Eight major aspects reveal the design process of small pitch LED display

In the indoor monitoring large-screen market, DLP splicing and LCD liquid crystal splicing are the first opportunities in the market. Although they have their own advantages, they all have a common problem, that is, the seams between the display units. High-density LED displays have inherent advantages for seamless splicing. The high-density display pixels are getting smaller and smaller, the resolution is getting higher and higher, and the display picture is clearer and more delicate. When displaying standard HD images, the resolution requirements can be fully met. If the price of high-density lamps is getting lower and lower, it is bound to have a high-density LED display that will occupy a larger market in the field of indoor video surveillance.

The high-density led display features high-definition display, high refresh rate, seamless splicing, good heat dissipation system, and convenient and flexible assembly and disassembly. With the pixel pitch becoming smaller and smaller, higher and higher requirements are placed on the mounting, assembling, splicing process and structure of the LED. This article discusses some of the process issues.

1, LED display LED selection: P2 or higher density display generally uses 1515, 2020, 3528 lights, LED pin shape uses J or L package. Lateral soldering of the pins, the soldering area will be reflective, the ink color is poor, and it is necessary to increase the mask to improve the contrast. The density is further improved, and the L or J package cannot meet the minimum electrical performance spacing requirement, and the QFN package must be adopted. Both the National Star 1010 and the Crystal 0505 are packaged in this package.

Unique QFN package welding unique process, this process is characterized by no lateral welding of the pins, the welding zone is not reflective, so that the color rendering effect is very good. In addition, the all-black integrated design is used for molding, and the screen contrast is improved by 50%. The display image quality is better than the previous display.

2, printed circuit board process selection: with high density trend, 4 layers, 6 layers of boards are used, printed circuit boards will use micro-perforation and buried hole design, printed circuit graphics wire fine, micro-hole narrow spacing, The mechanical drilling technology used in the machining has not been able to meet the requirements. The rapidly developing laser drilling technology will satisfy the micro hole processing.

3, LED display printing technology: too much, too little solder paste volume and printing offset directly affect the welding quality of high-density display lamps. The correct PCB pad design needs to be communicated with the manufacturer and implemented into the design. The correct size of the opening of the stencil and the correct printing parameters are directly related to the amount of solder paste printed. Generally, the 2020 RGB device adopts an electropolished laser steel mesh with a thickness of 0.1-0.12 mm, and a steel mesh of 1.0-0.8 thickness is recommended for devices below 1010 RGB. 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 functional printers with thickness detection and SPC analysis will play an important role in reliability.

4, LED display placement technology: high-density display RGB device position of the slight shift will lead to uneven display of the screen, it is bound to require higher accuracy of the placement equipment, Panasonic NPM equipment placement accuracy (QFN ± 0.03 Mm) will meet the P1.0 and above placement requirements.

5, LED display welding process: reflow soldering temperature rise too fast will lead to uneven wetting, which will inevitably cause the device to shift during the wetting imbalance. Excessive wind cycles can also cause displacement of the device. Try to choose a reflow soldering machine above 12 temperature zones, chain speed, temperature rise, circulating wind, etc. as a strict control project, that is, to meet the welding reliability requirements, but also to reduce or avoid the displacement of the device, try to control the demand range. Generally, the range of 2% of the pixel pitch is used as the control value.

6, LED display box assembly: the box is composed of different modules, the flatness of the box and the gap between the modules directly related to the overall effect of the box assembly. The aluminum plate processing box and the cast aluminum box are the types of boxes that are widely used at present, and the flatness can reach 10 wires. The splicing gap between the modules is evaluated by the spacing of the nearest pixels of the two modules, and the two pixels are too bright to be bright. Line, two pixels too far will lead to dark lines. Before assembly, it is necessary to measure and calculate the joint of the module, and then select the metal piece of relative thickness as the jig to be inserted in advance.

7. LED display screen assembly: The assembled cabinet needs to be assembled into a screen to display refined images and videos. However, the dimensional tolerances of the cabinet itself and the cumulative assembly tolerances cannot be ignored for the high-density display assembly. If the distance between the pixels of the nearest device between the cabinet and the cabinet 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 high-density displays cannot be ignored now and need to be urgently overcome. Some companies adjust the 3M tape and the box to adjust the nut to achieve the best results.

8, LED display system card selection: high-density display light and dark lines and uniformity, color difference is the difference between LED device differences, IC current differences, circuit design layout differences, assembly differences, etc. Some system card companies can be corrected by software Reduce the brightness and darkness, uneven brightness and chromaticity. Nova's introduction of brightness and chromaticity correction systems has been applied to high-density displays and has achieved good results.

Conclusion: The high-density display has a fine display effect, and must start from the material selection, circuit design, temperature rise control, process and other aspects. It is believed that with the advancement of technology and the popularization of prices, high-density displays will achieve greater and broader market share.