1, high-power LED chips
To get high-power LED devices, you must prepare suitable high-power LED chips. Internationally, the following methods are commonly used to manufacture high-power LED chips:
1 increase the size method. By increasing the effective light emitting area and size of the single LED, the current flowing through the TCL layer is promoted to be evenly distributed to achieve the desired luminous flux. However, simply increasing the light emitting area does not solve the problem of heat dissipation and light emission, and does not achieve the desired luminous flux and practical application effects.
2 silicon floor flip-up method. Firstly, a large size LED chip suitable for eutectic soldering is prepared, and correspondingly sized silicon bottom plates are prepared, and a gold conductive layer and a lead conductive layer for eutectic bonding are fabricated on the silicon substrate (ultrasonic gold ball solder joints). , and then use the eutectic welding equipment to weld the large size LED chip together with the silicon substrate. This kind of structure is more reasonable. It not only considers the light output problem but also considers the heat dissipation problem. This is the current mainstream high-power LED production method.
In 2001, the United States Lumileds Corporation developed AlGaInN power type flip-chip (FCLED) structure, the manufacturing process is: First, in the P-type GaN on the top of the epitaxial wafer deposit NiAu layer thickness greater than 500A, for ohmic contact and back Reflective; the mask layer is used to selectively etch away the P-type layer and the multiple quantum well active layer to expose the N-type layer; the N-type ohmic contact layer is formed by deposition and etching, and the chip size is 1 mmÃ—1 mm. The P-type ohmic contact In a square, the N-type ohmic contact is inserted into the comb, so that the current spreading distance can be shortened to minimize the extension resistance; then the AlGaInN chip with metallized bumps is flip-chip bonded to the silicon with ESD protection diode (ESD) On the carrier.
3 ceramic floor flip-up method. Firstly, an LED chip general equipment is used to prepare a LED chip with a large light emitting area suitable for a eutectic welding electrode structure and a corresponding ceramic substrate, and a eutectic soldering conductive layer and a leaded conductive layer are fabricated on the ceramic substrate, and then eutectic soldering is performed. The device solders large size LED chips to the ceramic base plate. Such a structure not only considers the light emission problem but also considers the heat dissipation problem, and the ceramic base plate used is a ceramic plate with high thermal conductivity. The heat dissipation effect is very satisfactory and the price is relatively low. Therefore, it is a relatively suitable bottom plate material and can be used for the future. The integrated circuit integrates the reserved space.
4 sapphire substrate transition method. After a PN junction is grown on a sapphire substrate according to a conventional InGaN chip manufacturing method, the sapphire substrate is excised and then connected to a conventional quaternary material to produce a large-sized blue LED chip having upper and lower electrode structures.
5AlGaInN silicon carbide (SiC) backside light extraction method. Cree Inc. is the only manufacturer in the world to manufacture AlGaInN ultra-high-brightness LEDs using SiC substrates. The AlGaInN/SiCa chips produced over the years have been continuously improved in structure and their brightness has been continuously improved. Since the P-type and N-type electrodes are located on the bottom and top of the chip, respectively, single-wire bonding is used, the compatibility is good, and the use is convenient, thus becoming another mainstream product of AlGaInN LED development.
2, power type 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 had two operating currents of 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 original bracket-type LED, and the thermal resistance is reduced to a fraction of the original. The tile level power LED is the core part of the future lighting device, so all major companies in the world have invested a lot of effort in the research and development of tile power LED packaging technology.
LED chips and packages are developing in the direction of high power and generate 10 to 20 times more luminous flux than Ï†5mm LEDs at high currents. Therefore, effective heat dissipation and non-deteriorating packaging materials must be used to solve the problem of light decay. Therefore, the package and package are The key technology, LED packages that can withstand several watts of power have emerged. The 5W series of white, green, blue-green, and blue power LEDs was introduced to the market from the beginning of 2003. The white LED has a light output of 187 lm and an optical efficiency of 44.3 lm/W. It is currently developing LEDs that can withstand 10W power, using a large area die with a size of 2.5mm x 2.5mm, which can operate at 5A and have a light output of 200lm.
Luxeon series power LED flip-chip AlGaInN power flip-chip die to the silicon carrier with solder bumps, and then flip-welded into the thermal carrier and the shell of the silicon carrier, bonding wire for packaging. The light extraction efficiency, heat dissipation performance, and design of increased operating current density of this package are all optimal.
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 board is used as the wiring of the device electrode connection, and the aluminum core interlayer can be used as the thermal liner. Obtain 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 lighting sources such as module type, light guide plate type, condenser type, and reflection type.
When the ultra-high brightness LED is used as a signal lamp and other auxiliary illumination light source, it is generally to assemble a plurality of monochrome and white LEDs of a plurality of Î¦5mm packages on a lamp panel or a standard lamp holder, and the service life can reach 100,000 hours. Researches in 2000 have pointed out that after Î¦5mm white LED work 6000h, its light intensity has been reduced to half. In fact, the light-emitting device using a Î¦5mm white LED array may have a lifetime of only 5000 hours. 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. Because Î¦5mm packaged LEDs were originally used only for indicator lights, the thermal resistance of the package was as high as 300Â°C/W, which did not allow sufficient heat dissipation, resulting in an increase in the temperature of the LED chip, which resulted in faster device light attenuation. In addition, the yellowing of the epoxy will also reduce the light output. High-power LEDs generate 10 to 20 times more luminous flux than Î¦5mm white LEDs at high currents. Therefore, light attenuation must be solved by effective heat dissipation design and use of non-deteriorated packaging materials. The package and package have become the development of high-power LEDs. One of the key technologies. The new LED power package design concept is mainly classified into two types, one is a single-chip power type package, and the other is a multi-chip power type package.
(1) Single Chip Package of Power LED In 1998, Lumileds of the United States developed Luxeon series high power LED single chip package structure. This power single chip LED package structure is totally different from the conventional Î¦5mm LED package structure. LED chips that are exposed on the front are soldered directly to the thermal lining, or the LED chips on the back side are flipped onto the silicon carrier with solder bumps first, and then they are soldered on the thermal lining so that the large area chips are under high current. The thermal characteristics of the work are improved. This package is optimized for light extraction efficiency, heat dissipation, and current density. Its main features are:
1 Low thermal resistance. Traditional epoxy packaging has a high thermal resistance, and the thermal resistance of this new package structure is generally only 14 Â°C / W, can be reduced to 1 / 20 of conventional LED.
2 high reliability. The internally filled, stable flexible gel body does not break the gold wire and the frame lead at 40 to 120Â°C due to the internal stress caused by the sudden temperature change. With this silicone rubber as the light-coupled sealing material, yellowing does not occur with ordinary optical epoxy resins, and metal lead frames are not fouled by oxidation.
3 The optimal design of reflector cups and lenses enables controlled radiation and highest optical efficiency. In the application, they can be assembled on a circuit board (aluminum core PCB board) with aluminum interlayers. The circuit board is used as the wiring for connecting the device electrodes. The aluminum core interlayer can be used as the thermal liner of the power LED. This not only can obtain a higher luminous flux, but also has a higher photoelectric conversion efficiency.
The single-chip tile power LED was first introduced by Lumileds in 1998. The feature of this package structure is the use of thermoelectric separation. The flip chip is directly welded to a thermal carrier with a silicon carrier, and a reflector cup is used. New structures and materials such as optical lenses and flexible transparent adhesives can now provide single-chip 1W, 3W, and 5W high-power LED products. OSRAM launched 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 and has good heat dissipation performance, and the input power can reach 1W.
(2) Multi-chip combination package of power type LED The hexagonal aluminum substrate has a diameter of 3.175 cm (1.25 inches) and the light emitting region is located at the center thereof, and has a diameter of approximately 0.9525 cm (0.375 inches) and can accommodate 40 LED chips. An aluminum plate was used as the thermal liner, and the bonding wires of the chip were connected to the positive electrode and the negative electrode through two contact points made on the substrate. The number of arrayed dies on the substrate is determined according to the required output optical power. The combined packaged ultra-high brightness chips include AlGaInN and AlGaInP, and their emitted light may be monochromatic, color (RGB), white (by RGB three Basic color synthesis or binary synthesis from blue and yellow). Finally, high-refractive materials are packaged according to the optical design shape, which not only has high light efficiency, but also enables chip and bonded leads to be protected. The lumen efficiency of the LED packaged by a combination of 40 AlGaInP(AS) chips is 20 lm/W. Combined module with RGB three-color synthetic white light, when the color mixing ratio is 0:43(R) 0:48(G):0.009(B), the typical value of the luminous flux is 100lm, the color temperature of the CCT standard is 4420K, and the color coordinate x It is 0.3612 and y is 0.3529. It can be seen that this type of power LED with a high-density combined package using a conventional chip can achieve a higher brightness level, has a low thermal resistance, can operate under a large current, and has a high optical output power.
Multi-chip packaged high-power LEDs have more structures and packages. In 2001, UOE of the U.S. launched a multi-chip combo-packaged Norlux series of LEDs with hexagonal aluminum as the substrate. In 2003, Lanina Ceramics introduced a high-power LED array packaged with the company's proprietary Low Temperature Sintered Ceramic (LTCC-M) technology. Panasonic launched a high-power white LED with a combination of 64 chips in 2003. In 2003, Nichia launched an ultra-high brightness white LED with a luminous flux of 600lm and an output beam of 1000lm. The power consumption is 30W, the maximum input power is 50W, and the luminous efficiency of the white LED module is 33lm/W. The feature of the MB series high-power LEDs encapsulated by Metal Bonding technology in China's Taiwan UEC (Metro National) company is that Si is used instead of a GaAs substrate. The heat dissipation effect is good, and the metal bonding layer is used as a light reflection layer to improve Light output.
The thermal characteristics of the power LED directly affect the operating temperature, luminous efficiency, light emission wavelength, and service life of the LED. Therefore, the packaging design and manufacturing technology of the power LED chip 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 a timely manner and the junction temperature of the PN junction is kept within the allowable range, a stable light output cannot be obtained and a normal device lifetime is maintained.
The thermal conductivity of silver is the highest among commonly used heat-dissipating materials, but the cost of silver is relatively high and it is not suitable as a universal heat sink. The thermal conductivity of copper is relatively close to that of silver, and its cost is lower than that of silver. Although the thermal conductivity of aluminum is lower than that of copper, its integrated cost is the lowest, which is favorable for large-scale manufacturing.
Through experimental comparison, it has been found that a more appropriate method is to use a copper-based or silver-based thermal lining to connect the chip part, and then connect the thermal lining to the aluminum-based heat sink, adopt a ladder-type heat-conducting structure, and utilize the high thermal conductivity of copper or silver. The heat generated by the chip is efficiently transferred to the aluminum-based radiator, and heat is then dissipated through the aluminum-based radiator (disposed by air-cooling or heat conduction). The advantages of this approach are: Full consideration of the cost-effectiveness of the heat sink, combining heat sinks with different characteristics, achieving efficient heat dissipation and rationalizing cost control.
It should be noted that the choice of materials for connecting the copper-based thermal liner and the chip is very important, and the commonly used chip connection material for the LED industry is silver paste. However, after investigation, it was found that the thermal resistance of the silver paste is 10-25 W/(mÂ·K). If silver paste is used as the connecting material, it is equivalent to artificially adding a thermal resistance between the chip and the thermal liner. In addition, the internal structure of the silver paste after curing is an epoxy resin skeleton + silver powder filled heat conductive conductive structure, this structure has a high thermal resistance and a low TG point, which is extremely unfavorable for the heat dissipation and physical characteristics of the device. The solution to this problem is to use tin solder as the connection material between the die and the thermal lining [the thermal conductivity of tin is 67 W/(mÂ·K)] to obtain a more ideal thermal conductivity (thermal resistance is about 16 Â°C/W). The thermal and physical properties of tin are far superior to silver paste.
2 light out. The traditional LED device packaging method can only use about 50% of the light energy emitted by the chip, due to the large difference in refractive index between the semiconductor and the closed epoxy resin, resulting in the internal total reflection critical angle is very small, the light generated by the active layer only A small part is taken out, and most of the light is absorbed by multiple reflections inside the chip, which is the fundamental reason for the low light extraction efficiency of the ultra-high brightness LED chip. How to use 50% of the light energy consumed by refraction and reflection between different materials inside is the key to designing the optical coefficient.
The flip chip technology (Flip Chip) can get more effective light than the traditional LED chip packaging technology. However, if a reflective layer is not added under the light emitting layer and electrodes of the chip to reflect the wasted light energy, about 8% of light loss will occur, so a reflective layer must be added to the base material. The light on the side of the chip must also be reflected off the mirror surface of the thermal liner to increase the light output of the device. In addition, a layer of silica gel should be added on the light-guiding surface of the flip-chip sapphire substrate and the epoxy resin to improve the refractive index of the chip.
After the improvement of the above optical packaging technology, the light output rate (light flux) of the high-power LED device can be greatly improved. The optical design of the top lens of the high-power LED device is also very important. The usual practice is to fully consider the optical design requirements of the final lighting device when designing the optical lens, and try to design with the optical requirements of the application lighting device.
Commonly used lens shapes include: convex lenses, conical 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 the shape of the lens is limited, a semi-hermetic package can also be used. Lens materials should be made of highly transmissive glass or acrylic materials, or they can be made of conventional epoxy resin modules. The secondary heat dissipation design can also basically increase the light output.
3, the progress of power LED
The development of power LEDs started with GaAs infrared light sources in the mid-1960s. Due to its high reliability, small size, and light weight, it can be used at low voltages. Therefore, it was first used in military night-vision systems to replace the original. Some incandescent lamps, InGaAsP/InP double heterojunction infrared light source of the 1980s were used for some special test instruments to replace the existing large-size, short-lived xenon lamp. This infrared light source DC operating current up to 1A, pulsed operating current up to 24A. Although the infrared light source is an early power type LED, it has been developed so far, the product is continuously updated, applied more widely, and has become the inheritable technology foundation for the development of the current optical power LED.
In 1991, the practical use of red, orange, and yellow AlGaInP power LEDs enabled the application of LEDs to move from indoor to outdoor and was successfully used in various traffic lights, car taillights, direction lights, and outdoor information displays. The successful development of blue and green AlGaInN ultra-brightness LEDs has led to the realization of ultra-high-brightness and full-color LEDs. However, lighting is another new area for the development of ultra-high-brightness LEDs. LED solid-state lamps replace incandescent and fluorescent lamps. Traditional glass bulb lighting sources have become the LED development goals. Therefore, the R&D and industrialization of power LED will become another important direction for future development. The key to its technology is to continuously improve the luminous efficiency and the luminous flux of each device (component). The epitaxial material used for power LED uses MOCVD epitaxial growth technology and multi-quantum well structure. Although its internal quantum efficiency still needs to be further improved, the biggest obstacle in obtaining high luminous flux is still low light extraction efficiency of the chip. At present, due to the use of a conventional indicator LED package structure, the operating current is generally limited to 20mA. Power LEDs designed and fabricated according to this conventional concept simply cannot achieve the requirements of high efficiency and high luminous flux. In order to improve the luminous efficiency and luminous flux of visible light power type LEDs, a new design concept must be adopted. On the one hand, the efficiency of light extraction is improved by designing a novel chip structure, and on the other hand, the chip area is increased, the operating current is increased, and the low thermal resistance is adopted. The package structure improves the photoelectric conversion efficiency of the device. Therefore, designing and fabricating new types of chips and package structures, and continuously improving the light extraction efficiency and photoelectric conversion efficiency of the devices have been a crucial issue in the development of power LEDs.
Power LED greatly expands the application of LED in various signal display and lighting sources. It mainly includes automotive interior and exterior lights and various traffic lights, including urban traffic, railways, highways, airports, harbor lighthouses, and security warning lights. Power-type white light LEDs have been used as reading lights in automobiles and aircraft as dedicated lighting sources, and have been increasingly used in portable lighting sources such as key lamps and flashlights, backlights, and miner's lamps. In addition to the synthesis of the three primary colors, white light may also be formed by applying a special phosphor on a GaN blue or ultraviolet wavelength power LED chip. The power LED shows its unique features in comparison with its similar products in building decorative lighting, stage lighting, shopping arcade lighting, advertising light box lighting, courtyard lawn lighting, and urban nightscapes. The use of power type RGB three-color LED can be made into a compact digital light source with higher luminous efficiency than traditional incandescent light sources. With the computer control technology, it can be extremely colorful and colorful. Power LEDs have the advantages of low voltage, low power consumption, small size, light weight, long life, and high reliability. They can also be used as a special solid-state light source for field, diving, aerospace, and aviation.
With the advancement of power LED structures, optimised design of the light and thermal linings, their luminous efficiency and luminous flux have been continuously improved. Lamps and lamp heads assembled from multiple 5mm LEDs will be replaced by wicks assembled from power LEDs. In the last 30 years from 1970 to 2000, luminous flux has increased by a factor of 2 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 the power LED, the LED lighting source has attracted more attention in the lighting field. 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 direction of development, LED solid state lighting will achieve significant market breakthrough in the next 5 to 10 years.
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