A new type of display device for outdoor full-color LED display screen

introduction

LED displays, with their high brightness, weather resistance and large size, have occupied the mainstream market for large-size indoor and outdoor displays, while cathode ray tube displays (CRT), liquid crystal displays (LCD) and plasma displays (PDP) have dominated the market for small and medium-sized displays that are viewed indoors at close range.

LED display screens have been widely used in advertising, stage, public information display, sports and transportation facilities, etc. LED display screens are divided into indoor, outdoor and semi-outdoor LED display screens according to the use environment; divided into monochrome screens, dual-color screens and full-color screens according to the display color; divided into digital dot matrix screens, direct-insert LED screens and SMD screens according to the selected devices.

This article describes the emerging outdoor full-color SMD display screen and the performance of its dedicated SMD devices.

1 Main advantages of outdoor full-color SMD display

As we all know, outdoor LED display screens all used direct-plug oval red, green and blue LED devices one or two years ago. However, in the past two years, outdoor full-color SMD display screens with a pixel pitch of 8 to 16 mm are gradually emerging and have been widely used.

In the past, due to the low brightness of SMD, and the lack of waterproof, moisture-proof and UV-proof performance in outdoor harsh environments, outdoor full-color display screens were dominated by direct-plug LED devices, and full-color SMD display screens could only be used indoors in large quantities. With the advancement of LED chip technology and LED packaging technology, the brightness and protection level of SMD can meet the needs of outdoor applications, and full-color SMD display screens have more advantages that full-color direct-plug LED display screens cannot achieve, so they have been quickly applied.

The advantages of outdoor full-color SMD display screen over outdoor direct-plug oval LED display screen are as follows:

(1) Wide viewing angle

The outdoor full-color SMD display has a wide viewing angle of 110° not only in the horizontal direction, but also in the vertical direction, which is particularly advantageous in some applications.

(2) Good lighting

The matching consistency of the three brightnesses of the red, green and blue oval LEDs at different angles is a very difficult indicator. The full-color SMD has a three-in-one design structure, with the red, green and blue chips all in one bracket bowl. Therefore, the matching consistency of the three brightnesses of red, green and blue at different angles is highly consistent, making the outdoor full-color SMD display screen have good brightness matching consistency at any angle, achieving better color fidelity.

Figure 1 and Figure 2 are the outdoor full-color SMD light distribution curve and the outdoor full-color elliptical LED light distribution curve respectively. By comparison, it can be seen that the light distribution consistency of SMD has obvious advantages over elliptical LED.

(3) Good light mixing

Due to the three-in-one design structure, the three chips are very close to each other and mix light in the same bracket bowl, rather than three separate elliptical LEDs. Therefore, the red, green and blue light mixing effect is better than the direct-insert elliptical LED screen, which is especially suitable for close-up viewing.

(4) High contrast

Due to the three-in-one design structure, the full-color SMD is small in size, so the luminous area is small and the black area is large, which improves the contrast of the LED display.

(5) Automated production

SMD can be automatically mounted using a fully automatic SMD machine, which has high production efficiency. The circuit design of the full-color SMD display module can realize the integration of the light board and the driver board, which reduces costs, improves reliability, and improves production efficiency. Automated SMD production can also improve the vertical accuracy of SMD on the circuit board, overcoming the vertical accuracy problem of direct-insert elliptical LED on the circuit board, thereby ensuring better optical effects.

2 Main parameters of outdoor full-color SMD

(1) Brightness

At present, the brightness of outdoor full-color SMD can meet the brightness requirements of outdoor full-color SMD display screens with a dot pitch of 8 to 16 mm, of which the green brightness can reach 1,200 to 1,600 mcd, and the brightness of the entire screen can be higher than 5,000 nits. To ensure the consistency of the brightness of the entire screen, the brightness range of each color should be below 1:1.3, and the average brightness ratio of red, green and blue is about 3:6:1.

(2) Wavelength

The wavelength range of red, green and blue should not be wider than 5nm, and the wavelength distribution of each color should be normally distributed.

(3) Attenuation

Outdoor full-color SMD uses relatively high current due to brightness. Since it is used outdoors, in order to achieve minimum attenuation, it has high requirements on chip size, heat dissipation design, waterproofness, and UV resistance.

At the same time, the brightness attenuation of the three colors of red, green and blue must be consistent, thereby ensuring that the brightness of the SMD display screen not only decreases slightly after working for a long time, but also maintains good brightness and color consistency. High-quality outdoor full-color SMD can generally ensure that the average attenuation of red, green and blue is less than 5% after working for 3,000 hours, and the discreteness of red, green and blue attenuation is less than +10%.

(4) Angle

The angle of outdoor full-color SMD is generally required to be greater than 110° in both horizontal and vertical directions.

(5) Waterproof and moisture-proof

The waterproof and moisture-proof performance of outdoor full-color SMD is a very important key parameter. The selection of SMD bracket materials and structural design determine the waterproof and moisture-proof performance. The use of new PPA materials, silicone materials and the design of new structures can solve the waterproof and moisture-proof problems. If the waterproof and moisture-proof problems are not well solved, problems such as failure, dim light, and rapid attenuation will occur. The waterproof and dustproof level of high-quality outdoor full-color SMD can reach IP67, and the moisture-proof level is Lever3.

(6) Anti-ultraviolet rays

Due to outdoor use, the bracket material and potting material of full-color SMD are required to be able to resist ultraviolet rays. SMDs that are not UV-resistant will have yellowing of PPA and turbidity of potting materials, resulting in brightness attenuation and failure. The yellowing ratio of high-quality outdoor full-color SMD should be no more than 5% after 10,000 hours of outdoor work.

(7) Failure rate

Outdoor full-color SMDs are used in outdoor environments and are prone to failure due to a number of factors. The failure rate of high-quality outdoor full-color SMDs should be less than 50ppm within 3,000 hours.

(8) Thickness

Taking the circuit board waterproofing into consideration, the outdoor full-color SMD display needs to be potted with waterproof glue below the SMD thickness. Therefore, the full-color SMD requires a certain height. The mainstream thickness on the market is 2.5mm.

3 Development trend of outdoor full-color SMD display

In terms of brightness and cost, the traditional outdoor full-color direct-plug oval LED display has an irreplaceable advantage in pixel pitch of 20mm and above. With the improvement of LED chip brightness and LED packaging process technology, the price gap between outdoor full-color SMD and oval LED has narrowed to 10-20%. Due to its advantages of wide viewing angle, good light distribution, good light mixing, high contrast and easy automated production, the market share of outdoor full-color SMD display is expected to develop from less than 10% now to a situation where it shares the market with outdoor full-color direct-plug oval LED display.