LED (light emitting diode) is just as important to the display screen as the engine of a car and the compressor of an air conditioner. Choosing a high-performance LED is the basic condition to complete a high-performance LED display. Whether the LED can be used well is the touchstone to test the display manufacturer. It is generally believed that the display screen has the following five key performance indicators, which are closely related to the LED quality parameters: brightness and viewing angle, uniformity and clarity, pixel out of control rate, service life, energy consumption and energy efficiency.
1. Brightness and viewing angle
The brightness of the display screen mainly depends on the luminous intensity and led density of the LED. In recent years, new technologies of LED in substrate, epitaxy, chip and packaging have emerged one after another, especially the stability and maturity of indium tin oxide (ITO) current expansion layer technology and process have greatly improved the luminous intensity of LED. At present, when the horizontal viewing angle is 110 degrees and the vertical viewing angle is 50 degrees, the luminous intensity of the world-class brand low-power LED has reached 4000mcd for the green tube, 1500mcd for the red tube and 1000mcd for the blue tube. When the pixel spacing is 20mm, the brightness of the display screen can reach more than 10000nit. 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: the observation angle of LED display screen, especially outdoor display screen, is basically from bottom to top. According to 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 index to measure the quality of display. It is often said that led display is "little brilliant, piece brilliant", which is an image metaphor for the serious uneven between pixels and modules. Professional terms are "dust effect" and "mosaic phenomenon".
The main causes of non-uniformity are: the inconsistency of LED performance parameters; Insufficient assembly accuracy of display screen during production and installation; The consistency of electrical parameters of other electronic components is not enough; Nonstandard module and PCB design, etc.
Among them, "inconsistency of LED performance parameters" is the main reason. The inconsistency of these performance parameters mainly includes: inconsistent light intensity, inconsistent optical axis, inconsistent color coordinates, inconsistent light intensity distribution curves of each primary color, inconsistent attenuation characteristics, etc.
At present, there are two main technical ways to solve the inconsistency of LED performance parameters: one is to further subdivide the LED specification parameters to improve the consistency of LED performance; The second is to improve the uniformity of the display screen through subsequent correction. The subsequent correction has also developed from the early module correction and module correction to today's point by point correction. The correction technology has developed from simple light intensity correction to light intensity color coordinate correction.
However, we believe that subsequent correction is not omnipotent. Among them, the inconsistency of optical axis, inconsistent light intensity distribution curve, inconsistent attenuation characteristics, poor assembly accuracy and non-standard design cannot be eliminated through subsequent correction, and even this subsequent correction will worsen the inconsistency of optical axis, attenuation and assembly accuracy.
Therefore, through practice, our conclusion is that the follow-up correction is only the treatment of the table, and the subdivision of LED parameters is the root cause and the mainstream of LED display industry in the future.
When it comes to the relationship between display uniformity and definition, there is often 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. Simply reducing the physical pixel spacing to improve the resolution and ignoring the uniformity is undoubtedly to improve the definition. Imagine a display screen with serious "dust effect" and "mosaic phenomenon", even if its physical pixel spacing is small and its resolution is high, it is impossible to get a good image definition.
Therefore, in a sense, the main reason restricting the improvement of LED display clarity is "uniformity" rather than "physical pixel spacing".
3. Display pixel out of control
There are many reasons for display screen pixels out of control, among which the main reason is "led failure".
The main causes of LED failure can be divided into two aspects: one is the poor quality of LED itself; The second is improper use. Through analysis, we conclude the corresponding relationship between LED failure mode and the above two main causes.
As mentioned above, the failure of many LEDs can not be found in the routine inspection and test of LEDs. In addition to the improper use of electrostatic discharge, high current (resulting in excessive junction temperature), external strength and so on, many led failures are caused by the difference of internal stress caused by the difference of thermal expansion coefficient of LED chip, epoxy resin, support, inner lead, solid crystal adhesive, PPA cup and other materials under high temperature, low temperature, rapid temperature change or other adverse conditions. Therefore, The quality detection of LED is a very complex work.
4. Life span
The factors affecting the service life of LED display screen include internal and external factors, including the performance of peripheral components, the performance of LED light-emitting devices and the fatigue resistance of products; The internal reason is the working environment with LED display screen, etc.
In addition to LED light-emitting devices, LED display also uses many other peripheral components, including circuit board, plastic shell, switching power supply, connector, shell, etc. any problem with any component may reduce the service life of the display. Therefore, the longest service life of the display screen is determined by the service life of the key component with the shortest service life. For example, led, switching power supply and metal shell are selected according to the 8-year standard, and the protective process performance of the circuit board can only support its work for 3 years. After 3 years, it will be damaged due to corrosion, so we can only get a display screen with a service life of 3 years.
2 Effect of on the performance of LED light emitting devices
LED light-emitting device is the most critical and life-related component of display screen. For LED, it mainly includes the following indexes: attenuation characteristics, waterproof vapor permeability characteristics and UV resistance. If the manufacturer of LED display screen fails to evaluate the index performance of LED devices, it will be applied to the display screen, which will lead to a large number of quality accidents and seriously affect the service life of LED display screen.
The fatigue resistance of LED display products depends on the production process. The fatigue resistance of the module produced by the poor three prevention treatment process is difficult to ensure. When the temperature and humidity change, cracks will appear on the protective surface of the circuit board, resulting in the decline of the protective performance.
Therefore, the production technology of LED display screen is also the key factor to determine the service life of display screen. The production processes involved in the production of display screen include: component storage and pretreatment process, furnace welding process, three prevention treatment process, waterproof sealing process, etc. The effectiveness of the process is related to material selection and proportioning, parameter control and operator quality. For the vast majority of LED display manufacturers, the accumulation of experience is very important. A factory with many years of experience will control the production process more effectively.
Due to different purposes, the working conditions of the display vary greatly. In terms of environment, the indoor temperature difference is small, without the influence of rain, snow and ultraviolet rays; The maximum outdoor temperature difference can reach 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 service life of the display screen.
The service life of LED display screen is determined by many factors, but the end of service life caused by many factors can be continuously extended through the replacement of parts (such as switching power supply). The LED cannot be replaced in large quantities. Therefore, once the LED life ends, it means the end of the display life.
We say that the LED life determines the life of the display screen, but we do not mean that the LED life is equal to the life of the display screen. Because not every led works at full load all the time when the display screen is working, when the display screen plays video programs normally, the service life of the display screen should be 6 10 times that of the LED. When the LED works at low current, the service life can be longer. Therefore, the service life of the LED display screen of this brand can reach about 50000 hours.
How to make the LED life longer? In general, we can start from two aspects: device manufacturing and device application. In terms of device manufacturing: select high-quality epitaxial materials; Increase the chip area and reduce the current density; Equalizing current density; Reduce thermal resistance; Choosing packaging materials with excellent performance and strong UV resistance can prolong the service life of LED.
In terms of device application: take heat dissipation as a central work from module design to project implementation and even future system maintenance; Reduce the working current of LED; The service life of the LED can be prolonged by correctly configuring the LED and making the synchronous attenuation of each primary color LED.
Whether the LED lamp is stable, the quality is very important with the heat dissipation of the lamp body itself. At present, the heat dissipation of 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 service life of the lamp itself will also be affected.
5. Energy consumption and energy efficiency
Improving LED light efficiency and reducing display energy consumption is an important development direction of LED display technology. It has the following positive significance: first, energy saving, emission reduction and environmental protection; Second, reduce the investment in power capacity increase, power equipment and heat dissipation equipment; Third, save electricity charges and reduce operating costs; Fourth, reduce the temperature rise of the display screen; Fifth, delay the attenuation speed of LED; Sixth, improve system reliability; Seventh, extend the service life of the display screen; Eighth, reduce the temperature drift of photoelectric parameters of the display screen and stabilize the image effect.
The luminous efficiency of LED (i.e. external quantum efficiency) is determined by the internal quantum efficiency and escape rate of LED. Nowadays, the internal quantum efficiency of LED has reached more than 90%, but due to the low escape rate, the external quantum efficiency has become the bottleneck to improve the light efficiency of LED. In order to break through the bottleneck restricting the development of the industry, many novel solutions have been proposed and verified theoretically. Most of them have entered the experimental stage, some have been successful, and have laid a solid foundation for the final industrialization.
As a green and energy-saving light source, LED is favored by people. It will also lead the future of display technology as a mainstream media.
In short, device manufacturing and device application itself are a complementary unity. The progress of device technology brings prosperity to the application market, and the demand of the application market is the eternal driving force of device technology progress. Only the joint efforts of upstream and downstream enterprises can create a new future of LED display technology.