LED packaging quality control

　　With the advent of high-brightness blue LEDs, white LEDs can be easily obtained by combining phosphors with blue LEDs. Currently, white LEDs have become the main backlight source for portable information products, and may even become general household lighting sources in the future. In addition, high-power near-ultraviolet LEDs have appeared in recent years, and they can also be converted into white LEDs using phosphors. LEDs are characterized by small size, low power consumption, and long life. If combined with phosphors that have the characteristics of color design freedom, stability, and easy handling, they can become a new lighting source.

　　When LEDs and phosphors are usually combined, the typical method is to place the phosphor near the LED. The main reason is that the phosphor is expected to efficiently convert the wavelength of the light generated by the LED. Placing the phosphor in an area with a high light radiation density is the simplest method for wavelength conversion. In addition, the phosphor packaging method determines the luminous efficiency and color tone of white light LEDs. Therefore, the packaging technology of LEDs and phosphors will be discussed in depth from the perspective of white light.

　　Blue LED + YAG phosphor white light packaging

　　It is a commercialized white light LED. Specifically, it disperses the YAG:Ce phosphor that can generate yellow light in a transparent epoxy resin, and then uses the light generated by the blue LED in the bowl to convert it into white light. The white light emission mechanism of this method is to use LED to generate blue light, part of which will excite the YAG phosphor to turn into yellow light, and the remaining blue light will directly mix with the yellow light outside to become white light. The characteristic of this method is that the structure is simple, and only the phosphor coating process needs to be added in the LED manufacturing process, so the production cost can be greatly suppressed. Another feature is that the color adjustment is very simple.

　　As long as the color coordinates are within the straight line range formed by the color coordinates of the LED and the YAG phosphor, the color tone can be adjusted arbitrarily. It can be seen that when the concentration of the YAG phosphor is low, the ratio of blue penetrating light is higher, and the overall white light will be blue-based; conversely, if the concentration of the YAG phosphor is high, the ratio of yellow converted light is higher, and the overall white light will be yellow-based.

　　As mentioned above, using part of the blue LED as a complementary color does not require high-density (percentage with resin) phosphor coating, so the amount of phosphor used can be effectively reduced. Generally speaking, the percentage of phosphor to resin will change with the conversion efficiency of the YAG phosphor and the shape of the bowl, but a low ratio of about 10 to 20wt% can produce white light. In addition, since the light intensity emitted by the blue LED is not distributed the same on the central axis and the surroundings, even if the density of the YAG phosphor around the LED chip is exactly the same, it will still cause uneven light on the axis and the surroundings, which is also one of the issues that must be overcome in the future.

　　Both the Lead Frame Type and the Chip Type place blue LEDs in a cup and then coat them with a resin mixed with a certain amount of YAG phosphor. Due to the characteristics of LEDs being small, energy-saving, and long-lasting, they have been widely used in the backlighting of portable information products such as mobile phones and PDAs, as well as in the field of trail guide lights.