Improve Light Extraction Efficiency, Reduce Thermal Resistance, Power LED Packaging Technology

- Aug 15, 2019-

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 for 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 flip chip soldering 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 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 reflective 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. If the new power chip adopts the traditional LED package form, the junction temperature will rise rapidly and the epoxy carbonization will turn yellow due to poor heat dissipation. Accelerated light decay until failure, even due to the stress caused by rapid thermal expansion caused by open circuit failure.


Therefore, for a power LED chip with a large operating current, a new package structure with low thermal resistance, good heat dissipation and low stress is the technical key of the power LED device. 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 part 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~560 nm, and the yellow-green light and 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 more 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.


We believe that the following technical problems must be solved in order to realize industrialization of W-class power LED products for lighting:


1. Powder coating control: The coating method used in the LED chip + phosphor process is usually to mix the phosphor with the glue and then apply it to the chip with a dispenser. During the operation, because the viscosity of the carrier gel is the dynamic parameter, the specific gravity of the phosphor is larger than that of the carrier gel, and the precision of the dispenser and the accuracy of the dispenser, the control of the uniformity of the coating amount of the phosphor is difficult, resulting in white light. Uneven color.


2, the film photoelectric parameters: the characteristics of the semiconductor process, the same material can determine the optical parameters (such as wavelength, light intensity) and electrical (such as forward voltage) parameter differences between the same wafer chip. This is especially true for RGB trichromatic chips, which have a large impact on white chromaticity parameters. This is one of the key technologies that must be solved in industrialization.


3. Control of light chromaticity parameters according to application requirements: For different purposes, the requirements for color coordinates, color temperature, color rendering, optical power (or light intensity) and spatial distribution of light of white LEDs are different. The control of the above parameters involves the cooperation of various factors such as product structure, process method and materials. In industrial production, it is important to control the above factors to obtain products that meet the application requirements and have good consistency.


Testing technology and standards


With the development of W-class power chip manufacturing technology and white LED technology, LED products are gradually entering the (special) lighting market. The traditional LED product parameter testing standards and test methods used for display or indication can no longer meet the needs of lighting applications. Semiconductor equipment manufacturers at home and abroad have also launched their own test instruments. There are certain differences in the test principles, conditions and standards used by different instruments, which increases the difficulty and complexity of the test application and product performance comparison work.


China Optical Optoelectronics Industry Association Optoelectronics Sub-Committee Industry Association released the "LED Test Method (Trial)" in 2003 according to the needs of LED product development. This test method has added regulations on LED colorimetric parameters. However, LEDs need to expand into the lighting industry. Establishing LED lighting product standards is an important means of industrial standardization.


Screening technology and reliability assurance


Due to the limitation of the appearance of the luminaire, the assembly space of the LED for illumination is sealed and limited, and the sealed and limited space is not conducive to heat dissipation of the LED, which means that the environment for illuminating the LED is inferior to the conventional display and indication LED products. In addition, the lighting LED is operated under high current drive, which puts higher reliability requirements on it. In industrial production, it is necessary to carry out appropriate thermal aging, temperature cycle shock, load aging process screening tests for different product uses, and to eliminate early failure products to ensure product reliability.


Electric protection technology


Since GaN is a wide bandgap material, the resistivity is high, and the induced charges generated by the static electricity in the production process are not easily lost, and accumulated to a considerable extent, and a high electrostatic voltage can be generated. When the material's ability to withstand is exceeded, a breakdown phenomenon occurs and discharge occurs. The blue chip of the sapphire substrate has positive and negative electrodes on the chip with a small pitch. For the InGaN/AlGaN/GaN double heterojunction, the InGaN active thin layer is only a few tens of nanometers, and the electrostatic withstand capability is small, which is extremely easy. It is broken down by static electricity to disable the device.


Therefore, in industrial production, the prevention of static electricity is appropriate, directly affecting the product yield, reliability and economic benefits. There are several techniques for preventing static electricity:


1. Take precautions against the transmission, stacking, etc. of human body, platform, ground, space and products for production and use. The means are anti-static clothing, gloves, bracelets, shoes, pads, boxes, ion fans, testing instruments, etc.


2. Design an electrostatic protection circuit on the chip.


3. Mount the protection device on the LED.


Current status of power LED packaging technology


Power LEDs are divided into power LEDs and W-class power LEDs. The input power of the power LED is less than 1W (except for tens of milliwatts of power LED); the input power of the W-class power LED is equal to or greater than 1W.


Foreign power LED packaging technology


(1) Power LED


At the earliest, HP introduced the LED of the "Piranha" package structure in the early 1990s, and introduced the improved "SnapLED" in 1994. It has two operating currents, 70mA and 150mA, and the input power can reach 0.3W. Then OSRAM introduced "PowerTOPLED". Later, some companies introduced a variety of power LED package structure. The power LEDs of these structures are several times higher than the LED input power of the original rack package, and the thermal resistance is reduced by a fraction.


(2) W-class power LED


W-class power LED is the core part of future lighting, so the world's major companies have invested a lot of power to research and develop the packaging technology of W-class power LED.


The single-chip W-class power LED was first introduced by Lumileds in 1998. The package structure is characterized by thermoelectric separation. The flip chip is directly soldered to the heat sink with a silicon carrier, and a reflective cup and optical are used. New structures and materials such as lenses and flexible transparent adhesives are now available in high-power LEDs with single-chip 1W, 3W and 5W. OSRAM launched the single-chip "GoldenDragon" series of LEDs in 2003, which is characterized by heat sinks and metals. The circuit board is in direct contact and has good heat dissipation performance, and the input power can reach 1W.


High-power LEDs with multi-chip package are available in many structures and packages. In 2001, UOE Corporation introduced the Norlux series of LEDs in a multi-chip package with a hexagonal aluminum plate as the substrate. In 2003, LaninaCeramics introduced a high-power LED array packaged on the company's proprietary low-temperature sintered ceramic (LTCC-M) technology on metal substrates. In 2003, Panasonic launched a high-power white LED packaged by a combination of 64 chips. In 2003, Nichia Corporation announced that it is the brightest white LED in the world, with a luminous flux of 600 lm and an output beam of 1000 lm. It is 30W, the maximum input power is 50W, and the white LED module that provides the exhibition has a luminous efficiency of 33lm/W.


Regarding high-power LEDs with multi-chip combination, many companies have continuously developed many new products with new structure and packaging according to actual market demand, and their development speed is very fast.


Domestic power LED packaging technology


Domestic LED packaging products are more complete. According to preliminary estimates, there are more than 200 LED packaging factories in China, with a packaging capacity of more than 20 billion/year, and the packaging capability is also very strong. However, many packaging factories are private companies, which are small in scale. However, the MB series high-power LEDs encapsulated by MetalConding technology in China's Taiwan UEC Corporation (National Union) are characterized by replacing the GaAs substrate with Si, providing good heat dissipation, and using a metal bonding layer as a light reflection layer to improve light output. .


For the research and development of high-power LED packaging technology, the country has not officially supported the investment, and domestic research units rarely intervene. The strength of investment and research (manpower and financial resources) of packaging enterprises is still not enough, forming a weak domestic development of packaging technology. The technical level of packaging is still quite different from that of foreign countries.