Authors: Honglei Ji1*, Pengfei Li1
1. TCL Electronics Holdings Limited., Shenzhen, China
* Corresponding author, jihl@tcl.com
Abstract
Today, with the rapid development of Mini LED backlight technology, what kind of technology can meet the needs of consumers, while the cost is controllable and can be accepted by the industry? POB are definitely such an option. POB do not require special transfer equipment at first and can be processed using current SMT machines. POB is superior to COB and COG in terms of stability, operability and process difficulty. Although there are certain technical bottlenecks, POB can be regarded as a technical form that gives consideration to both performance and cost in the terminal market. This paper hopes to objectively analyze the characteristics, status and future development direction of POB technology from the perspective of industrial chain.
Author keywords
Mini LED backlight, POB, COB
The concept of Mini LED comes from the development of Micro LED technology. Although the advantages of Micro LED for display are very prominent, it is still far from the market due to the development of technology such as mass transfer. Traditional LED are generally in millimeter size, while Micro LED are generally in micron size. There is a chip size gap between traditional LED and Micro-LED, known as Mini-LED technology. Mini-LED technology can be regarded as the transition from LED to Micro-LED technology. Due to the relatively low difficulty of Mini-LED technology, and thanks to the liquid crystal BLU technology, Mini-LED technology has a prospect of application in LCD backlight, be coming the main driving force of the development of Mini-LED. In addition, the huge amount transfer and detection technologies related to Mini-LED play a certain role in promoting the development of Micro-LED, and also become the technical driving force for the development of Mini-LED technology.
Mini-LED display technology is mainly divided into two categories. One is to directly drive Mini-LED through PM passive matrix or AM active matrix, which is the display technology closest to Micro-LED. There are still a number of technical problems to be overcome, and commercial application has not been achieved. The other is Mini-LED backlight technology, through the combination of Mini-LED backlight and LCD, can greatly improve the performance of LCD in contrast and motion blur, while maintaining ultra-thin thickness, so that LCD technology upgrade to a new level, improve the market competitiveness of LCD terminal products.
This paper only discusses the Mini-LED backlight technology that is closest to industrial application at present. According to the common technology realization form of Mini-LED backlight and the comparison between POB and COB, it is judged that the technology can become the mainstream application in Mini-LED backlight in the future.
2. Definition of Mini-LED backlight
At present, there is no Mini-LED standard in the industry. Different manufacturers have different definition standards for Mini-LED backlight technology due to their different positions in the industrial chain.
From the perspective of the term Mini-LED itself, it needs to be defined according to the size of the chip. At present, it is generally believed that Mini-LED refers to the chip whose short side size is between 75 and 300 microns.
As for the packaging plant, when the number of zones is small, the micron chip is used for the backlight scheme, the number of chips is greatly increased, the driving current is greatly reduced, and the luminous efficiency of the chip is also greatly reduced.
For TV product applications, Mini-LED backlight technology provides more zones to enable area dimming technology. Therefore, the number of zones per unit area (i.e. partition density) is an important parameter for terminal manufacturers to evaluate Mini-LED backlight.
For ordinary consumers, they pay more attention to the performance, function and price of the product, and do not care about whether the products they buy adopt Mini-LED backlight technology.
They are more concerned about whether the high quality parameters related to Mini-LED backlight can bring new additional functions. There are no clear standards in this regard.
The value of a technology depends on what it can empower the end user, which is to improve performance and add functionality. In this regard, the definition of Mini-LED backlight should be defined according to the value enhancement it can bring to the terminal. On this issue, the biggest problem in the industry at present is that the performance of high partition, high brightness, high contrast and other performance that Mini-LED backlight technology can achieve can also be achieved by using traditional straight down backlight (Sky Star scheme). That is, in terms of performance improvement of high picture quality, the recognition of Mini-LED backlight technology is lower than that of all-sky Backlight technology, which is the main reason why it is difficult to accurately define Mini-LED backlight technology in terms of application. As can be seen from the structure diagram of Mini-LED backlight, except for the difference of LED chip size, the structure of other parts is basically the same as that of direct LED backlight. As show in Figure 1
3. The definition of POB
The concept of POB is relative to COB, POB is packag on PCB while COB is chip on PCB. COG, compared with COB, changes the material of original PCB from FR4 and BT board to glass substrate. This paper focuses on discussing the difference between POB and COB technology, which one of them can become the future development trend. The differences between POB and COB are shown in Figure 2
4. The essence of Mini-LED technology
The essence of Mini-LED technology is to meet the problem of low contrast of LCD display technology, so how many zones is appropriate? Theoretically, the more zones of Mini-LED backlight, the better. If pixel-level zones can be achieved, the display effect can be equal to or even surpass that of OLED. But as the number of zones increases, the cost of driving them, the number of LEDs used, and the complexity of the backplane lines all cause costs to skyrocket.
In addition, display technology ultimately depends on human eyes to evaluate and perceive, and the improvement of theoretical parameter value does not mean that human eyes can also improve perception. The visual perception limit of Halo Effect was determined by designing and conducting subjective experiments. LabPSNR is an evaluation index used to quantify the difference between the display image and the target image, and the value should be greater than 47.4 dB in practical application. Based on this, the requirements for the number of zones can be put forward: for the LCD panel with high CR 55:1, more than 200 zones are sufficient; for the LCD panel with CR 200:1, more than 3000 dimming regions are required .
In the case of small number of zones, using POB technology to realize Mini-LED technology is more cost-effective than COB.
Therefore, how to choose a balance point between performance and cost, and minimize the number of zones on the premise of ensuring the perceived performance of the human eye is an important topic when the display terminal uses Mini-LED backlight technology.
The number of zones is an important indicator to measure Mini-LED backlight technology, and the number of zones is also a key parameter to determine performance, solution and cost.
5. POB VS COB
The current COB chip size is 0.2mmx0.2mm or 0.2mmx0.4mm, while package size, used in POB, is 3.5mmx2.8mm or 1mmx1mm. The LED package used by POB is 100 times larger in area than the chip, so we can understand that the pad area of POB is 100 times larger than that of COB. The difference in size directly determines the essential difference in the equipment, process, precision and quality of the materials used in the two technical schemes.
From the process point of view, COB requires 9 processes, while POB only needs 6 processes, which is 3 processes less. In the bulk transfer process, COB needs to reinvest in the purchase of long fixation machine, while POB only needs the traditional SMT machine, no additional investment is required, and its process is more reliable. The overall system yield of POB is close to 100%, while that of COB is 90%. Due to the low yield, maintenance process is required, which increases the maintenance cost.
Due to the improvement of processing accuracy, the price of PCB board required by POB technology is only 100 yuan/square meter, while the price of PCB board required by COB is 300~400 yuan/square meter. POB only needs the solder paste of no. 9 powder to meet the requirements, while COB needs no. 5 powder, which not only increases the technological difficulty, but also increases the material cost.
In addition, since the driving current of general POB is larger than that of COB, the photoelectric conversion efficiency of POB is higher from the perspective of backlight scheme. As the driving current becomes larger, the control accuracy of the driving current is reduced, which reduces the requirements of the driving chip and also helps to further reduce the cost.
6. Summary
The application of Mini-LED backlight technology in TV and other display terminals is the most promising and revolutionary event in recent years. Foreign display giants such as Samsung, LG and Apple have entered the game and released their own Mini-LED backlight products one after another. In China, TCL was the first to release TV products with Mini-LED backlight technology, and Xiaomi, Hisense, Skyworth and other companies gradually followed suit.
In terms of the specific implementation scheme, there has been a lot of discussion on the three routes of POB, COB and COG since this year. COB and COG are only different in substrate, and have the same properties in yield and cost. POB technology can use the current mature SMT process to meet the basic product performance at the lowest price, which is the mainstream technology direction of Mini-LED technology at present. This paper believes that Mini LED backlight products will be dominated by POB technology in 2022.
7. Acknowledgment
The authors would like to gratefully acknowledge the support of the Guangdong Provincial Key R&D program (2020B0101030008: Development and industrial application of low environmental pollution quantum dot luminescent materials and devices).
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Symp Digest of Tech Papers – 2022 – Ji – 27 1 POB technology applied in Mini LED backlight