A brief analysis of LED display classification and packaging technology requirements

In recent years, with the Beijing Olympics, Shanghai World Expo, and Guangzhou Asian Games, LED displays can be seen everywhere. LED displays can display changing numbers, text, graphics, and images; they can be used not only in indoor environments but also in outdoor environments, and have advantages that projectors, TV walls, and LCD screens cannot match.

LED has been widely valued and developed rapidly, which is inseparable from its own advantages. These advantages can be summarized as: high brightness, low operating voltage, low power consumption, miniaturization, long life, impact resistance and stable performance. The development prospects of LED are extremely broad, and it is currently developing towards higher brightness, higher weather resistance, higher luminous density, higher luminous uniformity, reliability, and full color.

1. Types of LED Displays

1. Classification by color

Single-color display: single red or single green; dual-color display: red and green dual-color, 256 levels of grayscale, can display 65,536 colors; full-color display: red, green and blue three primary colors, 256 levels of grayscale full-color display can display more than 16 million colors.

2. Classification based on pixel units

Digital display screen: The display pixels are 7-segment digital tubes, suitable for making clock screens, interest rate screens, etc.

Graphic display screen: The display pixels are dot matrix modules, suitable for playing text and image information;

Video display screen: The display pixels are composed of many light-emitting diodes and can display various video files such as videos and animations.

3. Classification by location of use

Indoor display screen: small light point, dense pixel pitch, suitable for close viewing;

Semi-outdoor display: between indoor and outdoor, not rainproof, suitable for door lintels for information guidance, etc.

Outdoor display: large luminous point, large pixel pitch, high brightness, can work in the sun, with windproof, rainproof and waterproof functions, suitable for long-distance viewing.

4. According to the driving mode, there are static, horizontal scrolling, vertical scrolling and page turning display.

2. Types and advantages and disadvantages of LEDs used in display screens

According to the classification of display screens, the pixel LEDs used can also be divided into the following categories:

1. Dot matrix module

Advantages: low cost, mature processing technology, stable quality; Disadvantages: brightness and color consistency are difficult to control, and mosaic phenomenon is easy to occur;

2. Plug-in lamp

Advantages: Color consistency is easier to control, and the pixel spacing can be adjusted freely as needed; Disadvantages: The red, green and blue color mixing effect is not good, the angle is not large, and it is difficult to control the angle consistency. The processing is prone to unevenness, and it is easy to misalign up and down, left and right;

3. Patch

Advantages: the display color, color mixing effect, angle consistency, etc. are the best; Disadvantages: high cost of packaging and processing;

3. Technical requirements for LED packaging for display screens

Different application locations will use display screens of different specifications, and different display screens require LED devices with different technical requirements. So what are the technical requirements for LEDs used in display screens? The following is an explanation of each one according to different categories:

1. SMD LED for indoor display screen.

SMD LEDs for indoor display screens are divided into "three-in-one" and "three-in-one". The former is mostly used in relatively low-cost fields with low display requirements, while the latter has better display effects and relatively higher costs. The following mainly explains "three-in-one";

1. Brightness:

The first thing to consider is the appropriate ratio of product brightness. At present, the red, green and blue brightness of most full-color display screens are matched in a ratio of 3:6:1 (that is, the red brightness accounts for 30%, green accounts for 60%, and blue accounts for 10% in a pixel). Generally speaking, packaging factories and upstream chip manufacturers almost all test products according to 20mA, but when it comes to actual display screens, this current is rarely used to drive LED lights. Generally speaking, the three types of LEDs, red, green and blue, are made of different chip materials, so their heat generation, light emission and brightness attenuation in actual use are also disproportionate. Generally speaking, the red attenuation ratio is smaller, and the blue-green attenuation is larger. Therefore, in order to ensure that the service life of the display screen is relatively long, the general designer will set the blue-green driving current lower than the red. For example: if red is driven at 18mA, green at 15mA, and blue at 12mA, according to the IV-If relationship of the LED chip:

We can roughly deduce that the actual required brightness of the LED, the relationship between red, green and blue is:

R:3÷90%

G:6÷75%

B:1÷60%

At the same time, the brightness must be graded. The purpose of the grading is to ensure that the brightness difference of the display screen cannot be seen by the naked eye. Generally speaking, the brightness difference that the human eye can sensitively distinguish is 30%. After deducting the test error of the equipment and the error of the installation mechanism, it is better to design the light splitting according to 10%~20% grading.

2. Color:

According to the optical effect, white light is decomposed into a variety of colors after passing through a prism. The colors are red, orange, yellow, green, cyan, blue, and purple. This is the visible spectrum. The human eye is most sensitive to red, green, and blue. The human eye is like a three-color receptor system. Most colors can be synthesized by red, green, and blue in different proportions. Similarly, most monochromatic light can also be decomposed into red, green, and blue.

This is the most basic principle of colorimetry, namely the principle of three primary colors. The three primary colors are independent of each other, and any one of the primary colors cannot be synthesized by the other two colors. Red, green and blue are the three primary colors, and the color range synthesized by these three colors is the widest. Adding the three primary colors of red, green and blue in different proportions to mix colors is called additive color mixing. The three primary colors are three colors that are independent of each other, and any one of them cannot be produced by mixing the other two colors.

The color obtained by adding the three primary colors in different proportions is called additive color mixing. The rule is:

Red + Green = Yellow

Red + Blue = Purple

Blue + Green = Cyan

Red + Blue + Green = White

The best color combination for LED display screens is: red 620-630nm, green 520-530nm, blue 465-475nm. Of course, this range can be narrowed according to actual needs, such as 3nm for red and 2.5nm for blue and green.

Angle: Actually, the bigger the better, but more importantly, the angle is not just big. The angle curve must be full and smooth, and the red, green and blue must overlap. Otherwise, after the display is completed, some positions will show dark areas, or one color will be particularly bright or dark.

3. Electrical parameters:

The electrical parameters of LED are mainly reflected in forward voltage (vf), reverse current (Ir) and anti-static discharge (ESD). Generally, 5V DC power supply with constant current is used for display screens. Therefore, the Vf of LED is generally not required to be too consistent, as long as it is controlled within a reasonable range (generally 1.8-2.2v for red, 3.0-3.4v for blue and green). Generally speaking, the larger the ESD, the better, because ESD is everywhere, so the larger the ESD of LED, the less likely it is to cause problems in display screen processing; the reverse current requires Vr=5v, Ir≦10uA.

Oval LED for outdoor display

Outdoor LED displays generally use elliptical plug-in lamps. Compared with indoor LED displays, outdoor elliptical lamps mainly have horizontal and vertical light-emitting angles. Generally, for outdoor displays, the horizontal visible range is relatively large, while the vertical visible range is relatively small.

Generally speaking, the horizontal angle of the LED outdoor display is about 110 degrees, and the vertical angle is about 50 degrees. The angle of the direct plug-in lamp is affected by many factors, mainly manifested in:

(1) The appearance and shape of the LED package;

(2) The bowl-cup structure of the bracket;

(3) The chip and its structure;

(4) Refractive index of encapsulation glue;

(5) Consistency of material quality;

(6) Packaging process (i.e., the light-emitting height position of the chip);

Therefore, the angle of LED for outdoor display screens is more difficult to control, which requires not only good design, but also good process design and quality control level.

As technology continues to improve, the requirements for display screens will become higher and higher, and of course the requirements for LED packaging technology will be equally high.