In the information society, people receive a large amount of information each day. As the main interface facilitating exchange between people and information, display products have a significant impact on people’s lives. Given the rapid increase of information and speed of upgrades—in particular the shift from traditional single text and video information to multimedia digital audio and video information—people have developed more sophisticated display product requirements. Such specifications include improved image quality, resolution and etc.; therefore, various new display technologies are booming, especially the new generation of display technology Micro LED, which has become a technical highland for display manufacturers. In response to this shift, Absen released the Micro LED display technology worldwide on 12 March 2021.
1 Comparison of mainstream display technologies
Display product performance requirements differ depending on the usage scenarios. Accordingly, different mainstream display technologies have been selected. Display products can be divided into 2C displays and 2B displays based on their intended use.
2C display products mainly refer to TVs, monitors, laptops, in-vehicle products, mobile phones, wearable products etc. The LCD display is the most traditional technology scheme applied to 2C display products, but suffers from issues such as low contrast, small color gamut and limited viewing angle. With the development of material technologies, OLED technology has emerged. Compared with LCD, OLED display products boast significant advantages including simpler frames, thinner overall thickness, higher brightness and contrast, larger viewing angle, wider color gamut range and lower power consumption. These strengths have quickly enabled OLED display products to dominate the consumer market. However, OLED display products also have disadvantages such as short life. Micro LED is characterized by display technology that minimizes the LED display to a micron scale, which matches the long life and reliability of inorganic LED. This technology results in displays that feature high brightness, high contrast, wide color gamut, long service life and high reliability. Due to these qualities, the micro LED display is considered to be a near perfect display technology. Its brightness and energy-saving capabilities have broad application prospects on 0.x-inch and x-inch wearable products. For example, Apple expects to apply Micro LED technology to 2C products such as the Apple Watch; and the advantage of seamless splicing renders the Micro LED the best solution for 85-inch and above large screen TV products. At present, Sony, Samsung and some other domestic display manufacturers have all introduced Micro LED-based large screen consumer products.
2B display products are mainly utilized as large indoor screens and outdoor displays. Common usage scenarios or locations include cinemas, shopping malls, conference rooms and control rooms. Mainstream display technologies selected for use in these situations include LCD splicing displays, projection displays and LED large screen displays. It is impossible for LCD splicing displays to achieve seamless splicing and the projection display has problems such as poor brightness, poor contrast etc., but LED displays are characterized by high brightness, high contrast, wide color gamut, available seamless splicing and other advantages. Since minimizing the dot pitch, especially with the development of Micro LED technology, it has become the trend to push LED display development towards UHD. This development has coincided with improvements in display image quality that is more and more exquisite; while application scenarios have been extended from traditional industrial usage to commercial display and high-end consumer usage; for example, 2B display application scenarios include large indoor screens and outdoor displays used in cinemas, shopping malls, conference rooms, control rooms etc. It can be seen that Micro LED is highly competitive in both 2C and 2B markets because of its unique advantages. Traditional panel manufacturers and LED display manufacturers, including Absen, a global leading supplier for true LED display applications and services, are all actively making overall arrangements for this technology. At present, Absen has launched the following Micro LED commercial display products worldwide: 2K 55-inch (P0.6), 2K 73-inch (P0.8), 2K 82-inch (P0.9), 4K 110-inch (P0.6), 4K 138-inch (P0.7), 4K 165-inch (P0.9) and 8K 220-inch (P0.6).
Micro LED display technology can be deemed as the micro pitch and HD of traditional LED displays. Micro LED takes tiny LED crystal particles as pixel light emitting points; the LED chip structure and encapsulation method can directly affect the performance of Micro LED display devices.
2 Structural comparison of Micro LED light emitting chips
Generally, the LED chip is composed of the substrate, P-type semiconductor layer, N-type semiconductor layer, P-N junction and positive and negative electrodes. When the positive voltage is applied between the positive electrode and negative electrode, the electron hole implanted from P area to N area is combined with the electron implanted from N area to P area at the P-N junction, so that the electric energy is converted into light energy to emit different wavelengths of light. LED chips are primarily composed of the following three structures: wire-bonding structure, flip structure and vertical structure. Figure 1 shows the schematic design of three chip structures.
(1) Structure of wire-bonding chip
As the earliest chip structure, the wire-bonding chip is composed of the following from top to bottom: electrodes, P-type semiconductor layer, light emitting layer, N-type semiconductor layer and substrates. In this structure, the heat generated at the P-N function can be transmitted to the heat sink only through the sapphire substrate, but the poor thermal conductivity of the sapphire leads to poor thermal conductivity of this structure. This in turn reduces the efficiency of light emission and the overall reliability of the chip. In the structure of the wire-bonding chip, both p electrode and n electrode are located on the light emitting surface of the chip; due to the shielding of the electrode, it will affect the light emitting capability of the chip, resulting in low light emitting efficiency; when both the positive electrode and negative electrode are located on the same side, it can also easily cause current crowding and reduce the efficiency of light emission; in addition, electrode metal drift may be caused by factors such as temperature, humidity and etc.; with the reduction of chip size, the pitch between the positive electrode and negative electrode is decreased, so electrode drift may result in short circuiting.
(2) Structure of flip chip
The flip chip is composed of the following from top to bottom: sapphire substrate, N-type semiconductor layer, light emitting layer, P-type semiconductor layer and electrodes. Compared with the wire-bonding chip, the heat generated at the P-N junction of this structure can be directly transmitted to the heat sink without passing through the substrate; therefore, it has good heat dissipation performance, high light emitting efficiency and high reliability; in the flip structure, both p electrode and n electrode are located on the bottom, which avoids obstructing light emission, and therefore maintains high light emitting efficiency; in addition, it is relatively far between electrodes of the flip chip, which can reduce the risk of short circuiting due to electrode metal drift.
(3) Structure of vertical chip
Compared with the wire-bonding chip, the vertical chip uses the substrate with high thermal conductivity (Si, Ge or Cu substrate) instead of sapphire substrate, which greatly improves the heat dissipation performance of the chip. At the same time, the positive electrode and negative electrode of the vertical chip are located on the top and bottom of the chip respectively, which distributes the current more evenly and prevents overheating in an isolated area and further improves the reliability of the chip. At the present however, due to high costs, mass production capacity of the vertical chip is low.
3 Comparison of Micro LED Encapsulation Schemes
It is impossible for a single LED light emitting chip to meet use requirements, so it requires encapsulation. With a reasonable encapsulation structure and process, it can not only provide the light emitting chip with electrical input, mechanical protection and effective heat dissipation channel, but can also be good for realizing the high-efficiency and high-quality output of light.
Due to the reduction of LED chip size and display ditch, it necessitates higher requirements for encapsulation. At present, typical Micro LED encapsulation methods include Chip-type SMD encapsulation, N-in-one IMD encapsulation and COB encapsulation, as shown in Figure 2.
(1) Chip-type SMD encapsulation
Chip-type SMD encapsulation is to die bond a single LED pixel on the BT board and secure the light emitting chip using the encapsulation glue to achieve a single Chip-type encapsulation pixel; and then patch the single Chip-type encapsulation pixel on the printed circuit board (PCB) using SMD technology to create the LED display module. Chip-type SMD encapsulation is a single pixel encapsulation with a small size profile, small solder joint areas and more solder joints. As the size of the LED chip and pixel pitch of the display decrease, the single SMD device suffers from poor air tightness control which makes it vulnerable to water and vapor damage, bumping and overall poorer protection. Close proximity of the solder joints can also result in greater risk of short circuiting; therefore, it is not suitable for micro-pitch Micro LED displays.
(2) N-in-one IMD encapsulation
The N-in-one IMD encapsulation method is to die bond N pixel units (mostly 2 or 4) on the BT board and then secure such pixel units using encapsulation glue as a whole; compared with single pixel SMD discrete encapsulation, it boasts higher integration, which can effectively improve the air tightness and protection. It is easy to water and vapor erosion, bump and poor protection. At the same time, it inherits the mature process, technical difficulty and low cost of the single SMD device, but integration of the N-in-one IMD encapsulation is still low; for micro pitch displays below 0.6mm the N-in-one IMD encapsulation process is difficult, and its display effect, reliability and service life are poor.
(3) COB encapsulation
The COB encapsulation scheme is to directly die bond multi-pixel naked chips on the PCB and then secure them using encapsulation glue as a whole. Compared with Chip-type SMD encapsulation and the N-in-one IMD encapsulation, COB encapsulation secures multiple LED chips as a whole, which greatly improves its protection and air tightness; so it is more suitable for small chip encapsulation and micro-pitch display products. At the same time, COB encapsulation requires no BT board, but secures LED chips directly on the PCB, which can provide a shorter heat conduction channel and better heat dissipation performance, so it is more suitable for high pixel density displays.
4 Conclusion
With the advantages of high brightness, high contrast, wide color gamut and seamless splicing, Micro LED is deemed as a near perfect display technology. It has broad application prospects for 85-inch or above large screen displays. Therefore, all large display manufacturers at home and abroad are making active arrangements in the Micro LED display industry. The chip structure and encapsulation method directly determine the performance of Micro LED display products. At present, three structures, namely the wire-bonding structure, flip structure and vertical structure, are mainly adopted in the industry. It can be seen from the comparison of those three structures that the flip chip can provide high light emitting efficiency, good heat dissipation performance, high reliability and high mass production capacity, so it is more suitable to Micro LED display products. Typically, Micro LED encapsulation methods include the SMD single pixel encapsulation, N-in-one IMD encapsulation and COB encapsulation. Among these three encapsulation methods, COB encapsulation has the highest integration, which can realize the minimum pixel pitch, highest reliability and longest display life, so it is recognized as the best encapsulation scheme for Micro LED.
The full range of Micro LED products released by Absen are based on Absen proprietary HCCI technology.
1) Use the inverted COB encapsulation technology
2) Integrate Absen’s core algorithm HDR3.0
3) Integrate the intelligent display technology
A professional team has been continuously working on the development of display effects such as optical design and image quality processing. These efforts have resulted in improved technologies characterized by smooth display images, high color resolution, soft image and good consistency effects. These results have been applied to various products series’ and are widely used in scenarios such as large control centers and conference centers.