In the last post, we’ve discussed what’s LCD and the manufacturing process of TFT LCD. And now, what if we encounter some defective LCD screens? What kind of defect may occur and can they be avoided during the process of manufacturing? Let’s learn about it together.
There’re more than 300 procedures to produce TFT LCD. The most advanced LCD, in which the array and cell process are highly automatic. Technically, every step in the process can lead to defects, and most of the defects have been eliminated through the development of TFT LCD technology.
In the following paragraphs, we’ll mention some common defects that related to manufacturing process.
1. Bubbles on screen or LC leakage
If you see bubbles or something liquid-like inside the screen, that’s a defect related to LC filling.
In the LC filling process, if the quantity of LC injected is not enough, the spare space will form bubbles. And loose LC containing sealant will result in LC leakage.
To solve this problem, we have to calibrate the sealant inject machine and then the LC inject machine.
2. Point defect
Point defect is a kind of defect that some point on your screen don’t display correctly. There are mainly three situations: the point keeps displaying black or white when the screen is working or the point can only display a single color.
For the first two situations, that’s because the circuit on the TFT and CF controlling that defective pixel point is shorted or broken. While the third situation is caused by damaged color pixel.
Currently, we can’t avoid point defect completely in the manufacturing process, and this defect can’t be repaired.
The production of the circuit and color pixel is under micro scale, and the technology is similar to semiconductor technology. Hundreds of thousands micro materials will be printed during the production process, you can understand some of the materials not being printed correctly, which result in the point defect.
Thus, we always hear a word ‘yield’ in semiconductor industry. To increase product yield, we have to wait for the technical innovation.
To conclude, if you find your screens have point defect, just hand back to manufacturer, and they will replace them according to standard.
3. Line or block defect
Line or block defect means a larger scale display abnormality like a dark line, strip or block appear on your screen.
Unlike point defect, this larger scale defect is caused by the failure of external FPC or PCBA, or a bad connection between FPC and cell. Therefore, a bunch of pixels connected to these IC are out of control, and we see those defects.
Usually, assembly of cell and IC is under heat and should be positioned accurately. Problems with IC connection will be checked out very soon, followed by the adjustment on machine parameter.
4. Newton’s rings
It’s a phenomenon caused by interference of light.
In LCD, newton’s rings may occur on screen when two glass substrate haven’t been sealed well, so that one of the glass may form a convex lens and lead to light interference.
To avoid this problem, we have to pay attention to the gap distance parameter in sealing process. By the way, newton’s rings has became a method to exam the glass sealing process in reverse .
5. Light leakage
Light leakage usually refers to the extra bright area on the edge of screen.
The black matrix on CF glass or a additional shield bar is used to avoid this problem. So if this problem occurs, we have to check the CF deposition process.
You may notice there are some screens have uneven display, which means some white area appears in dark picture or vice versa. We call this ‘mura’, a word originated from Japanese.
Mura is very common but it doesn’t affect the screen function severely, however it still bring bad look. Hence, many high end display manufacturers have their own standards of mura, and the displays without mura are of the best quality.
Causes of mura includes:
- light system doesn’t provide uniform light;
- uneven thickness of LC cell;
- mismatch sealing of TFT and CF glass;
- uneven thickness of the film on CF;
- IC issue on TFT;
- uneven thickness of the whole cell.
Among the causes listed, thickness of the whole cell is the most critical one, and there are many factors related to that. Researches provide a lot of advice to adjust the thickness of the whole cell:
- thicker glass;
- less LC;
- better material of spacer bar;
- better design of glass and sealant structure;
- better matching rate of glass sealing;
- better pressure control of glass sealing.
So much for this topic. If you find it useful or want to know more about TFT LCD, just leave your comment below or contact us. We’re hearing!
Note: We do not own the images used in this post. Feel free to contact us if they belong to you, and we’ll take them down as quickly as we possibly can.