Analysis and improvement of light extraction efficiency, thermal resistance, power LED packaging technology

The application of ultra-high-brightness LEDs continues to expand, first entering the market for specialty lighting and moving toward the general lighting market. Due to the continuous increase of the input power of LED chips , higher requirements are placed on the packaging technology of these power LEDs. Power LED packaging technology should meet the following two requirements: First, the package structure should have high light extraction efficiency, and second, the thermal resistance should be as low as possible, so as to ensure the photoelectric performance and reliability of the power LED.

If a semiconductor LED is to be used as an illumination source, the luminous flux of a conventional product is far from that of a general-purpose light source such as an incandescent lamp or a fluorescent lamp. Therefore, in order to develop LEDs in the field of lighting, the key is to increase the luminous efficiency and luminous flux to the level of existing lighting sources. The epitaxial materials used in power LEDs use MOCVD epitaxial growth technology and multiple quantum well structures. Although the internal quantum efficiency needs to be further improved, the biggest obstacle to obtaining high luminous flux is the low light extraction efficiency of the chip. The existing power LED design adopts a new structure of flip chip soldering to improve the light extraction efficiency of the chip, improve the thermal characteristics of the chip, and increase the photoelectric conversion efficiency of the device by increasing the chip area and increasing the operating current. Higher luminous flux. In addition to the chip, the packaging technology of the device is also important. The key packaging technology processes are:

Heat dissipation technology

The traditional indicator type LED package structure generally uses conductive or non-conductive glue to mount the chip in a small-sized reflector cup or on a carrier table. The gold wire is used to complete the internal and external connection of the device and is encapsulated with epoxy resin. Its thermal resistance is as high as 250 °C / W ~ 300 °C / W, the new power chip if the traditional LED package form, due to poor heat dissipation, the chip junction temperature rises rapidly and the epoxy carbonization turns yellow, resulting in the device Accelerated light decay until failure, even due to the stress caused by rapid thermal expansion caused by open circuit failure.

Therefore, for power LED chips with large operating current, the new package structure with low thermal resistance, good heat dissipation and low stress is the key technology of power LED devices. The chip can be bonded with a material with low resistivity and high thermal conductivity; a copper or aluminum heat sink is added to the lower portion of the chip, and a semi-encapsulated structure is used to accelerate heat dissipation; and even a secondary heat sink is designed to reduce the thermal resistance of the device. Inside the device, filled with flexible silicone rubber with high transparency, within the temperature range of silicone rubber (generally -40 ° C ~ 200 ° C), the gel will not open due to sudden changes in temperature, and will not appear yellow phenomenon. Part materials should also take into account their thermal and thermal properties to achieve good overall thermal properties.

Secondary optical design technology

In order to improve the light extraction efficiency of the device, an additional reflective cup and multiple optical lenses are designed.

Power LED white light technology

There are three common methods for achieving white light:

(1) The blue chip is coated with YAG phosphor, the blue light of the chip excites the phosphor to emit yellow-green light of 540 nm to 560 nm, and the yellow-green light and the blue light synthesize white light. The method is relatively simple to prepare, high in efficiency and practical. The disadvantage is that the consistency of the cloth is poor, the phosphor is easy to precipitate, the uniformity of the light surface is poor, the color tone is not uniform, the color temperature is high, and the color rendering is not ideal.

(2) RGB three primary colors Multiple chips or multiple devices emit light to form white light, or use blue + yellow green dual chip complementary color to produce white light. As long as the heat is dissipated, the white light produced by the method is more stable than the former method, but the driving is complicated, and the different light decay speeds of different color chips are also considered.

(3) Apply RGB phosphor on the ultraviolet light chip, and use the violet light to excite the phosphor to produce three primary colors to form white light. Due to the low efficiency of current UV chips and RGB phosphors, it has not yet reached the practical stage.

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