Selection and application of special LED Lamp for Full Color 4K LED Video Wall

1、 Overview

Full color 4K LED Video Wall is a new type of electronic communication media. It has irreplaceable advantages in large size, high brightness and environmental adaptability. It can be used for indoor and outdoor large-size graphic and video display. It is mainly used in advertising media, stage setting, building decoration, traffic information, sports venues and so on. With the rapid improvement of LED chip technology, packaging technology and 4K LED Video Wall manufacturing technology and the further decline of price, full-color 4K LED Video Wall is crossing the application threshold of cost performance and obtaining huge market application and demand.

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2、 Selection of special LED for full color display

The key component of full-color 4K LED Video Wall is led device. There are three reasons: first, LED is the most used key device in the full-color screen, and thousands to tens of thousands of LEDs will be used per square meter; Second, LED is the main body that determines the performance of the whole screen optical display, which directly affects the audience’s evaluation of the display screen; Third, LEDs account for the largest proportion of the overall cost of the display screen, ranging from 30% to 70%.

LED is the key device of full-color 4K LED Video Wall screen, which is equivalent to the CPU of computer. The selection of LEDs has determined the quality of more than 50% of the whole display screen. If the LED is not selected well, no matter how good other parts of the display can make up for the defect of display quality.

The quality and parameters of special LED for full-color 4K LED Video Wall can be summarized into the following five elements:

1. Inefficiency

Because the full-color display screen is composed of tens of thousands or even hundreds of thousands of pixels composed of red, green and blue LEDs, the failure of any color LED will affect the overall visual effect of the display screen. Generally speaking, according to industry experience, the failure rate of 4K LED Video Wall screen 72 hours before shipment shall not be higher than 3 / 10000 (referring to the failure caused by the LED device itself).

2. Antistatic ability

LED is a semiconductor device, which is sensitive to static electricity and is easy to cause static failure. Therefore, the anti-static ability is very important to the service life of the display screen. Generally speaking, the human body electrostatic mode test failure voltage of LED shall not be lower than 2000V.

3. Attenuation characteristics

Red, green and blue LEDs have the characteristics of brightness attenuation with the increase of working time. The attenuation speed of LED is determined by the quality of LED chip, auxiliary materials and packaging technology. Generally speaking, after 1000 hours and 20 mA normal temperature lighting test, the attenuation of red LED shall be less than 10%, and that of blue and green LED shall be less than 15%. The consistency of red, green and blue attenuation has a great impact on the white balance of full-color 4K LED Video Wall in the future, and then 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 the current used, which is good for saving power consumption and maintaining the stability of the LED. The LED has different angle values. When the chip brightness is fixed, the smaller the angle, the brighter the LED, but the smaller the viewing angle of the display screen. Generally, 100 degree led shall be selected to ensure sufficient viewing angle of the display screen. For displays with different point spacing and different viewing distance, a balance should be found in brightness, angle and price.

5. Consistency

The full-color display screen is composed of numerous pixels composed of red, green and blue LEDs. The consistency of brightness and wavelength of each color LED determines the brightness consistency, white balance consistency and chromaticity consistency of the whole display screen. Generally speaking, the display manufacturer requires the device supplier to provide LEDs with a wavelength range of 5nm and a brightness range of 1:1.3. These indicators can be graded by the device supplier through the light and color splitter. Voltage consistency is generally not required.

Because the LED is angular, the full-color 4K LED Video Wall also has angular directionality, that is, when viewed from different angles, its brightness will increase or decrease. In this way, the angle consistency of red, green and blue LEDs will seriously affect the consistency of white balance at different angles and directly affect the fidelity of video color on the display screen. In order to achieve the matching consistency of brightness changes of red, green and blue LEDs at different angles, it is necessary to strictly carry out scientific design in packaging lens design and raw material selection, which depends on the technical level of packaging suppliers. For the display screen with good white balance in the normal direction, if the angle consistency of the LED is not good, the white balance effect of the whole screen at different angles will be bad. The angle consistency characteristics of LED devices can be measured by LED angle comprehensive tester, which is particularly important for medium and high-grade display screens.

3、 Use of special LED for full-color display:

High quality led also needs a good use method and environment, which can be summarized into the following eight key points:

1. Antistatic

The display assembly factory shall have good anti-static measures. Special anti-static ground, anti-static floor, anti-static soldering iron, anti-static table pad, anti-static ring, anti-static clothing, humidity control, equipment grounding (especially foot cutting machine) are the basic requirements, and should be tested regularly with an electrostatic instrument.

2. Over wave soldering temperature and time

The temperature and furnace passing time of wave front welding must be strictly controlled. It is recommended that the preheating temperature is 100 ℃± 5 ℃ and the maximum temperature shall not exceed 120 ℃, and the preheating temperature rise shall be stable. The welding temperature is 245 ℃± 5 ℃, and the welding time is recommended not to exceed 3 seconds. Do not vibrate or impact the LED after furnace passing until it returns to normal temperature. The temperature parameters of wave soldering machine shall be detected regularly, which is determined by the characteristics of LED. Overheating or fluctuating temperature will directly damage led or cause hidden dangers of LED quality, especially for small-size circular and Oval LEDs such as 3mm.

3. Design current value

The nominal current of LED is 20mA. Generally, it is recommended that the maximum use current should not exceed 80% of the nominal value. Especially for the display screen with small point spacing, the current value should be reduced due to poor heat dissipation conditions.

According to experience, due to the inconsistency of attenuation speed of red, green and blue LEDs, the current value of blue and green LEDs shall be reduced to maintain the consistency of white balance after long-term use of the display screen.

4. Mixed lamp

LEDs with different brightness levels of the same color need to be mixed, or inserted according to the lamp insertion diagram designed according to the discrete law, so as to ensure the consistency of the brightness of each color on the whole screen. If there is a problem in this process, the local brightness of the display screen will be inconsistent, which will directly affect the display effect of the 4K LED Video Wall screen.

5. Control the verticality of the lamp

For in-line led, there should be enough technology to ensure that the LED is perpendicular to the PCB when passing through the furnace. Any deviation will affect the brightness consistency of the set led, and color blocks with inconsistent brightness will appear.

6. Heat dissipation design

LED will generate heat when working, and too high temperature will affect the attenuation speed and stability of LED. Therefore, the heat dissipation design of PCB and the ventilation and heat dissipation design of box will affect the performance of LED.

7. False soldering control

When the LED on the LED display screen is not lit, there is often more than 50% probability that it is caused by various types of false soldering, such as LED pin false soldering, IC pin false soldering, pin and bus false soldering, etc. The improvement of these problems needs to strictly improve the process and strengthen the quality inspection. The vibration test before leaving the factory is also a good inspection method.

8. Driving circuit design

The layout of the driving circuit board and driving IC on the display module will also affect the brightness of the LED. Because the driving IC output current is transmitted too far on the PCB, the voltage drop in the transmission path will be too large, affecting the normal working voltage of the LED and reducing its brightness. We often find that the LED brightness around the display module is lower than that in the middle, which is why. Therefore, to ensure the consistency of display brightness, it is necessary to design the distribution diagram of driving circuit.

The above introduces the relevant knowledge of special LED for full-color display from the perspective of selection and use. Full color LED display is a system integrated product, which requires close cooperation between LED packaging manufacturers and display application manufacturers to strengthen technical exchange. China has now become the manufacturing center of LED display screens all over the world and is moving closer to a powerful LED display manufacturing country. So far, the packaging level of Chinese mainland full color screen LED has been greatly improved. Many indicators are close to the level of Japanese manufacturers, and individual indicators have surpassed. This will greatly promote the development of LED display business in China.

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