shown in the figure, the etching solution used should generally be able to etch away the ITO without damaging the photoresist on the glass surface. Generally, a certain proportion of HCl, HNO3 (or FeCl3), and a mixture of water are used. 

The etching temperature and time have a great influence on the etching effect.

Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. … The plate is then dipped in a bath of acid, known as the mordant (French for “biting”) or etchant, or has acid washed over it.

With an appropriate acid etching solution, the ITO film without photoresist covering is etched off. Thus, the required ITO electrode pattern is obtained.

7. Demolding

Demolding

Use high-concentration lye as a stripping solution to peel off the remaining photoresist on the glass, so that an ITO pattern that is exactly the same as the film pattern is formed on the ITO glass, as shown in the figure below. 

Generally, a brush is used to scrub at the same time to enhance the stripping effect. Then use DI water to clean and dry the glue crumbs. Generally, the above processes are called lithography processes, which play a key role in LCD manufacturing. 

the high concentration of alkali solution (NaOH solution) is used as the film removal solution to peel off the remaining photoresist on the glass, thus forming ITO graphics exactly consistent with the lithography mask on the ITO glass.

8. Medium Cleaning

Medium Cleaning

According to product requirements, prepare ITO glass (thickness, surface resistance, substrate type) suitable for cleaning. The front of the glass is placed in the same direction, that is, the identification angles of all the glass to be cleaned are placed in the same direction. Work in the same direction.

Rinse remaining lye and remaining photoresist and other impurities with high purity water.

9. PI Silk Screen Printing

PI Silk Screen Printing

Generally, the orientation materials for the production of low- and medium-grade LCDs are made of PA, that is, polyimide acid, which is made by the polymerization reaction of dianhydride and diamine at low temperature. It is dehydrated and cured at high temperature (the upper chemical layer is a kind of ring Chemical reaction), it becomes polyimide (PI). Polyimide has good chemical stability, excellent mechanical properties, high insulation, high-temperature resistance, high dielectric constant, radiation resistance, and non-flammability. 

Screen printing is used for seal printing and thick layer printing. Flexo-Printing is used for polyimide layer printing and printing of thin layers with high precision.

Screen printing is a technology, where the print material is pressed by a squeegee through a mesh that is fixed on a frame. The mesh carries a photoresist layer with openings defining the pattern to be printed.

Flexo printing is offered in order to form polyimide layers in glass plates for LCD manufacturing. We can integrate these machines in an in-line production street or in a polyimide cluster, consisting of a polyimide printing machine, a coupled hot plate curing system and related automation incl. loading and unloading devices.

10. High-temperature baking

High temperature baking

The purpose of the pre-baking is to promote the full volatilization of the solvent in the adhesive film and to dry the adhesive film to enhance the adhesion between the adhesive film and the ITO glass surface and the wear resistance of the adhesive film. 

The glass coated with a photoresist is baked for a period of time at a certain temperature to volatilize the solvent in the photoresist and increase its adhesion to the glass surface. the glass will be processed by high temperature again, making the photoresist stronger.

11. Friction

Friction

Use flannel materials to rub the surface of the alignment layer in a specific direction so that the liquid crystal molecules can be aligned along the rubbing direction of the alignment layer in the future.

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Your content goes here. Edit or remove this text inline or in the module Content settings. You can also style every aspect of this content in the module Design settings and even apply custom CSS to this text in the module Advanced settings.

Your content goes here. Edit or remove this text inline or in the module Content settings. You can also style every aspect of this content in the module Design settings and even apply custom CSS to this text in the module Advanced settings.

12. Punctuation silkscreen

Punctuation silk screen

Punctuation silkscreen is used to connect the circuits of the LCD because there are 2 glasses of the LCD. up one and down. Only the pin side glass has an electricity connection, that dots are for supplying electricity for the glass which doesn’t have electricity. that dots are for supplying electricity for the glass which doesn’t have electricity.

13. Powder Coating

Powder Coating

14. Lamination

Lamination

Roller laminating system
The system of roller lamination is that after an inclined stage which sucks and holds the film, then touches the roller at the film tip portion, and laminates the film to the cover glass by roller pressure and the movement of the lamination stage. The screen lamination system is a method of entirely sucking and holding the film in an adsorption mesh sheet and laminating it by a roller pressure through a mesh.

15. Press Baking

Press Baking

Upper stitch or hot pressing paper 

When users have products with pins or hot press paper, install appropriate metal electrode pins or hot press paper. 

As described above already, the seal has to be finally cured after the cell assembly process. This has to be done under pressure in order to make sure that the seal thickness is properly related to the spacer diameter and the calculated liquid crystal thickness can be reached with low tolerances. Hotpress ovens are available as a batch process tool and as a single panel press oven. The batch oven requires a previous collection of panels and preparation of a larger pile of panels that are pressed together. The pressed pile of wafers is then cured in a clean convection oven. The single panel hot press oven is easier to integrate with automatic lines and works continuously.

16. Glass cutting

Glass cutting

Usually, a pair of ITO glasses can make multiple liquid crystal cells. In order to expose the LCD filling port, the glass must be appropriately cut into strips or granules.

The two glass substrates must be cut to the proper size, polished, and washed. Cutting can be done with a diamond saw or scribe while polishing involves a process called lapping, in which the glass is held against a rotating wheel that has abrasive particles embedded in it.

17. Crystal Infusion

Crystal Infusion

This stage includes filling the liquid crystal into the prepared empty cell and sealing the filling hole so that the liquid crystal cell is basically made. Generally, a special liquid crystal filling machine is used to form a vacuum in the liquid crystal cell, and then the liquid crystal is filled into the LCD cell by the pressure difference and the capillary action of the liquid crystal cell.

18. Sealing

Sealing

Use a sealing material (such as a sealing resin) to block the opening of the filled liquid crystal box. Wipe clean the liquid crystal on the sealing surface of the filled LC liquid crystal box, apply a certain amount of UV sealing glue, and then irradiate it with ultraviolet rays to cause the glue itself to chemically react and interact. Coupling and polymerization to form a firm seal to prevent the liquid crystal in the screen from leaking to the outside, and also prevent external contaminants from intruding into the liquid crystal in the screen.

19. Post-cleaning

Post-cleaning

20. Visual Inspection

Visual Inspection

Detection Visually inspects the appearance and background color of semi-finished LCD products under the polarized light table and picks out the unqualified products. 

  • Rainbow Internal pollution 
  • Internal scraping 
  • Poor orientation 
  • and many more Electrical measurements

21. Electrical Inspection

Electrical Inspection

The electrical test is to check the display graphics, electrical and functional defects, and other appearance defects of the LCD screen under the power-on state.

  1. Short circuit 
  2. Missing plane 
  3. High current 
  4. Show bumps, draw more 
  5. Show white point (pinhole) and many more.

22. Silk Screen Printing

Silk Screen Printing

This is the most important point in LCD manufacturing, where silkscreens are used for printing. High-resolution silkscreens have more than 130 meshes per cm, enabling them to print lines with a width down to 50µm. The layer thickness is in the range of 10µm normally, which is regarded as a “thick” layer in LCD manufacturing.

For creating the image, the mesh is coated by a photosensitive emulsion, which is exposed using a mask, made from a glass or polymer film. The unexposed portion of the coating is washed off afterward. This portion lets the seal material go through and you have its pattern on the substrate.

23. Mounting

Mounting

Adhesive force enhancement. This surface improvement is used for various materials like plastics, metals, and inorganic compounds. It can be used for many applications e.g before like aquarium tank sealing, sticking or mounting plastic parts or rubbers, before attachment of aluminum foils in micro condensers, mounting of magnetic heads, semiconductor lead frames, etc.
Adhesive property enhancement. Optimum fixing of HD lubricants, lens protecting films, liquid crystal backlight diffusion panel, etc.
Coating property enhancement. This effect is used for mudguards, fender frames, and airbags in cars, propulsion coils for superconductive linear motor cars, outdoor structures for power facilities, etc.
Mounting means mounting polarizer

24. Spraying code

Spraying Codes

Some customers will need us to spray their codes on the LCD module.

25. Mounting pins

Mounting pins

Mounting a pin means assembling the pins to the glass.

26. Mounting pin inspection

Mounting pin inspection

27. Packaging

Packaging

28. Delivery

Delivery

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