With the continuous advancement of LED display technology, indoor small-pitch screens are becoming more and more popular in the market. Now some LED display manufacturers have launched high-density LED displays of P1.4 and P1.2, and have begun Applied in the field of command control and video surveillance, this paper sees whether there are peers who can conduct some discussions on technological issues:
1. LED selection: Displays with a density above P2 generally use 1515, 2020, 3528 lights, and the LED pin shape adopts J or L packaging. If the pin is welded sideways, the welding area will reflect light, and the ink color effect will be poor. It is necessary to add a mask to improve the contrast. The density is further increased, and the L or J package cannot meet the minimum electrical performance spacing requirements, and the QFN package must be used. National Star's 1010 and Jingtai's 0505 both use this package.
The 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, the all-black integrated design is used for compression molding, and the contrast ratio of the screen is increased by 50%, and the image 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, printed circuit boards will adopt micro-via and buried hole design, printed circuit graphics with fine wires, micro-holes and narrow spacing, The mechanical drilling technology used in processing can no longer meet the requirements. The rapid development of laser drilling technology will meet the processing of micro holes.
3. Printing technology: Too much or too little solder paste and printing offset directly affect the welding quality of high-density display lamps. The correct PCB pad design needs to be implemented in the design after communicating with the manufacturer. Whether the opening size of the stencil and the printing parameters are correct or not is directly related to the amount of solder paste printed. Generally, 2020RGB devices use an electropolished laser steel mesh with a thickness of 0.1-0.12MM, and devices below 1010RGB are recommended to use a steel mesh with a thickness of 1.0-0.8. Thickness, opening size increases proportionally to tin amount. The quality of high-density LED soldering is closely related to solder paste printing, and 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 position of each RGB device of the high-density display will lead to uneven display of the screen, which will inevitably require higher precision mounting equipment. The mounting accuracy of Panasonic NPM equipment (QFN±0.03mm) will Meet the placement requirements above P1.0.
5.Soldering process: If the reflow soldering temperature rises too fast, it will lead to uneven wetting, which will inevitably cause the device to shift during the process of wetting imbalance. Excessive wind circulation can also cause device displacement. Try to choose a reflow soldering machine with a temperature zone above 12, chain speed, temperature rise, circulating wind, etc. as strict control items, that is, to meet the welding reliability requirements, and to reduce or avoid device 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. For maintenance tools and equipment, SMD lamps with small dot pitches can only be reflowed once in principle, and the pins can only be tinned once for no more than 3 seconds, and the temperature has strict requirements, so the maintenance requirements are very high. But from a technological point of view, it needs to be done at one time, so the quality of the previous process should be well controlled to prevent the occurrence of defective products.
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