How to better select and use dedicated LEDs for full-color displays

Full-color LED display is a new type of electronic communication media. It has irreplaceable advantages in terms of large size, high brightness, environmental adaptability, etc. It can be used for graphic and video display of indoor large size and outdoor large size. It is mainly used in advertising media, stage scenery, building decoration, traffic information, sports venues and other fields. With the rapid improvement of LED chip technology, packaging technology and LED display manufacturing technology and the further decline in prices, full-color LED displays are crossing the application threshold of cost performance and gaining huge market applications and demand.

1. The choice of special LEDs for full-color display
The most critical component of full-color LED displays is LED devices. There are three main reasons: First, LEDs are the most used key components in the full-color screen machine, using thousands to tens of thousands of LEDs per square meter; Second, LEDs are the main body that determines the optical display performance of the entire screen, which directly affects the audience's evaluation of the display; third, LEDs account for the largest proportion of the overall cost of the display, ranging from 30% to 70%.
LEDs are the most critical device in full-color LED displays, equivalent to the CPU of a computer. The choice of LEDs directly determines the quality of the entire display screen by more than 50%. If the LED is not selected well, no matter how good the other parts of the display are, they cannot make up for the defects in the quality of the display.
The quality and parameters of LEDs dedicated to full-color LED displays can be attributed to the following five factors:

1. Failure rate: Since the full-color display consists of tens of thousands or even hundreds of thousands of groups of pixels composed of red, green and blue LEDs, the failure of any color LED will affect the overall visual effect of the display.Generally speaking, according to industry experience, the failure rate before the LED display starts to be assembled and shipped for 72 hours after aging should not be higher than 3/10,000 (referring to the failure caused by the LED device itself).
2. Anti-static ability: LEDs are semiconductor devices that are sensitive to static electricity and can easily cause static failure. Therefore, antistatic ability is crucial to the life of the display screen.In general, the human electrostatic mode test failure voltage of LEDs should not be less than 2000V.
3. Attenuation characteristics: Red, green and blue LEDs all have the characteristic of attenuating brightness with the increase of working time.The quality of LED chips, the quality of auxiliary materials and the level of packaging process determine the attenuation speed of LEDs. Generally speaking, after 1000 hours, 20 mA room temperature lighting test, the attenuation of red LEDs should be less than 10%, and the attenuation of blue and green LEDs should be less than 15%. The consistency of red, green and blue attenuation has a great impact on the future white balance of full-color LED displays, which in turn affects the display fidelity of the display.
4. Brightness: LED brightness is an important determinant of display brightness.The higher the brightness of the LED, the greater the margin of current used, which is beneficial to saving power consumption and keeping the LED stable. LEDs have different angle values. When the brightness of the chip is fixed, the smaller the angle, the brighter the LED will be, but the smaller the viewing angle of the display screen. Generally, a 100-degree LED should be selected to ensure sufficient viewing angle of the display. For displays with different dot spacing and different viewing distances, a balance point should be found in terms of brightness, angle and price.
5. Consistency: The full-color display is made up of countless pixels composed of red, green and blue LEDs. The consistency of the brightness and wavelength of each color LED determines the brightness consistency, white balance consistency, and chroma consistency of the entire display.

Generally speaking, display manufacturers require device suppliers to provide LEDs in a wavelength range of 5 nm and a brightness range of 1: 1.3. These indicators can be graded by the device supplier through a dichroic machine. Voltage consistency is generally not required.
Since LEDs are angled, full-color LED displays also have angular directionality, that is, when viewed at different angles, their brightness will increase or decrease. In this way, the angular consistency of the three color LEDs of red, green and blue will seriously affect the consistency of white balance at different angles and directly affect the fidelity of the video color of the display screen. In order to achieve consistency in matching brightness changes among the three LEDs at different angles, strict scientific design is needed in terms of packaging lens design and raw material selection, which depends on the technical level of the packaging supplier. No matter how good the white balance in the normal direction is, if the angle consistency of the LEDs is not good, the white balance effect at different angles of the entire screen will be poor. The angle consistency characteristics of LED devices can be measured by an LED angle comprehensive tester, which is particularly important for medium and high-end displays.
3. The use of special LEDs for full-color displays
High-quality LEDs also require a good use method and environment. In summary, there are eight key points for use:

1. Anti-static: Display assembly factories should have good anti-static measures.Special anti-static floor, anti-static floor, anti-static iron, anti-static table mat, anti-static ring, anti-static clothing, humidity control, equipment grounding (especially foot cutting machines), etc. are all basic requirements, and must be checked regularly with an electrostatic meter.
2. Wave soldering temperature and time: The temperature and passing time of wave front soldering must be strictly controlled. The recommendation is: the preheating temperature is 100 ℃ 5 ℃, the maximum shall not exceed 120 ℃, and the preheating temperature rise is required to be stable. The welding temperature is 245 ℃ 5 ℃, and the welding time is recommended not to exceed 3 seconds. After passing through the furnace, avoid vibrating or impacting the LED until it returns to normal temperature.±± The temperature parameters of the wave soldering machine must be tested regularly, which is determined by the characteristics of the LED. Overheating or fluctuating temperature will directly damage the LED or cause hidden dangers to the LED's quality, especially for round and oval LEDs with small size such as 3 mm.
3. Design current value: The nominal current of the LED is 20 mA. It is generally recommended that the maximum current should not exceed 80% of the nominal value. Especially for displays with small dot spacing, due to poor heat dissipation conditions, the current value should also be reduced.

Based on experience, due to the inconsistent attenuation speeds of red, green and blue LEDs, the current values of blue and green LEDs are reduced in a targeted manner to maintain the consistency of white balance of the display after long-term use.
4. Mixed lights: LEDs with different brightness levels of the same color need to be mixed, or the lights should be plugged in according to a light insertion diagram designed according to the discrete rule to ensure the consistency of the brightness of each color on the entire screen.If there is a problem in this process, local brightness of the display will be inconsistent, which directly affects the display effect of the LED display.
5. Control the verticality of the lamp: For direct-inserted LEDs, sufficient process technology must be used to ensure that the LEDs are perpendicular to the PCB board when passing through the furnace.Any deviation will affect the brightness consistency of the LED that has been set, and color blocks with inconsistent brightness will appear.
6. Heat dissipation design: LEDs will heat when working. Excessive temperatures will affect the attenuation speed and stability of the LEDs. Therefore, the heat dissipation design of the PCB board and the ventilation and heat dissipation design of the box will affect the performance of the LEDs.
7. Virtual welding control: When the LED does not light up on the LED display, there is often a probability of more than 50% caused by various types of virtual welding, such as virtual welding on LED pins, virtual welding on IC pins, and virtual welding on needle rows and mother rows.Improvement of these problems requires strict improvement of the process and strengthening of quality inspection to solve them. Vibration testing before leaving the factory is also a good inspection method.
8. Driver circuit design: The layout of the driver IC on the driver circuit board on the display module will also affect the brightness of the LED.Because the output current of the driver IC is transmitted too far on the PCB, the voltage drop in the transmission path will be too large, which will affect the normal working voltage of the LED and cause its brightness to be reduced. We often find that the brightness of the LEDs around the display module is lower than that in the middle. This is the reason. Therefore, to ensure the consistency of display brightness, it is necessary to design the driving circuit distribution diagram.
The above introduces the relevant knowledge of LEDs dedicated to full-color displays from the perspective of selection and use. Full-color LED displays are a system-integrated product that requires close cooperation between LED packaging manufacturers and display application manufacturers to strengthen technical exchanges. China has now become the world's LED display manufacturing center and is moving closer to a powerful LED display manufacturing country. So far, Chinese mainland full color LED packaging level has been greatly improved, many indicators have been close to the level of Japanese manufacturers, individual indicators have exceeded, which will greatly promote the development of China's LED display industry.