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With the rapid development of LED display technology, the dot pitch is getting smaller and smaller. The LED package continues from 3528, 2020, 1515, 1010, 0505 to a smaller size to meet the high density demand, becoming a hot product in the LED industry. High-density LEDs have four trends of high pixel density, high scan ratio, high refresh rate, and high gray level. High-density LEDs can be used to combine touch, naked-eye 3D, smart applications, cloud broadcast control and other concepts to broaden the application of LED display.
Currently, the market for small-pitch LED displays is huge. The small-pitch LED display has the characteristics of no seam, low energy consumption, long life and excellent display effect. With the continuous improvement of light efficiency and the mature control technology, the LED small-pitch screen will gradually replace the original DLP and LCD. The size of the market space is also expanding.
However, the display effect of small-pitch LED display, large-resolution loading, and splicing gaps restrict the development of LED display. At the same time, the spacing of high-density LED displays is getting smaller and smaller, and the heat dissipation problem is also prominent, which seriously affects the color uniformity and display life. In order to solve the heat dissipation problem of high-density LED display, we propose a new packaging method in this paper, which can effectively solve the problems of heat dissipation and mounting difficulties.
1, the traditional packaging method
The conventional display screen mounts a driver integrated circuit (IC) on one side of a printed circuit board (PCB), and the lamp body is mounted on the other side, and the lamp body is driven to emit light through a constant current chip. Whether the layout of the constant current chip is reasonable directly affects the display effect of the display screen, and the generated heat affects the normal illumination characteristics of the LED; thereby affecting the color uniformity of the entire display screen. In addition, the miniaturization of LED lamp body size will lead to difficulty in mounting process and rework of surface mount package technology. In addition to the pixel pitch of the high-density display screen is getting smaller and smaller, the output pin of the constant current chip is also increased, and the heat dissipation problem becomes a difficult problem to be solved. The poor heat dissipation may cause unevenness of the screen body, which affects the uniformity and life of the display. .
The LED body of the LED package 1515, 2020, 3528, the pin shape is J or L package. Both the National Star 1010 and the Crystal 0505 are available in a quad flat no-lead package (QFN). QFN is a new surface mount package technology with small pad size, small size and plastic as sealing material. The exposed lead frame and the pad with direct heat dissipation channel release the heat of the chip inside the package for excellent heat dissipation. At the same time, since the conductive path between the internal pins and the pads is short, the self-inductance coefficient and the wiring resistance in the package are low, so that good Electrical performance can be provided and the overall electromagnetic interference is small. However, the solder joint is completely at the bottom, and the repair needs to remove the entire device. It is difficult to repair the high-density LED display.
2, the new packaging method
In order to solve the two difficulties of heat dissipation and rework in the traditional packaging method, this paper proposes a new packaging method, which can effectively solve these problems, and can greatly improve the reliability of the product, making it possible to arrange extremely high-density pixel LEDs. . The general rule of the development of LED technology is that it can withstand a larger current drive in a smaller LED chip area, and obtain better luminous flux and thinning characteristics to achieve better performance.
The new package is a new LED package based on ball grid array structure and chip scale package, as shown in Figure 1. The constant current driving logic chip and the LED wafer are packaged together, and the structure mainly has seven parts: epoxy filling, LED wafer, bracket body, IC chip, interconnect layer, solder ball (or bump, solder column) and The protective layer. The interconnect layer is formed by automatic soldering, wire bonding, flip chip bonding, etc. to achieve internal connection between the chip and the solder ball (or bump, solder post), which is a key component of the package.
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