In the indoor surveillance large-screen market, DLP splicing and LCD liquid crystal splicing are the two display technologies that dominate the market. Although they have their own advantages, they all have a common problem, that is, the joints between the display units. High-density LED displays have inherent advantages and can achieve seamless splicing. The pixels of high-density display screens are getting smaller and smaller, the resolution is getting higher and higher, and the display pictures are more clear and delicate. When displaying standard high-definition images, it can fully meet the resolution requirements. If the price of high-density lamps becomes 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 rate, seamless splicing, good heat dissipation system, convenient and flexible disassembly and assembly. As the pixel pitch becomes smaller and smaller, higher and higher requirements are placed on the mounting, assembly, and splicing process and structure of LEDs. This article discusses some process issues.
1. LED selection: display screens with density above P2 generally use lights of 1515, 2020, 3528, and the shape of LED pins uses J or L packaging. When the pins are welded from the side, there will be reflections in the welding area, and the ink color effect is poor. It is bound to increase the mask to improve the contrast. With further increase in density, L or J packaging cannot meet the minimum electrical performance spacing requirements, and QFN packaging must be used. NationStar’s 1010 and Jingtai’s 0505 both use this package.
Original QFN package welding unique process, this process is characterized by no side welding pins, no reflection in the welding area, so that the color rendering effect is very good. In addition, it adopts all-black integrated design and compression molding, the contrast of the picture is increased by 50%, and the picture quality of the display application is better than that of the previous display.
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 the design of micro via and buried hole, and the printed circuit pattern will have fine wires and narrow spacing. The mechanical drilling technology used in processing can no longer meet the requirements. The rapidly developing laser drilling technology will meet the needs of micro-hole processing.
3. Printing technology: Too much or too little solder paste and printing offset directly affect the welding quality of high-density display tubes. The correct PCB pad design needs to be communicated with the manufacturer and implemented into the design. The opening size of the stencil and the correct printing parameters are directly related to the amount of solder paste printed. Generally, 2020RGB devices use electro-polished laser stencils with a thickness of 0.1-0.12MM, and 1.0-0.8 thickness stencils are recommended for devices below 1010RGB. The thickness and opening size increase in proportion to the amount of tin. The quality of high-density LED soldering is closely related to solder paste printing. The use of printers with functions such as thickness detection and SPC analysis will play an important role in reliability.
4. Mounting technology: The slight deviation of the position of each RGB component of the high-density display will cause the display of the screen to be uneven, which will inevitably require the mounting equipment to have higher precision, and the mounting accuracy of Panasonic NPM equipment (QFN±0.03mm) will Meet the mounting requirements above P1.0.
5. Soldering process: Excessive temperature rise in reflow soldering will result in uneven wetting, which will inevitably cause the device to shift during the process of wetting imbalance. Excessive wind circulation will also cause displacement of the device. Try to choose a reflow soldering machine above 12 temperature zones, chain speed, temperature rise, circulating wind power, etc. as strict control items, that is, to meet the requirements of soldering reliability, and to reduce or avoid component displacement, and try to control it within the required range. Generally, the range of 2% of the pixel pitch is used as the control value.
6. Box assembly: The box body is made up of different modules. The flatness of the box body and the gap between the modules are directly related to the overall effect of the box body after assembly. Aluminum processing boxes and cast aluminum boxes are currently widely used box types, and the flatness can reach within 10 wires. The splicing gap between the modules is evaluated by the distance between the nearest pixels of the two modules, and the two pixels are too close to light up and then bright Line, two pixels too far will cause dark lines. Before assembling, it is necessary to measure and calculate the seam of the module, and then select the metal sheet of relative thickness as the jig to be inserted in advance for assembling.
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 box itself and the cumulative tolerance of assembly cannot be ignored for the high-density display assembly effect. If the pixel pitch of the nearest device between the cabinet and the cabinet is too large or too small, it will cause dark and bright lines to be displayed. Dark lines and bright lines are problems that cannot be ignored in high-density displays and need to be overcome urgently. Some companies make adjustments by attaching 3M tapes and fine-tuning nuts on the cabinet to achieve the best results.
8. System card selection: high-density display screen light and dark lines and uniformity, color difference are accumulated criticisms of LED device differences, IC current differences, circuit design layout differences, assembly differences, etc. Some system card companies can reduce light and dark lines through software correction And uneven brightness and chromaticity. Nova introduced the brightness and chromaticity correction system that has been applied to various high-density display screens and achieved better display results.
Conclusion: The high-density display has a refined display effect, and it must start from all aspects such as material selection, circuit design, temperature rise control, and craftsmanship. It is believed that with the advancement of technology and the popularization of prices, high-density display screens will gain a larger and wider market share.
