Introduction to LCD
The composition of the liquid crystal:
The liquid crystal used in LCD generally refers to mixed liquid crystal, which is composed of a variety of liquid crystal monomers and chiral agents.
Characteristics of LCD:
TN liquid crystals generally have short molecular chains and difficult to adjust characteristic parameters, so the difference in characteristics is obvious. STN liquid crystal is to display the data model of STN, calculate the required length of liquid crystal molecules, and its optical and electrical performance parameters, and then chemically synthesize a variety of monomers with similar molecular linking structures and different polar molecular groups, and mix each other into one Series of LCDs with similar characteristics. Different series of STN liquid crystals often have completely different molecular chains, therefore, different series of STN liquid crystals cannot be adjusted with each other unless the manufacturer states that they can be adjusted with each other.
There are polar groups and non-polar groups in the liquid crystal molecules. The liquid crystal monomers with polar group molecules mainly determine the threshold voltage parameters of the mixed liquid crystal. The liquid crystal monomers without polar group molecules It mainly determines the refractive index and clear point of the mixed liquid crystal. In the liquid crystal, monomers with polar groups and monomers without polar groups will exhibit isomeric chromatography under static conditions.
In order to increase the standby time of the machine itself and enhance the driving capability of the liquid crystal display, liquid crystal manufacturers have developed low-threshold voltage liquid crystals that can be used under low-voltage and low-frequency conditions. It has the following characteristics:
In the low threshold voltage liquid crystal, the monomer with polar group and the monomer without polar group have a shorter time for isomeric chromatography under static conditions.
More monomer components with polar groups also mean that the liquid crystal is more likely to bind water molecules and other polar free ions, thereby reducing the capacitive resistance of the liquid crystal, resulting in increased leakage current and power consumption Big.
When the molecular chain of the polar liquid crystal monomer is excited by ultraviolet light, the polar molecular groups are easily entangled with each other to form a neutral molecular group, which becomes a non-stratified misalignment state, thus causing the threshold voltage to increase and anchor the guide layer It is insensitive to fixed action and loses low-voltage drive capability.
1. Classification of LCD:
According to the display type: TN type liquid crystal, STN type liquid crystal, HTN type liquid crystal;
According to the clear points: ordinary LCD, wide temperature LCD;
According to the threshold voltage: low threshold voltage liquid crystal, ordinary liquid crystal, high threshold voltage liquid crystal.
2. The main parameters that affect the performance of the LCD:
Clearing point; refractive index Δn; threshold voltage; purity; viscosity constant K; dielectric constant ε; pitch Ï
3. Factory adaptive test method and judgment standard of LCD:
Resistivity: A. Test method: Use a high resistance meter to test the resistance value of the liquid crystal to be tested.
B. Judgment standard: The test result is within the scope of product requirements (factory standard ≥ 8X107).
Photoelectric performance: A. Test method: test the product and test its photoelectric performance.
B. Judgment standard: The Von and Voff values ​​of the test samples are consistent with the supplier ’s parameters, and the viewing angle, contrast, and background color meet the requirements of the production product.
Clearing point: A. Test method: Heat the liquid crystal to be measured and measure the temperature when it reaches the clearing point.
B. Judgment standard: The temperature of the measurement result is consistent with the clearing point temperature provided by the supplier.
Ultraviolet resistance performance: A. Test method: put the liquid crystal to be tested as a trial product, put it under the sealing UV machine, irradiate it twice according to the UV intensity and time specified by the sealing process, and test the photoelectric performance change before and after irradiation.
B. Judgment standard: After UV irradiation, the Voff value rises within 0.1V (low voltage liquid crystal within 0.15V), the current value changes within 2 times, and the contrast decline is not obvious.
Reliability: A. Test method: test the liquid crystal to be tested as a product and test its reliability performance.
B. Judgment standard: The photoelectric performance changes after the reliability test are within the product requirements.
4. Selection rules of LCD:
According to the background color required by the customer, select the appropriate liquid crystal type in the range of Δn, and then select the appropriate liquid crystal type in the voltage range according to the data of the customer's IC circuit. The liquid crystal that meets the above conditions can be used after being adjusted at a reasonable ratio and used. Meet customer requirements.
5. How to use LCD:
The liquid crystal must be fully stirred before use before pouring. The liquid crystal added with a solid chiral agent should be heated to 60 degrees Celsius, and then quickly cooled to room temperature and fully stirred. Moreover, it cannot be left for too long during use. Especially for low threshold voltage liquid crystals, since low threshold voltage liquid crystals have these different characteristics, you should pay attention to the following aspects when using these liquid crystals:
The liquid crystal should be fully stirred before use, and the prepared liquid crystal should be immediately put into production to minimize the storage time and avoid the phenomenon of chromatography.
The prepared liquid crystal should be covered with shading and stored, and used as much as possible within one shift (eight hours). The unused liquid crystal needs to be recycled and stirred before re-testing the voltage before use. Generally, as time goes on, the driving voltage will increase.
After the liquid crystal is taken from the original bottle, the original bottle should be sealed and shaded in time to reduce the exposure time in the air. Generally, the exposure time in the air is too long, which will increase the leakage current of the liquid crystal.
It is best to fill the empty cell of the low threshold voltage liquid crystal display chip from the PI curing to the liquid crystal filling process, and store the empty cell within 24 hours of production time. Generally, a relatively low filling rate is used during the filling operation.
Low-threshold voltage liquid crystals must be covered with a suitable hood during sealing, and should be kept as far away from the UV source as possible during the entire liquid crystal filling period except during the curing of the sealing glue. Otherwise, the phenomenon of misdirection and increase of the threshold voltage will occur near the ultraviolet.
Liquid crystal is an organic polymer material, which is easy to dissolve in various solvents or react with other chemicals. Liquid crystal itself is also a good solvent, so it should be kept away from other chemicals as much as possible during use and storage.
6. Liquid crystal storage and handling methods:
The liquid crystal should be sealed, moisture-proof, and shading when stored. It should be stored at room temperature, and cannot be stored and used in a low-temperature environment to avoid irreversible crystallization. Do not mix with other chemicals.
Control according to chemical regulations when handling.
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