Five key elements that determine the quality of LED display

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

LED (Light Emitting Diode) is as important to the display screen as the engine of the car and the compressor of the air conditioner. Selecting an LED with excellent performance is the basic condition for completing a high-performance LED display. Whether the LED can be used well is a touchstone for testing display manufacturers. It is generally believed that the following five key performance indicators of the display are closely related to the LED quality parameters: brightness and viewing angle, uniformity and clarity, pixel runaway rate, life, energy consumption and energy efficiency.

1. Brightness and viewing angle

The brightness of the display mainly depends on the luminous intensity and LED density of the LED. In recent years, LEDs have emerged with new technologies in substrates, epitaxy, chips, and packaging. In particular, the stability and maturity of indium tin oxide (ITO) current spreading layer technology and technology have greatly improved the luminous intensity of LEDs. At present, when the horizontal viewing angle is 110 degrees and the vertical viewing angle is 50 degrees, the luminous intensity of the green tube has reached 4000mcd, the red tube is 1500mcd, and the blue tube is 1000mcd. When the pixel pitch is 20mm, The brightness of the display can reach more than 10000nit. The display can work around the clock in any environment.

When it comes to the viewing angle of the display screen, there is a phenomenon worthy of our consideration: LED display screens, especially outdoor display screens, people's viewing angles are basically from the bottom up, and from the existing LED display product form, there are Half of the luminous flux disappears into the vast sky.

2. Uniformity and clarity

With the development of LED display technology to this day, uniformity has become the most important indicator for measuring the quality of display screens. It is often said that the LED display is "a little bit splendid and brilliant", which is an image metaphor for the serious unevenness between pixels and modules. A professional statement is "dust effect" and "mosaic phenomenon".

The main causes of unevenness are: the inconsistency of LED performance parameters; the lack of assembly accuracy of the display during production and installation; the consistency of the electrical parameters of other electronic components; the irregular design of modules and PCBs.

Among them, "the inconsistency of LED performance parameters" is the main reason. The inconsistency of these performance parameters mainly includes: inconsistency of light intensity, inconsistency of optical axis, inconsistency of color coordinates, inconsistency of distribution curve of light intensity of each primary color, and inconsistency of attenuation characteristics.

How to solve the inconsistency of LED performance parameters, there are currently two main technical approaches in the industry: one is to further subdivide the LED specification parameters to improve the consistency of LED performance; the second is to improve the display through subsequent correction Uniformity. Subsequent corrections have also evolved from early module corrections and module corrections to point-by-point corrections today. The correction technology is developed from simple light intensity correction to light color coordinate correction.

However, we believe that subsequent corrections are not omnipotent. Among them, inconsistent optical axes, inconsistent light intensity distribution curves, inconsistent attenuation characteristics, poor assembly accuracy, and design irregularities cannot be eliminated by subsequent corrections, and even such subsequent corrections will cause optical axis, attenuation, and assembly accuracy. The inconsistency worsens.

Therefore, through practice, our conclusion is that the subsequent correction is only a cure, and the LED parameter subdivision is the root cause, and it is the future mainstream of the LED display industry.

When it comes to the relationship between display uniformity and resolution, there is often a misunderstanding in the industry that resolution is used instead of resolution. In fact, the definition of the display screen is the subjective experience of the human eye on the display screen resolution, uniformity (signal-to-noise ratio), brightness, contrast and other factors. Simply reducing the physical pixel pitch to increase the resolution, while ignoring the uniformity, has no doubt about improving the clarity. Imagine a display screen with serious "dust effect" and "mosaic phenomenon". Even if the physical pixel pitch is small and the resolution is high, it is impossible to get a good image clarity.

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

3. The display pixels are out of control

There are many reasons for the pixel out of control of the display screen, and 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 other is the improper use. Through analysis, we summarized the correspondence between the LED failure mode and the above two main causes.

As we mentioned above, the failure of many LEDs is usually not found in the routine inspection and testing of LEDs. In addition to improper use due to electrostatic discharge, high current (causing excessive junction temperature), external strength, etc., many LED failures are due to high temperature, low temperature, rapid temperature change or other harsh conditions, due to LED chips, epoxy resin, brackets , Inner lead, die bonding glue, PPA cup body and other materials thermal expansion coefficient difference, caused by the difference in its internal stress and generated, therefore, LED quality inspection is a very complicated task.

4. Life

The factors that affect the life of LED display screen are internal and external factors. The internal factors include the performance of peripheral components, the performance of LED light-emitting devices, and the product's anti-fatigue performance; the internal factors include the working environment of the LED display screen, and so on.

1). Peripheral components

In addition to LED light-emitting devices, LED display screens also use many other peripheral components, including circuit boards, plastic housings, switching power supplies, connectors, chassis, etc. Any problem with any component may lead to the life of the display screen reduce. Therefore, the longest life of the display screen is determined by the life of the key component with the shortest life. For example, the LED, switching power supply, and metal casing are all selected according to the 8-year standard, and the protection process performance of the circuit board can only support its work for 3 years. After 3 years, it will be damaged due to corrosion, then we can only get a piece of 3 years Lifetime display.

2). Influence of LED light emitting device performance

LED light-emitting devices are the most critical and most relevant components of the display screen. For LEDs, the main indicators are: attenuation characteristics, waterproof vapor penetration characteristics, and ultraviolet resistance. If LED display manufacturers fail to evaluate the performance of the LED devices, they will be applied to the display, which will cause a large number of quality accidents and seriously affect the life of the LED display.

3). Impact of product's anti-fatigue performance

The fatigue resistance of LED display products depends on the production process. The anti-fatigue performance of the module produced by the poor three-prevention treatment process is difficult to guarantee. When the temperature and humidity change, cracks will appear on the protective surface of the circuit board, resulting in a decline in the protective performance.

Therefore, the production process of the LED display is also a key factor in determining the life of the display. The production processes involved in the production of display screens are: component storage and pretreatment process, furnace welding process, three-proof treatment process, waterproof sealing process, etc. The effectiveness of the process is related to material selection and ratio, 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 be more effective in controlling the production process. .

4). Influence of working environment

The working conditions of the display screen vary greatly due to different uses. From an environmental point of view, the indoor temperature difference is small, without the influence of rain, snow and ultraviolet rays; the outdoor temperature difference can be up to 70 degrees, plus the sun and rain. The harsh environment will exacerbate the aging of the display, and the working environment is an important factor affecting the life of the display.

The life of the 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 is unlikely to be replaced in large quantities, so once the LED's end of life, it means the end of the display's life.

We say that the LED life determines the life of the display, but it does not mean that the LED life is equal to the life of the display. Because the display screen is not working, every LED is working at full load at all times. When the display screen is playing a video program normally, the life span of the display screen should be 6 to 10 times that of the LED. When the LED The service life can be longer under the condition of small current. Therefore, the life span of the display screen using the brand LED can reach about 50,000 hours.

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

From the aspect of device application: heat dissipation is a central task from module design to project implementation and even future system maintenance; reducing LED operating current; properly configuring LEDs, so that the primary color LEDs are simultaneously decayed, etc. can extend the life of LEDs.

Whether the LED lamp is stable, the quality of the lamp and the heat dissipation of the lamp body are very important. At present, the heat dissipation of high-brightness LED full-color screens on the market usually uses natural heat dissipation, and the effect is not ideal. If the heat dissipation is not ideal, the 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, reducing power expansion, power equipment, and heat dissipation equipment. Investment; three is to save electricity costs and reduce operating costs; four is to reduce the temperature rise of the display; five is to delay the LED attenuation rate; six is ​​to improve system reliability; seven is to extend the life of the display; eight is to reduce the temperature drift of the photoelectric parameters of the display , To stabilize the image effect.

The luminous efficiency (ie external quantum efficiency) of an LED is determined by the internal quantum efficiency and the escape rate of the LED. Nowadays, the internal quantum efficiency of LEDs has reached more than 90%, but due to the low escape rate, the external quantum efficiency has become the bottleneck for improving LED light efficiency. In order to break through this bottleneck restricting the development of the industry, many novel solutions have been proposed and theoretically verified. Most of them have entered the experimental stage, and some have been successful, and have laid a solid foundation for the final industrialization.

LED is favored by people as a green and energy-saving light source, and will surely lead the future of display technology as a mainstream media.

In short, device manufacturing and device application are a complementary unity. The advancement of device technology brings prosperity to the application market, and the demand of the application market is the eternal power of device technology progress. Only the joint efforts of upstream and downstream companies can create a new future for LED display technology.