With the rapid progress of LED display technology, the dot pitch of LED display screens is getting smaller and smaller. Now the market has launched P1.4, P1.2 high-density LED display screens, and began to apply in the field of command control and video surveillance.
In the indoor monitoring large-screen market, the two display technologies of DLP splicing and LCD liquid crystal splicing take the lead in the market. Although they have their own advantages, they all share a common problem, that is, the joint between display units. High-density LED display screen has inherent advantages to achieve seamless splicing. The pixels of the high-density display screen are getting smaller and smaller, the resolution is getting higher and higher, and the display screen is clearer and more delicate. When displaying standard high-definition images, it can fully meet the resolution requirements. If the price of high-density lamps is getting lower and lower, high-density LED displays will inevitably occupy a larger market in the field of indoor video surveillance.
The high-density LED display has the characteristics of high-definition display, high refresh frequency, seamless splicing, good heat dissipation system, convenient and flexible disassembly. As the pixel pitch becomes smaller and smaller, higher and higher requirements are placed on the LED mounting, assembly, splicing process and structure. This article discusses some technical issues.
1. LED selection: The display with density above P2 generally adopts 1515, 2020, and 3528 lamps, and the LED pin shape is J or L package. Welding the pins laterally, there will be reflections in the welding area, and the ink color effect is poor. It is necessary to increase the mask to improve the contrast. The density is further improved. The L or J package cannot meet the minimum electrical performance spacing requirements, and the QFN package must be used. Both National Star's 1010 and Crystal Table's 0505 use this type of package.
The unique QFN package welding unique process, this process is characterized by no lateral welding pins, no reflection in the welding area, so that the color rendering effect is very good. In addition, the all-black integrated design is used for compression molding, and the screen contrast is increased by 50%. The image quality of the display application is better than the previous display screen.
2. Printed circuit board process selection: With the trend of high density, 4-layer and 6-layer boards are adopted. The printed circuit board will adopt micro-vias and buried vias. The mechanical drilling technique used in the processing can no longer meet the requirements. The rapidly developed laser drilling technology will meet the processing of micro holes.
3. Printing technology: Too much and too little solder paste amount and printing offset directly affect the welding quality of high-density display lamp tubes. The correct PCB pad design needs to be communicated with the manufacturer and implemented in the design. The size of the opening of the stencil and the printing parameters are directly related to the amount of solder paste printed. Generally 2020RGB devices use 0.1-0.12MM thickness electropolished laser steel mesh, and devices below 1010RGB recommend 1.0-0.8 thickness steel mesh. The thickness and opening size increase in proportion to the amount of tin. The soldering quality of high-density LEDs is closely related to solder paste printing. The use of printing machines with functions such as thickness detection and SPC analysis will play an important role in reliability.
4. Mounting technology: The slight deviation of the RGB device position of the high-density display will cause the display of the screen to be uneven. It is bound to require the mounting equipment to have higher accuracy. Meet the mounting requirements above P1.0.
5. Soldering process: Excessive temperature rise of reflow soldering will cause uneven wetting, which will inevitably cause the device to drift during the unbalanced wetting process. Excessive wind circulation can also cause displacement of the device. Try to choose a reflow soldering machine above 12 temperature zone, chain speed, temperature rise, circulating wind, etc. as strict management and control items, that is to meet the welding reliability requirements, but also to reduce or avoid the displacement of the device, try to control to the demand. Generally, the 2% range of the pixel pitch is used as the control value.
6. Box assembly: The box is assembled by different modules. The flatness of the box and the gap between the modules directly affect the overall effect of the box after assembly. Aluminum plate processing box and cast aluminum box are widely used box types at present, and the flatness can reach within 10 wires. The stitching gap between modules is evaluated based on the distance between the closest pixels of the two modules. When two pixels are too close, they are bright lines. If two pixels are too far, dark lines will result. Before assembling, it is necessary to measure and calculate the module seam, and then select a metal sheet with a relative thickness as a jig and insert it in advance for assembly.
7. Screen assembly: The assembled cabinet needs to be assembled into a screen before it can display refined pictures and videos. However, the dimensional tolerance of the cabinet itself and the cumulative tolerance of assembly cannot be ignored for the assembly effect of the high-density display. If the pixel distance between the cabinet and the closest device is too large or too small, dark lines and bright lines will be displayed. The problem of dark lines and bright lines is a problem that cannot be ignored in high-density displays and needs to be overcome urgently. Some companies make adjustments by pasting 3M tape and fine adjustment nuts on the cabinet to achieve the best results.
8. System card selection: high-density display light and dark lines and uniformity, color difference is the accumulation of LED device differences, IC current differences, circuit design layout differences, assembly differences, etc. Some system card companies can reduce the light and dark lines through software correction And uneven brightness and chroma. Nova introduced the brightness and chromaticity correction system has been applied to various high-density display screens, and achieved good display results.
Conclusion: The high-density display has a refined display effect, and it must start from various links such as material selection, circuit design, temperature rise control, and technology. I believe that with the advancement of technology and the popularization of prices, high-density display screens will gain a larger and wider market share.
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