From small pitch to Micro LED, what changes will occur in the form of packaging products?
Jingtai Technology Director: Shao Pengrui
From the perspective of packaging, I divide LED displays into three eras: small pitch, Mini and Micro. Different packaging eras have different product forms of LED display devices.
In the small-pitch display generation, packaged devices have the following typical forms:
1. Single pixel 3-in-1 discrete device SMD: 1010 is a typical representative;
2. Array packaged discrete device AIP: Four in one is a typical representative;
3. Surface glue filling GOB: SMD normal temperature liquid glue filling is a typical representative;
4. Integrated packaging COB: normal temperature liquid glue is a typical representative.
In the MiniLED era, there are two main categories of product forms: all-in-one discrete devices and integrated packaging.
SMT is a typical representative of all-in-one, separate devices; physical module splicing is a typical representative of integrated packaging. Integrated packaging technology still has problems such as ink color and color consistency, yield, and cost. The 0505 discrete device has reached the limit of SMD. It is currently facing problems such as reliability, SMT efficiency, thrust, etc. It may have lost the mainstream of technology in the Mini LED era. In the MicroLED era, there is no doubt that it will be integrated packaging, but the focus of the problem is on chip transfer.
As for predicting the future technology trends of LED displays, I think there are four main points:
1. Packaging technology has evolved from point technology packaging to surface technology packaging. In the face of LED miniaturization, this will be the way to reduce manufacturing processes and reduce system costs.
2. From One in one, Four in one to N in one, the packaging form is reduced to simplicity.
3. From the perspective of chip size and dot pitch, there is no doubt that it is moving from Mini LED to micro LED.
4. From the perspective of the terminal market, in the future LED display screens will shift from the engineering and rental markets to the commercial display market, and from the display "screen" to the display "device".
In the era of MiniLED and MicroLED, what should we do with phosphors?
Director of Youyan’s Rare Earth Luminescence Division: Liu Ronghui
MiniLED/MicroLED full-chip displays are generally favored by the industry. However, the problems of huge mass transfer, multi-color chip control and uneven attenuation in the manufacturing process are also very prominent. Before the above problems are completely solved, developing new phosphors excited by blue MiniLED/MicroLED to avoid the shortcomings of existing technologies while giving full play to their technical advantages is also a technical approach being considered by the industry. However, it is necessary to solve the problem of small particle size of phosphor and the loss of efficiency caused by small particle size.
At present, MiniLED is still suitable for the LCD industry as a backlight source, but it currently does not have a cost advantage. Nowadays, the industrialization level of liquid crystal display color gamut based on new LED backlight sources has exceeded 90% NTSC. Youyan Rare Earth has achieved mass production and widespread application of narrow-band fluoride, and is further conquering new narrow-band emitting red and green phosphors and LED backlight sources. This will help further increase the liquid crystal display color gamut to 110% NTSC, comparable to OLED/QLED technology.
Also, perhaps quantum dot light-emitting materials could also play a role. However, quantum dot luminescent materials "look beautiful" and have always been given high hopes. However, their stability issues, luminous efficiency, environmental issues, and high application costs have not been well solved. In addition, photoluminescent quantum dots are transitional, and the real application of quantum dots is in QLED. At present, Yanyan Rare Earth has also laid out the development of luminescent materials for QLED.
Why does the original LED display driving method no longer work in the era of Mini and MicroLED?
General Manager of Rouhao Electronics: She Qingwei
Yes, when LED displays enter MicroLED and MiniLED, traditional LED display driving methods cannot be used. The main reason is the problem of available location. Generally speaking, a traditional LED display driver IC can drive up to 600 pixels, and because LED displays are usually used in an area of more than 120 inches, the IC size will not cause a problem. However, if the same pixels fit into the size of a notebook or a mobile phone, the same size and number of ICs will not fit into the notebook or mobile phone device, so MicroLED and MiniLED require different driving methods.
General display driving modes can be roughly divided into two types. The first type is passive matrix (Passive matrix). Matrix), usually passive means that it will emit light only when the scanned pixels are affected by current or voltage, and will be inactive during the rest of the time when they are not scanned. Since this method only affects one column during the conversion time of each frame, it will be very difficult to achieve the requirements of high resolution and high brightness on a single panel, and as long as one of the pixels is short-circuited, it will easily cause signal crosstalk.
There are also designs that use an extra transistor as a switch to avoid signal interference caused by component problems. No matter which method is used, it is still driven in a passive manner. Currently, this driving method is relatively simple in circuit design and low in cost, so it is mostly used in low-resolution applications, such as sports wearable bracelets. If there is a demand for high-resolution panels, multiple low-resolution modules can be used for combination, such as large display screens.
Another type of driving mode is Active Matrix. Active, as its name implies, can continuously maintain the existing voltage or current state through the storage device of the pixel itself within one frame of the picture. The storage device design can be further divided into two different modes. One is called analog modulation driving (Analog Driving), and its architecture is usually similar to DRAM. The design is done in a way, and most of them achieve different gray scale changes by adjusting the actual current or voltage. This method is also commonly used in existing LCD and OLED displays, and the transistor design composition of the pixels is usually 1T1C or 2T1C. There are drive architectures for equal voltage or current sources. Since they use capacitors for storage, there are also problems with leakage and signal crosstalk, but they are much smaller than passive drives. Analog drive methods usually still have uniformity problems caused by the thin film transistor process and the light-emitting component itself at high resolutions. Therefore, there are also more complex circuits such as 7T1C or 5T2C. Flow source architecture to solve its uniformity problem.
There is also a type called digital modulation drive (Digital Driving). Its architecture is designed in the form of SRAM. Its pixel design is based on 6T to provide a voltage or current source. In this design, there is no capacitance factor, so the problems of poor image quality caused by signal crosstalk and leakage are less likely to occur. Its different grayscale expressions are achieved by fast switching, that is, 0 and 1. This type of digital driving method is mostly used in microdisplays. Its mode can usually avoid the differences in the transistor itself due to process factors at high resolution, thereby maintaining a better overall uniformity performance. Such driving also needs to be based on a substrate that can quickly switch CMOS changes, such as a silicon-based CMOS backplane circuit with higher electron mobility compared to a-Si TFT or LTPS TFT.
Microdisplays are mostly used in projection environments, so their panels are mostly smaller than 1 inch, and some are designed between 1 inch for special needs. Mainly considering the effective utilization of silicon wafers, the resolution requirements of the panels are at least qHD or even FHD or 4K. The PPI requirements are also greater than 2000 to 6000, which means that the pixel size will be less than 10um. Even 5um, so most of the backplane circuits are designed based on the process parameters of silicon semiconductors to achieve high resolution and high PPI requirements, so that the projected image can still maintain a good viewing effect. When the pixel size is small to a certain extent and the resolution requirements are high, digital drive methods will be used to design to meet the uniformity issues mentioned above. Pulse width modulation (PWM) is generally used to adjust gray levels to generate different gray scales.
The PWM method mainly uses pulse segments distributed at time intervals to produce different gray scale changes by changing the on and off durations. This technology can also be called duty cycle modulation. Since LED is a component mainly driven by current, most designs of Micro-LED microdisplays are designed with independent fixed current sources to drive each independent pixel to achieve the requirements of uniform brightness and stable wavelength. In addition, if the technology of transferring independent Micro-LEDs of different colors is used, the operating voltages of different RGBs need to be considered. Therefore, independent voltage supply control circuits must be designed inside the pixels.
Currently, Micro-LED microdisplays using silicon-based backplane circuits can already demonstrate various excellent characteristics, and due to their high brightness and low power consumption, they can also be implemented in products such as augmented reality (AR), virtual reality (VR), micro projectors, and head-up displays (HUD). Also, due to their high uniformity, they have the opportunity to become key components used in industry, such as maskless exposure machines, or 3D For products such as printers, with improvements in brightness and power in the future, they can be used in markets such as communication components (VLC) for signal transmission and smart car headlights.
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