Five key technologies of LED display quality 2019-10-24

LED display is composed of a row of light-emitting diodes, so the quality of led directly affects the overall quality of the display.

1. Brightness and Perspective

The brightness of the display mainly depends on the luminous intensity and led density. In recent years, the new technologies of LED in substrate, epitaxy, chip and package are emerging in endlessly, especially the stability and maturity of ITO current expansion layer technology and technology, which greatly improves the luminous intensity of LED. At present, under the condition of 110 degree horizontal view angle and 50 degree vertical view angle, the luminous intensity of green LED is as high as 4000 MCD, that of red LED is 1500 MCD, and that of blue LED is 1000 MCD. When the pixel spacing is 20 mm, the brightness of the display can reach more than 10000 nit. The display can work 24 hours a day in any environment.

When it comes to the perspective of display screen, there is a phenomenon worthy of our consideration: LED display screen, especially outdoor display screen, people's observation angle is basically from the bottom to the top, and in terms of the product form of existing LED display screen, half of the luminous flux disappears in the vast sky.

2. Uniformity and clarity

With the development of LED display technology, uniformity has become the most important indicator to measure the quality of display. It is often said that led display "a little bit brilliant, brilliant pieces", is a kind of image metaphor of serious uneven between pixels and between modules. The professional ones are "dust effect" and "mosaic phenomenon".

The main causes of the uneven phenomenon are: the inconsistency of various performance parameters of LED; the insufficient assembly accuracy of display screen in the process of production and installation; the insufficient consistency of electrical parameters of other electronic components; the nonstandard design of modules and PCB, etc.

Among them, "led performance parameters are inconsistent" is the main reason. The main differences of these performance parameters include: the light intensity is not consistent, the optical axis is not consistent, the color coordinates are not consistent, the intensity distribution curve of each primary color is not consistent, and the attenuation characteristics are not consistent.

At present, there are two main technical ways to solve the inconsistency of LED performance parameters in the industry: one is to further subdivide the LED specification parameters to improve the consistency of LED performance; the other is to improve the uniformity of display screen through subsequent correction. The follow-up correction also develops from the early module correction and module correction to the point by point correction today. The correction technology has developed from light intensity correction to light intensity color coordinate correction.

However, we don't think the follow-up correction is omnipotent. Among them, the inconsistent optical axis, the inconsistent light intensity distribution curve, the inconsistent attenuation characteristics, the poor assembly accuracy and the nonstandard design can not be eliminated by subsequent correction, or even this subsequent correction will make the inconsistency in optical axis, attenuation and assembly accuracy worse.

Therefore, through practice, our conclusion is: the follow-up correction is only to cure the table, and led parameter subdivision is the cure, which is the mainstream of LED display industry in the future.

As for the relationship between display uniformity and definition, there is a misunderstanding in the industry, that is, to replace definition with resolution. In fact, the definition of the display screen is the subjective feeling of the human eye on the resolution, uniformity (signal-to-noise ratio), brightness, contrast and other factors of the display screen. There is no doubt that simply reducing the physical pixel spacing to improve the resolution and ignoring the uniformity to improve the resolution. Imagine a display screen with serious "dust effect" and "mosaic phenomenon". Even if its physical pixel spacing is no longer small and its resolution is no longer high, it is impossible to get a good image clarity.

Therefore, in a sense, "uniformity" rather than "physical pixel spacing" is the main reason restricting the improvement of LED display clarity.

3. Display pixel out of control

There are many reasons for the display pixels out of control, the most important one is "led failure".

The main causes of LED failure can be divided into two aspects: one is the poor quality of LED itself; the other is the improper use. Through analysis, we conclude the corresponding relationship between LED failure mode and the above two main factors.

We mentioned above that many led failures are usually not found in the routine inspection and test of LEDs. In addition to the improper use of electrostatic discharge, high current (resulting in high junction temperature), external strength and so on, many led failures are caused by the difference of thermal expansion coefficient of LED chip, epoxy resin, bracket, internal lead, solid crystal glue, PPA cup and other materials under high temperature, low temperature, rapid temperature change or other adverse conditions, resulting in different internal stress. The quality inspection of ED is a very complex work.

4, life expectancy

The factors that affect the life of LED display include internal and external factors, internal factors include the performance of peripheral components, the performance of LED light-emitting devices, product fatigue resistance, internal factors include the working environment of LED display, etc.

(1) influence of peripheral components

In addition to LED light-emitting devices, LED display screen also uses many other peripheral components, including circuit board, plastic shell, switching power supply, connector, shell, etc. any problem of any component may cause the life of the display screen to be reduced. Therefore, the longest life of the display screen is determined by the life of the key component with the shortest life. For example, led, switching power supply and metal shell are all selected according to the 8-year standard, while the protection process performance of circuit board can only support its work for 3 years. After 3 years, due to corrosion, it will be damaged, so we can only get a 3-year-old display screen.

(2) the influence of LED performance

LED light-emitting device is the most critical and life-related component of display screen. For LED, it mainly includes the following indicators: attenuation characteristics, water-proof vapor permeability characteristics, UV resistance. If the LED display manufacturers fail to pass the performance evaluation of the indicators of LED devices, it will be applied to the display, which will lead to a large number of quality accidents, seriously affecting the life of the LED display.

(3) influence of fatigue resistance of products

The anti fatigue performance of LED display depends on the production process. It is difficult to guarantee the anti fatigue performance of the module made by the poor three proofing process. When the temperature and humidity change, cracks will appear on the protection surface of the circuit board, which will lead to the degradation of the protection performance.

Therefore, the production technology of LED display screen is also the key factor to determine the life of the display screen. The production technology involved in the production of display screen includes: component storage and pretreatment technology, furnace pass welding technology, three prevention treatment technology, waterproof sealing technology, etc. The effectiveness of the process is related to the material selection and ratio, parameter control and operator quality. For the most large LED display manufacturers, experience accumulation is very important. A factory with many years of experience will be more effective in controlling the production process.
4) impact of working environment

Due to different purposes, the working conditions of the display screen vary greatly. In terms of environment, the indoor temperature difference is small, without the influence of rain, snow and ultraviolet rays; the outdoor temperature difference can reach up to 70 degrees, plus wind, sun and rain. The bad environment will aggravate the aging of the display screen, and the working environment is an important factor affecting the life of the display screen.

The life of LED display is determined by many factors, but the end of life caused by many factors can be continuously extended through the replacement of parts (such as switching power supply). The LED can not be replaced in large quantities, so once the LED life is over, it means the end of the display life.

We say that led life determines the life of the display, but not that led life is equal to the life of the display. Because the display screen does not work at full load all the time when it is working, the life span of the display screen should be 6-10 times of the life span of the LED when it is playing video programs normally, and the life span can be longer when the LED is working at low current. Therefore, the life span of the LED screen of this brand can reach about 50000 hours.

How to make LED life longer? Generally, we can start from two aspects: device manufacturing and device application. From the aspect of device manufacturing: choosing high-quality epitaxial materials; increasing chip area, reducing current density; balancing current density; reducing thermal resistance; choosing packaging materials with excellent performance and strong UV resistance can make LED life longer.

From the aspect of device application, it can extend the service life of led to take heat dissipation as a central work from module design to project implementation and even system maintenance in the future, reduce the working current of LED, correctly configure led and make each primary color LED synchronously attenuate.

Whether the LED lamp is stable, whether the quality is good or not and whether the heat dissipation of the lamp body itself is very important. At present, the heat dissipation of the high brightness LED full color screen in the market usually adopts natural heat dissipation, and the effect is not ideal. If the heat dissipation is not ideal, the life of the lamp itself will be affected.

5. Energy consumption and energy efficiency

Improving LED light efficiency and reducing energy consumption of display screen is an important development direction of LED display screen technology, which has the following positive significance: first, energy conservation, emission reduction and environmental protection; second, reducing the input of power capacity increase, power equipment and heat dissipation equipment; third, saving electricity and reducing operating costs; fourth, reducing the temperature rise of display screen; fifth, delaying the LED attenuation speed; sixth, improving the system reliability The seventh is to prolong the life of the display screen; the eighth is to reduce the temperature drift of the photoelectric parameters of the display screen and stabilize the image effect.

The luminous efficiency (i.e. external quantum efficiency) of LED is determined by the internal quantum efficiency and escape rate of LED. Nowadays, the internal quantum efficiency of LED is more than 90%, but due to the low escape rate, the external quantum efficiency becomes the bottleneck to improve the light efficiency of LED. In order to break through the bottleneck that restricts the development of the industry, many novel solutions have been proposed, and have been verified by theory. Most of them have entered the experimental stage, some of them have been successful, and laid a solid foundation for the final industrialization.

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