High light extraction rate and low thermal resistance power LED packaging technology

The application of ultra-high brightness LEDs is constantly expanding. They first entered the market field of special lighting and then moved towards the general lighting market. As the input power of LED chips continues to increase, higher requirements are placed on the packaging technology of these power LEDs. The packaging technology of power LEDs should mainly meet the following two requirements: first, the packaging 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 power LEDs.
If semiconductor LEDs are to be used as lighting sources, the luminous flux of conventional products is far from that of general light sources such as incandescent lamps and fluorescent lamps. Therefore, the key to the development of LEDs in the field of lighting is to increase their 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 multi-quantum well structure. Although their internal quantum efficiency needs to be further improved, the biggest obstacle to obtaining high luminous flux is still the low light extraction efficiency of the chip. The existing design of power LEDs uses a new flip-chip structure 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 working current, thereby obtaining a higher luminous flux. In addition to the chip, the packaging technology of the device is also very important. The key packaging technology processes are:
heat dissipation technology
The traditional indicator LED packaging structure generally uses conductive or non-conductive glue to install the chip in a small-sized reflective cup or on a wafer stage, and the internal and external connections of the device are completed by gold wire and then encapsulated with epoxy resin. Its thermal resistance is as high as 250℃/W~300℃/W. If the new power chip adopts the traditional LED packaging form, the chip junction temperature will rise rapidly and the epoxy carbonization will turn yellow due to poor heat dissipation, thereby causing the device to accelerate light decay until failure, and even fail due to the stress caused by rapid thermal expansion.
Therefore, for power LED chips with large working currents, new packaging structures with low thermal resistance, good heat dissipation and low stress are the technical keys of power LED devices. Low-resistance and high-thermal conductivity materials can be used to bond the chip; copper or aluminum heat sinks can be added to the bottom of the chip, and a semi-encapsulated structure can be used to accelerate heat dissipation; even secondary heat dissipation devices can be designed to reduce the thermal resistance of the device. The inside of the device is filled with flexible silicone rubber with high transparency. Within the temperature range that silicone rubber can withstand (generally -40℃～200℃), the colloid will not cause the device to open due to sudden temperature changes, nor will it turn yellow. The material of the parts should also fully consider its thermal conductivity and heat dissipation properties to obtain good overall thermal properties.
Secondary optical design technology
In order to improve the light collection efficiency of the device, an additional reflector cup and multiple optical lenses are designed.
Power LED white light technology
There are three common process methods for achieving white light:
(1) YAG phosphor is coated on the blue chip, and the blue light of the chip excites the phosphor to emit yellow-green light of 540nm～560nm, and the yellow-green light and blue light are combined to form white light. This method is relatively simple to prepare, efficient and practical. The disadvantages are that the consistency of the amount of glue is poor, the phosphor is easy to precipitate, resulting in poor uniformity of the light output surface and poor color consistency; the color temperature is high; the color rendering is not ideal.
(2) Multiple chips or multiple devices of the three primary colors of RGB emit light and mix them into white light, or use blue + yellow-green dual chips to complement the color to produce white light. As long as the heat dissipation is done properly, the white light produced by this method is more stable than the previous method, but the driving is more complicated, and the different light decay speeds of chips of different colors must also be considered.
(3) Apply RGB phosphors on the ultraviolet light chip, and use ultraviolet light to excite the phosphors to produce three primary colors of light to form white light. Due to the low efficiency of current ultraviolet light chips and RGB phosphors, it has not yet reached the practical stage.