Causes of light leakage in LCD module

Posted by sere on March 13th, 2020

LCD is a plane display device widely used in the information age. It has the characteristics of low energy consumption, thin and light appearance and light weight. TV and liquid crystal display often can appear the phenomenon of panel leakage, what reason causes this after all, normal? Light-leaking what is, in fact light-leaking with panel structure has very big relations, first is mainly composed of liquid crystal display (TFT LCD) and backlight (BLU) and flexible circuit board (FPC) of the three most liquid crystal display module (TFT LCM) in the general case light-leaking is a very normal phenomenon, because light-leaking will not affect the brightness of the LCD TV and LCD itself, more will not affect the response speed of LCD TV and LCD monitor and service life, the only affect is the whole panel display effect. When the brightness of the backlit LED lamp port is significantly higher than that of other areas, the bright spot or column in the lamp port position is called: light leakage.

Most LCDS have a bright spot at the border seam, but that's not a leak! LCD leakage and screen frame "leakage" are two different things!

Display leakage mainly appears on the panel, some panel quality is not good or transport problems, will produce more serious leakage. Such panels are checked out during production and are not allowed to leave the factory. The performance of the panel that leaks light is more apparent commonly, normal use can see. And our common leakage of light, mainly when the assembling of the border pressure on the display caused by the performance of the halo around or a corner, but this leakage of light in the general use of basic can not be found, only in the full black environment display detection picture can be seen.

Reasons for light leakage of LCD module:

1. Causes of light leakage in LCD module

According to the statistical analysis of a few manufacturer, the reason that liquid crystal TV and liquid crystal display produce leak light is bad, among them 93% is because glare is bad and cause, accordingly, can conclude the main key problem that leak light is bad is to want to solve glare problem.

2. Reasons for glare

One of the reasons for glare is that the light emitted by the LED lamp in the backlight of the LCD module is emitted directly, rather than diffusing evenly through the light guide plate as normal. At the same time, there is another case that the poor production of the light guide plate will produce a similar situation.

From the above two cases further subdivision, can lead to six aspects of the reasons.

(1)LED light is thicker than light guide plate

When the thickness of LED lamp exceeds the light guide plate, the light in the excess part will radiate directly from the top of the light guide plate. It is the light that does not diverge evenly through the light guide plate that forms the glare. Through the use of screw micrometer to conduct measurement and statistics on the light guide plate and LED lamp respectively, the statistical data obtained from the comparison of the mean value of the statistical measurement shows that there is a significant difference in the mean value of the measured data: LED lamp is thicker than the light guide plate.

(2) the reflexive force of FPC is greater than the adhesive force of FPC

When the reflexes of the FPC are too large, the LCD will be pulled up, resulting in the misalignment between the light guide plate and the LED lamp, resulting in the light cannot normally diverge evenly through the light guide plate and generate glare. However, due to the large reflexes of FPC, there is no quantitative method to test the reflexes. Therefore, the viscosity coefficient and adhesive area of the double-sided adhesive are both reduced to test whether it will cause glare. By comparing the rate of glare generation under different adhesive areas, it is confirmed that the hypothesis that the reflexing force on FPC is greater than the adhesive force on FPC is not valid.

(3) staff extrusion module

By observing the operation gimmick than production line staff found that there is a staff in the machining process of extrusion die set happens by accident, which, through carries on the squeezing pressure on the module test, found that pinch pressure will make the LED and LCD contact, when come into contact with the LED and LCD are squeezed, LED at the top of the double-sided adhesive will be LED stick and then move. Therefore, the hypothesis of glare caused by the extrusion of the module by employees is valid.

(4) there is gap between LCD and LED

When the LCD module is pressed to make the gap between the LCD and the LED disappear, the double-sided adhesive will stick to the LED and drive it to offset, resulting in light leakage. In order to confirm the size of the clearance fit, we use the 0.1 mm plug gauge clearance for the different thickness of the test, the final conclusion is: 0.1 + 0.06 + 0.02 vinyl combination can into space, and use the vinyl combination of 0.15 + 0.06 can't into space, because black rubber itself is the biggest thickness: 0.06 mm, so the size of the gap is: 0.12 mm, note: LCD and LED gap exists between the glare of hypothesis.

(5) the distance between the window and the luminous surface of LED lights is small, resulting in glare

With backlit D2 design length of 2.9 mm model, for example, due to the operation staff can lead to effective D2 deviation, we of D2 length is 3.1 mm and 2.9 mm two different conditions of glare rate compares the detection, check its differences in different circumstances, the test results: under the condition of the two, with the increase of D2 distance, the glare of the defective rate is on the decline. Therefore, the small distance between Windows and LED lights is one of the factors of glare generation.

(6) possibility of glare caused by bad mesh point of light guide plate and bad sawtooth of light guide plate

IQC is required to detect the bad mesh point and bad sawtooth of the light guide plate. The two lengths of D2 are 3.1mm and 2.9mm, respectively, and the defect rate of 20 light guide plates is detected. The results are as follows: 10 are acceptable for the light guide plate effect, and 10 are unacceptable for the light guide plate effect. Two inspectors conducted extraction test on 500 light guide plates respectively, and no bad light guide plate was found. Therefore, bad light guide plate mesh point and bad light guide plate sawtooth were not the main factors causing glare.