From small to small applications, small-pitch LED displays are the result of joint innovation and joint efforts of upstream industrial chains such as chips and packages, as well as peripheral equipment such as drives, power supplies, and control systems. In the selection of actual projects, we have a clear understanding of lamp beads, power supply, driver IC, and display control, but the importance of LED chips in the role of small-pitch LED products is obviously insufficient.
This kind of neglect can be used as an example to illustrate that when engineers or integrators choose small-pitch products, we only care about whether the lamp bead brand is Guoxing, Jingtai or Yiguang, Hongqi, etc., but they are rarely concerned with packaging. The chip is Huacan, Silan or Jingyuan, Sanan. In this case, it is completely unthinkable to push back one or two years. Although the appearance of small pitches has given the LED packaging factory access, it has set a high threshold in technology, equipment, management control, etc., but it is undeniable that the quality and performance of LED chips in the whole small-pitch LED system. It is closely related to its high stability and reliability, low brightness, high gray and high brush. It is the gene that determines whether the small spacing of LEDs is excellent or not.
The performance of LED chips is closely related to the following qualities of small pitch:
Low light and high gray
Low-light, high-gray is the first threshold for small-pitch LEDs to enter the indoor display. For small-pitch products, the LED display will no longer compare who has high brightness, but who has lower brightness. Compared to the display screen of anybody, the brightness can be reduced without losing grayscale and image quality. Achieving low-light, high-gray, small-pitch LEDs is essential for LED chips in addition to high-performance driver ICs.
In order to protect the human eye and ensure a good display effect, the small-pitch LED display has a clear requirement for the total brightness per unit area, and the increase in the number of bit-area chips means that the brightness of a single LED chip needs to be reduced in proportion. The overall brightness of the LED display is generally achieved by adjusting the drive current of the LED chip. In some small-pitch LED environments, the forward current of the LED chip has been reduced to 1 mA or even 0.5 mA. This requires low-current operation, RGB three-color chip brightness and wavelength have a good consistency, but also requires adjustment of the operating current, RGB tri-color chip brightness, wavelength changes characteristics must be basically consistent, otherwise A flower screen will appear.
LED chip small current use conditions, various display problems will be concentrated
Because the epitaxial materials and components of RGB three color chips are quite different, the crystal quality and physical properties of the materials are different. To achieve the consistency of small currents, it is necessary to better match the extension of the three chips. And adjustment to ensure high gray levels at low brightness. Low-brightness and high-gray are the first requirements for small-pitch LEDs. The conventional chip cannot achieve high-light consistency at low brightness without achieving uniform current.
High refresh
Small-pitch LED displays require a higher refresh rate than conventional displays due to the need for some camera and shooting. The higher the refresh rate, the higher the tolerance of the chip to high frequency pulse current. For a screen made by an existing ordinary chip, after the refresh rate is higher, various display inconsistencies may occur. To cope with high refresh, not only does the LED chip have the shortest response time, but also the high consistency of the turn-on capacitor, as well as the fast and always off time.
In the above two cell boards, the left side is a chip with a small capacitor to open the consistency, the right side is a common chip, the package and spectroscopic conditions are the same, the conditions for the manufacture of the cell board are the same. It can be seen that when only the chip is different, when the current is reduced to a few milliamps by zero, which is the effect shown now, the display difference between the two chips is relatively large.
Severe dead light rate
The industry standard for traditional LED displays is that the LED dead light rate is one in ten thousand, but the small pitch LED display lamp has a high density of beads. According to the traditional standard, if there is one dead light in 10,000, it cannot be seen. The future dead light rate needs to be controlled to one in 100,000 or even one in a million to meet the needs of long-term use. Otherwise, it took a while for the user to be unacceptable. One hundred thousandth, or even one in a million, such a severe dead light rate puts higher demands on the quality control system of LED chip factories.
It is necessary to introduce the evaluation indicators commonly used in the two chip fields. One is ESD, Electro-Static discharge, which means "electrostatic discharge", which refers to the ability of the chip to be damaged under electrostatic shock. Due to the geometric multiple of the number of LED chips used in small-pitch LEDs, in order to achieve such a severe dead rate of one in 100,000 or even one in a million, all chips must pass the full test of ESD 2000v to maximize The guarantee of limits is not broken by static electricity. The other is IR, Reverse Leak Current, which means “reverse leakage currentâ€. The smaller the leakage current of the LED chip, the higher the reliability, and the longer the life that can be used, the less likely it is to die. At present, the highest standard of this parameter in the industry is less than 0.05uA@-12V, only a few chip factories can do it, and stricter implementation is even less.
In addition to the above points, the larger exit angle required for wide viewing angles, the back pressure resistance under high sweep and high brushing, and the higher resistance to high temperature and high humidity environments all determine that conventional chips are no longer sufficient. Performance and quality requirements for pitch LEDs. In response to this demand, in 2014, the chip factory has introduced LED chips for small pitch. After more than a year of market promotion, as well as diluting process R&D investment and improving R&D power, the cost performance of small-pitch dedicated LED chips has been greatly improved, and it has been widely used in packaging companies. The vast number of engineering and integrators can make comparative choices in the selection of specific projects.
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