Macular generation mechanism and improvement measures for TFT-LCD liquid crystal display module
In TFT-LCD LCD display modules, there are many disadvantages related to optical display effects and are relatively common, such as: white and light leakage on L0 full black picture, COG mura of L0 full black picture, greyscale picture frame Mura, white picture yellow, blue, black, etc.
There are many reasons for these adverse phenomena, and the degree of influence and acceptance of different causes on the LCD display module are also different. After all, some adverse phenomena exist stably and will not aggravate over time; while some adverse phenomena will aggravate over time.
Therefore, for adverse phenomena related to optical display effects, it is generally necessary to first find the real cause, and then risk assessment is carried out based on the cause to avoid inestimable losses.
01 LCD macular phenomenon and characteristics LCD macular: Usually refers to the fact that local positions appear yellow and yellow-brown in the display effect relative to other positions under a white picture. LCD macula usually has several key characteristics:
① The shape of the LCD macula is irregular and has poor regionality. The shape of LCD macula is usually a bit, strip, and frame-like; at the same time, the location where LCD macula occurs is regional, not yellowing on the entire surface.
It is divided according to the shape of the macula and the area where it occurs, usually: point-shaped yellowing, local yellowing in the corners, and yellowing in the surrounding edges. The causes of poor LCD macular deficiencies in different shapes and areas are also different.
②Irreversibility of the macular phenomenon of LCD. The LCD macular is usually caused by external force to squeeze the LCD display module, causing physical damage to the inside of the LCD display panel. Once this damage occurs, it is usually permanent and basically does not disappear on its own. Therefore, the LCD macular phenomenon has irreversible characteristics.
③The color of LCD macula is relatively yellow and obvious. The color of LCD macular spots usually shows more obvious yellow, light yellow and yellow-brown, which contrasts sharply with the white around the LCD display module.
02 The mechanism of LCD macular production
Regardless of the phenomenon, location and degree of the LCD macular phenomenon, the mechanism of its occurrence is basically the same. Before talking about the mechanism of LCD macular generation, it is necessary to explain the relationship between the transmittance of LCD display panel and its Cell Gap (box thickness).
The LCD transmittance and box thickness have a certain relationship. According to the relationship between LCD transmittance and box thickness, it can be seen that the LCD transmittance T is positively correlated with the optical path difference △nd; that is, when the effective birefringence △n of the liquid crystal is the same, the larger the Cell Gap d of the LCD, the higher the transmittance of the LCD.
When the LCD box thickness is in the best state, the transmittance of RGB red, green and blue light is also in the best state. At this time, the color of the three RGB lights after mixing light is white, and the LCD display module screen is white.
Due to the influence of LCD liquid crystal characteristics, different LCDs Cell Gap (box thickness) will have different transmittances to the three colors of RGB.
When the LCD Cell Gap (box thickness) increases, the transmittance of blue light B will be significantly reduced. At this time, red light R and green light G will still maintain a certain transmittance. After the red light R and green light B are mixed, it will be yellow, so the LCD will show yellowing.
When the LCD Cell Gap (box thickness) decreases, the transmittance of blue light B is the highest, and the LCD will appear blue.
Therefore, the mechanism of LCD macular production is: LCD Cell Gap (box thickness) increases. The reason for the increase in the thickness of the LCD box is basically that after the LCD display module is squeezed by external force, the liquid crystal in the cell box gathers to the squeezed peripheral position, and the liquid crystal gathering position is extended, which eventually causes the LCD display module to have poor yellowing.
03 Common causes and improvement measures for LCD macular production. The mechanism of LCD macular production has been explained earlier. Basically, after the LCD display module is squeezed by external force, the LCD Cell Gap becomes larger, causing the LCD display module to have poor yellowing.
During the full process of LCD display module and the whole machine testing process, the extrusion pressure of LCD display module comes from many aspects, but common extrusion pressures lead to the occurrence of macules in LCD mainly come from: extrusion during the mechanical testing of the whole machine, extrusion during the OCA bonding process, extrusion during the LCD cell formation process, etc.
① Extrusion of LCD by mechanical testing (yellowing around):
After the whole machine is installed, various mechanical tests are usually performed to evaluate the mechanical properties of the whole machine. The mechanical test of the whole machine generally includes: roller, slight drop, directional drop, squeezing, etc.
During the mechanical testing of the whole machine, due to the influence of the liquid crystal quantity, main PS height, PS density inside the LCD display panel, the flatness of the LCD display module and the entire machine case, the matching gap in the Z direction, the strength of the entire machine case, the LCD display module will cause LCD macular defects after being squeezed by the entire machine case.
a. Causes of yellowing around:
The specific process of causing macularity in the LCD display module after mechanical testing of the whole machine is as follows (the risk of macularity in the LCD caused by micro-falling test of the whole machine is the highest):
1) The overall thickness of the LCD display module is thicker and has poor flatness, which will cause the gap between it and the entire machine case in the Z direction. Repeated mechanical tests of the entire machine will cause the housing to squeeze the LCD display module.
2) When the LCD display module is squeezed, the Main PS in the middle area of the LCD display panel will usually be compressed, causing the Cell Gap in the middle area to become smaller, and the PS in the frame sealing glue around the LCD will be supported, resulting in the surrounding Main PS basically not compressed.
3) After the intermediate area of the LCD is squeezed, the Cell Gap will become smaller, and the LCD will flow and gather around the LCD, thereby enlarging the Cell Gap on the edge of the LCD, which will eventually lead to poor yellowing around the LCD.
b. Countermeasures to improve yellowing around:
The improvement measures for yellowing around LCDs are usually divided into two aspects: on the one hand, it is to improve the LCD display panel monomer, that is, to improve the stability of LCD Cell Gap (box thickness);
On the other hand, it is improved from the matching of the LCD display module and the case. Common improvement measures are as follows:
1) Reduce the amount of liquid crystal in the LCD cell box. When the amount of liquid crystal is reduced (the ability of the LCD to support the thickness of the Cell box will be weakened, and it mainly depends on PS to achieve support), the initial state of the Main PS will be compressed first, reducing the gap between the Sub PS.
When the LCD is squeezed by external forces, the Main PS can be quickly compressed and deformed. At this time, the Sub PS can quickly provide effective support to resist external forces.
2) The liquid crystal quantity remains unchanged, and the height of Main PS is increased. After the height of Main PS is increased, the compression amount equivalent to Main PS will increase when the LCD is squeezed by external force. At this time, the Cell Gap in the middle area of the LCD can be suppressed during the mechanical testing of the whole machine, and ultimately ensure the stability of the Cell Gap in each area of the LCD box.
3) Increase the density of PS per unit area. After the density of PS is increased, the LCD's compressive and deformation resistance can be improved, ensuring the stability of the Cell Gap in various areas of the LCD box.
4) Ensure the Z-direction clearance between the LCD display module and the entire machine case. The overall thickness of the LCD display module is in the middle and lower limit, the flatness of the LCD display module and the entire machine case, and the strength of the entire machine case are also common improvement measures to reduce the macular spot around the LCD.
② OCA full patch extrusion on LCD (local yellowing):
The local position is uneven during OCA bonding and the presence of stress are also a common cause of local yellowing of LCDs.
At the same time, due to interference inside the LCD display module (such as silicone glue exceeding the standard thickness, UV glue and conductive silver paste thickness exceeding the upper polarizer, etc.), interference between the LCD display module and the entire machine case is also a common reason for local yellowing of LCD. Due to the space limitations of the article, we will not go into details.
a. Causes of local yellowing:
Taking the local position uneven after OCA bonding as an example, when the force is uneven during the soft corresponding OCA bonding process, the local position of OCA is concave. The concave part will pull the LCD during the hard-to-hard bonding and vacuuming process, causing the Cell Gap (box thickness) to become larger (similar to the nozzle effect).
At this time, the transmittance of blue light B will be significantly reduced, but the red light R and green light G will still maintain a certain transmittance, which will eventually lead to local macules in the LCD.
b. Countermeasures to improve local yellowing:
DOE verification is carried out during the OCA full bonding process to avoid local yellowing of LCD due to uneven local positions of OCA and stress.
During the soft to hard bonding process, verify and formulate the best bonding pressure and bonding speed; at the same time ensure the flatness of the bonding platform;
During the hard to hard bonding process, verify and formulate the optimal bonding pressure, bonding time, and bonding vacuum; at the same time, confirm the parallelism of the bonding cavity to reduce the risk of local yellowing after OCA squeezes the LCD.
③Extrusion of LCD by the Cell boxing process (point yellowing):
The LCD cell is squeezed during the process of forming a box, causing the LCD to turn yellow. It is relatively rare. It is enough to control the cleanliness of the cell cell.
The generation mechanism of LCD dot-shaped yellowing is similar to the above, that is, when the CF and TFT substrates are combined, foreign objects will be generated after the equipment is worn, and foreign objects will cause the LCD cell to be lifted up, and the position where the foreign objects are lifted up will become larger, which will eventually lead to the generation of dot-shaped yellowing.
