From a practical point of view, high-power LED devices that are simple to install and relatively small in size will replace traditional low-power LED devices in most lighting applications. In order to meet the needs of lighting, lighting fixtures composed of low-power LEDs must concentrate the light energy of many LEDs to meet the design requirements, but the disadvantages are that the wiring is extremely complicated and the heat dissipation is not smooth. In order to balance the current between the LEDs, For voltage relationships, complex power supply circuits must be designed. In contrast, the power of the high-power single-chip LED is much larger than the sum of the power of several small-power LEDs, the power supply line is relatively simple, the heat dissipation structure is perfect, and the physical characteristics are stable. Therefore, the packaging method and packaging material of the high-power LED device cannot simply apply the packaging method and packaging material of the conventional low-power LED device. Large dissipated power, large heat generation and high light extraction efficiency put new and higher requirements on LED packaging technology , packaging equipment and packaging materials.
1, high power LED chip
In order to get a high-power LED device, it is necessary to prepare a suitable high-power LED chip. The international methods for manufacturing high-power LED chips are as follows:
1 Increase the size method. By increasing the effective light-emitting area and size of the individual LEDs, the current flowing through the TCL layer is evenly distributed to achieve the desired luminous flux. However, simply increasing the light-emitting area does not solve the heat dissipation problem and the light-emitting problem, and does not achieve the expected luminous flux and practical application effects.
2 Silicon floor flip method. Firstly, a large-sized LED chip suitable for eutectic soldering is prepared, and a silicon substrate of a corresponding size is prepared, and a gold conductive layer for eutectic soldering and a conductive layer (ultrasonic gold ball solder joint) are formed on the silicon substrate. Then, the eutectic soldering device is used to solder the large-sized LED chip to the silicon substrate. This kind of structure is more reasonable, considering both the light-emitting problem and the heat-dissipation problem, which is the current mainstream high-power LED production method.
In 2001, Lumileds developed the AlGaInN power flip-chip (FCLED) structure. The manufacturing process is: firstly deposit a NiAu layer with a thickness greater than 500A on the P-type GaN on top of the epitaxial wafer for ohmic contact and back. Reflection; then using a mask to selectively etch away the P-type layer and the multi-quantum well active layer, exposing the N-type layer; forming and etching to form an N-type ohmic contact layer, the chip size is 1 mm × 1 mm, P-type ohmic contact In a square shape, an N-type ohmic contact is inserted in a comb shape, which shortens the current spreading distance and minimizes the expansion resistance; then, the metal bumped AlGaInN chip is flip-chip bonded to the silicon with an antistatic protection diode (ESD) On the carrier.
3 ceramic floor flip method. Firstly, an LED chip with a large light-emitting area suitable for the eutectic soldering electrode structure and a corresponding ceramic substrate are prepared by using an LED chip general-purpose device, and a eutectic soldering conductive layer and a conductive layer are formed on the ceramic substrate, and then eutectic soldering is performed. The device solders large LED chips to the ceramic backplane. Such a structure not only considers the light-emitting problem but also the heat-dissipation problem, and the ceramic bottom plate is a high-heat-conducting ceramic plate, the heat dissipation effect is very ideal, and the price is relatively low, so it is a suitable floor material at present, and can be used for the future. Integrated circuit integrated package reserved space.
4 Sapphire substrate transition method. After the PN junction is grown on the sapphire substrate according to the conventional InGaN chip fabrication method, the sapphire substrate is cut away, and then the conventional quaternary material is connected to fabricate a large-sized blue LED chip having the upper and lower electrode structures.
5 AlGaInN silicon carbide (SiC) back light extraction method. Cree is the only manufacturer in the world to manufacture AlGaInN ultra-high brightness LEDs using SiC substrate. The structure of AlGaInN/SiCa chips produced in recent years has been continuously improved and the brightness has been continuously improved. Since the P-type and N-type electrodes are respectively located at the bottom and the top of the chip, single-wire bonding is adopted, which has good compatibility and convenient use, and thus becomes another mainstream product of AlGaInN LED development.
2, power package
Power LEDs first began with HP's introduction of "Piranha" packaged LEDs in the early 1990s. The company's improved "Snap LED" introduced in 1994 has two operating currents, 70mA and 150mA, respectively. Power up to 0.3W. The input power of the power LED is several times higher than the input power of the LED of the original bracket package, and the thermal resistance is reduced by a fraction of the original. The tile-level power LED is the core part of the future lighting device, so the world's major companies have invested a lot of effort to research and develop the packaging technology of the tile-level power LED.
The LED chip and package are developed in the direction of high power. Under high current, the luminous flux is 10~20 times larger than that of φ5mm LED. It is necessary to use effective heat dissipation and non-degrading packaging materials to solve the problem of light decay. Therefore, the package and package are Key technologies, LED packages that can withstand several watts of power have emerged. The 5W series of white, green, blue-green and blue power LEDs have been introduced to the market since the beginning of 2003. The light output of white LEDs is 187lm and the luminous efficiency is 44.3lm/W. At present, LEDs capable of withstanding 10W power are being developed, using a large-area die with a size of 2.5mm×2.5mm, which can work at a current of 5A and a light output of 200lm.
The Luxeon series power LEDs are flip-chip soldered to a silicon carrier with solder bumps, and then the flip-chip soldered silicon carrier is placed in a thermal liner and a package, and the bonding leads are packaged. The design of this package is optimal for light extraction efficiency, heat dissipation and increased operating current density.
In the application, the packaged product can be assembled on a metal core PCB with aluminum interlayer to form a power density LED. The PCB is used as the wiring for the device electrode connection, and the aluminum core interlayer can be used as a thermal lining. Achieve higher luminous flux and photoelectric conversion efficiency. In addition, the packaged SMD-LEDs are small in size and can be flexibly combined to form a variety of illumination sources such as a module type, a light guide plate type, a condensing type, and a reflection type.
When used as a signal light and other auxiliary illumination sources, ultra-high-brightness LEDs are typically assembled in a variety of Φ5mm packaged monochromatic and white LEDs on a single lamp or standard lamp holder for a lifetime of up to 100,000 hours. According to research in 2000, after 6,000 hours of Φ5mm white LED operation, its light intensity has dropped to half. In fact, the illuminating device using a Φ5mm white LED array may have a lifetime of only 5000h. Different colors of LEDs have different light attenuation speeds, among which red is the slowest, blue and green are centered, and white is the fastest. Since the LED of the Φ5mm package was originally only used for the indicator light, the thermal resistance of the package is as high as 300 ° C / W, and the heat cannot be sufficiently radiated, so that the temperature of the LED chip rises, and the light attenuation of the device is accelerated. In addition, yellowing of the epoxy resin will also reduce the light output. High-power LEDs generate 10 to 20 times more luminous flux than Φ5mm white LEDs at high currents. Therefore, it is necessary to solve the problem of light decay through effective heat dissipation design and packaging materials without deterioration. The package and package have become high-power LEDs. One of the key technologies. The new LED power package design concept is mainly divided into two categories, one is a single-chip power package, and the other is a multi-chip power package.
(1) Single-chip package of power LED
In 1998, Lumileds Company of the United States developed the Luxeon series of high-power LED single-chip package structure. This power type single-chip LED package structure is completely different from the conventional Φ5mm LED package structure. It is to directly solder the LED chip with front side light to the thermal lining. The LED chip on the back side is flipped on the silicon carrier with solder bumps, and then soldered to the thermal lining, so that the thermal characteristics of the large-area chip operating at a large current are improved. This package is optimized for light extraction efficiency, thermal performance and current density. Its main features are:
1 Low thermal resistance. Traditional epoxy packages have high high thermal resistance, and the thermal resistance of this new package is typically only 14 ° C / W, which can be reduced to 1 / 20 of conventional LEDs.
2 High reliability. The internal filled stable flexible gel does not break the gold wire and the frame lead due to internal stress caused by sudden temperature changes at 40~120 °C. By using this silicone rubber as a light-coupled sealing material, yellowing phenomenon like ordinary optical epoxy resin does not occur, and the metal lead frame is not contaminated by oxidation.
3 The optimal design of the reflector cup and lens makes the radiation controllable and optically efficient. In the application, they can be assembled on a circuit board with aluminum interlayer (aluminum core PCB), the circuit board is used as the wiring for the device electrode connection, and the aluminum core interlayer can be used as the thermal lining of the power LED. This not only achieves higher luminous flux, but also has higher photoelectric conversion efficiency.
The single-chip watt-level 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 thermal lining with a silicon carrier, and a reflective cup is used. New structures and materials such as optical lenses and flexible transparent adhesives are now available in high-power LED products with single-chip 1W, 3W and 5W. OSRAM introduced the single-chip Golden Dragon series LED in 2003. Its structural feature is that the thermal lining is in direct contact with the metal circuit board. It has good heat dissipation performance and the input power can reach 1W.
(2) Multi-chip package of power LED
The hexagonal aluminum substrate has a diameter of 3.175 cm (1.25 inches) and the light-emitting area is located at the center thereof, and has a diameter of about 0.9525 cm (0.375 inch) and can accommodate 40 LED chips. An aluminum plate was used as the thermal lining, and the bonding wires of the chip were connected to the positive and negative electrodes through two contact points made on the substrate. The number of aligned dies on the substrate is determined according to the required output optical power. The packaged ultra-high brightness chips include AlGaInN and AlGaInP, and their emitted light can be monochromatic, color (RGB), white (by RGB three). Primary color synthesis or binary synthesis from blue and yellow). Finally, the high refractive index material is packaged according to the optical design shape, which not only has high light extraction efficiency, but also protects the chip and the bonded leads. The LED packaged by 40 AlGaInP (AS) chip packages has a lumen efficiency of 20 lm/W. A combination package module using RGB three primary colors to synthesize white light, when the color mixture ratio is 0:43 (R) 0:48 (G): 0.009 (B), the luminous flux is typically 100 lm, and the CCT standard color temperature is 4420 K, color coordinate x It is 0.3612 and y is 0.3529. It can be seen that the power LED of the high-density combination package using the conventional chip can achieve a high brightness level, has the characteristics of low thermal resistance, high current operation and high light output power.
The high-power LEDs of the multi-chip combination package have 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, Lanina Ceramics 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 introduced a high-power white LED packaged by a combination of 64 chips. In 2003, Nichia introduced ultra-high brightness white LEDs with a luminous flux of 600 lm. When the output beam is 1000 lm, the power consumption is 30 W, the maximum input power is 50 W, and the luminous efficiency of the white LED module is 33 lm/W. The MB series high-power LEDs encapsulated by UEC (China Union) Corporation using Metal Bonding technology are characterized by replacing the GaAs substrate with Si, and the heat dissipation effect is good, and the metal bonding layer is used as the light reflection layer to improve The light output.
The thermal characteristics of power LEDs directly affect the operating temperature, luminous efficiency, wavelength of illumination, and lifetime of LEDs. Therefore, the packaging design and manufacturing technology of power LED chips are particularly important. The main issues to consider in high-power LED packages are:
1 heat dissipation. Heat dissipation is critical for power LED devices. If the heat generated by the current cannot be dissipated in time, keeping the junction temperature of the PN junction within the allowable range, stable light output and normal device life will not be obtained.
Silver has the highest thermal conductivity among commonly used heat-dissipating materials, but the cost of silver is high and it is not suitable for general-purpose heat sinks. The thermal conductivity of copper is closer to silver and its cost is lower than silver. Although the thermal conductivity of aluminum is lower than that of copper, its overall cost is the lowest, which is conducive to large-scale manufacturing.
After experimental comparison, it is more appropriate to use a copper-based or silver-based thermal lining on the connecting chip, and then connect the thermal lining to the aluminum-based heat sink, using a stepped heat-conducting structure, using a high thermal conductivity of copper or silver. The heat generated by the chip is efficiently transferred to the aluminum-based heat sink, and the heat is dissipated through the aluminum-based heat sink (dissipated by air cooling or heat conduction). The advantage of this method is: fully consider the cost performance of the radiator, and combine the radiators with different characteristics to achieve efficient heat dissipation and rationalize cost control.
It should be noted that the choice of materials for connecting the copper-based thermal lining and the chip is very important, and the chip connecting material commonly used in the LED industry is silver glue. However, after research, it is found that the thermal resistance of silver glue is 10~25W/(m·K). If silver glue is used as the connecting material, it is equivalent to artificially adding a thermal resistance between the chip and the thermal lining. In addition, the internal basic structure of the silver paste after curing is an epoxy resin skeleton + a silver powder filled thermal conductive structure, and the structure has extremely high thermal resistance and low TG point, which is extremely disadvantageous for heat dissipation and physical stability of the device. The solution to this problem is to use tin plate welding as the connecting material between the die and the thermal lining [the thermal conductivity of tin is 67W/(m·K)], and the ideal thermal conductivity can be obtained (the thermal resistance is about 16). °C/W). Tin has much better thermal conductivity and physical properties than silver.
2 light. The traditional LED device packaging method can only use about 50% of the light energy emitted by the chip. Because the refractive index difference between the semiconductor and the sealing epoxy resin is large, the critical angle of total internal reflection is small, and the light generated by the active layer is only A small portion is taken out, and most of the light is absorbed by multiple reflections inside the chip, which is the root cause of the low light-receiving efficiency of the ultra-high-brightness LED chip. How to use 50% of the light energy consumed by the refraction and reflection between different materials inside is the key to designing the light coefficient.
Through the chip flip chip technology (Flip Chip) can get more effective light output than the traditional LED chip packaging technology. However, if a reflective layer is not added under the luminescent layer of the chip and under the electrode to reflect the wasted light energy, about 8% of the light is lost, so a reflective layer must be added to the substrate material. The light on the side of the chip must also be reflected by the mirror processing of the thermal lining to increase the light extraction rate of the device. Moreover, a layer of silica gel material should be added on the sapphire substrate portion of the flip chip and the light-guide bonding surface of the epoxy resin to improve the refractive index of the chip light.
Through the improvement of the above optical packaging technology, the light extraction rate (light flux) of the high-power LED device can be greatly improved. The optical design of the top lens of high-power LED devices is also very important. The usual practice is to fully consider the optical design requirements of the final lighting fixture when designing the optical lens, and design it as far as possible to meet the optical requirements of the lighting fixture.
Common lens shapes include: convex lenses, concave cone lenses, spherical mirrors, Fresnel lenses, and combined lenses. The ideal assembly method for lenses and high-power LED devices is to adopt a hermetic package. If limited by the shape of the lens, a semi-hermetic package can also be used. The lens material should be made of synthetic materials such as glass or acrylic with high light transmittance. It can also be packaged in a traditional epoxy resin module, and the secondary heat dissipation design can basically achieve the effect of improving the light extraction rate.
3. Progress of power LED
The development of power LEDs began in the mid-1960s with GaAs infrared light sources. Because of its high reliability, small size, and light weight, it can work at low voltages. Therefore, it was first used in military night vision devices to replace the original. Some incandescent lamps, in the 1980s InGaAsP / InP double heterojunction infrared source was used in some special test equipment to replace the original large, short-lived xenon lamp. The infrared light source has a DC operating current of up to 1A and a pulsed operating current of up to 24A. Although the infrared light source is an early power LED, it has been developed ever since, and its products have been continuously updated and applied more widely, and it has become the inherited technical foundation for the development of optical power LED.
In 1991, the practical application of red, orange and yellow AlGaInP power LEDs enabled the application of LEDs from indoors to outdoors, and was successfully used in various traffic lights, taillights, directional lights and outdoor information displays. Blue and green AlGaInN ultra-high-brightness LEDs have been successfully developed, achieving ultra-high brightness full-color LED. However, lighting is another new field of ultra-high-brightness LED expansion, replacing incandescent and fluorescent lamps with LED solid-state lamps. Such traditional glass bulb lighting sources have become the development goal of LED. Therefore, the development and industrialization of power LEDs will become another important direction for future development. The key to the technology is to continuously improve the luminous efficiency and the luminous flux of each device (component). 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. At present, the operating current is generally limited to 20 mA due to the conventional LED-type LED structure. Power LEDs designed and fabricated in accordance with this conventional concept simply cannot achieve high efficiency and high luminous flux requirements. In order to improve the luminous efficiency and luminous flux of visible light power LEDs, a new design concept must be adopted. On the one hand, the design of the new chip structure is used to improve the light extraction efficiency, and on the other hand, by increasing the chip area, increasing the operating current, and adopting low thermal resistance. The package structure improves the photoelectric conversion efficiency of the device. Therefore, designing and fabricating new chips and package structures, and continuously improving the light extraction efficiency and photoelectric conversion efficiency of devices, has always been a crucial issue in the development of power LEDs.
Power LEDs greatly expand the application of LEDs in various signal display and illumination sources, including automotive interior and exterior lights and various traffic lights, including urban traffic, railways, highways, airports, harbor lighthouses, and security warning lights. Power-type white LEDs have been used as reading lights in automobiles and airplanes as dedicated lighting sources, and are increasingly used in applications such as portable lighting sources (such as key lights, flashlights), backlights, and miners. In addition to being synthesized from three primary colors, white light can also be formed by coating a special phosphor onto a GaN blue or ultraviolet wavelength power LED chip. Power LEDs show its unique characteristics compared with similar products in building decorative light source, stage lighting, shopping mall window lighting, advertising light box lighting, courtyard lawn lighting, and urban night scenes. The power type RGB trichromatic LED can be used to make a digital dimming light source with a compact structure and higher luminous efficiency than the traditional incandescent light source. With computer control technology, an extremely colorful luminous effect can be obtained. The low voltage, low power consumption, small size, light weight, long life and high reliability of the power LED make it a special solid light source for field, diving, aerospace and aviation.
Advances in power LED architectures, optimized design of light extraction and thermal lining have resulted in improved luminous efficiency and luminous flux. Lamp panels and bases assembled from multiple 5mm LEDs will be replaced by wicks assembled from power LEDs. From the last 30 years from 1970 to 2000, the luminous flux has doubled every 18 to 24 months. Since the introduction of the Norlux series of power LEDs in 1998, the increase in luminous flux has been even faster.
With the improvement of the performance of power LEDs, LED lighting sources have attracted greater attention in the field of lighting. The demand for the general lighting market is huge, and the power LED white light technology will be more suitable for general lighting applications. As long as the LED industry can continue this development direction, LED solid lighting will achieve significant market breakthroughs in the next 5 to 10 years.
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