The packaging methods and packaging materials of high-power LED devices cannot simply be applied to the packaging methods and packaging materials of conventional low-power LED devices. 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.
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 Ã— 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.
5AlGaInN 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 Ï†5mm 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 2.5mmÃ—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 Î¦5mm 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 Î¦5mm white LED operation, its light intensity has dropped to half. In fact, the illuminating device using a Î¦5mm 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 Î¦5mm package was originally only used for the indicator light, the thermal resistance of the package is as high as 300 Â° 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 Î¦5mm 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 Î¦5mm 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 Â° 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 Â°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 best design of the reflector cup and lens makes the radiation controllable and the optical efficiency is the highest. 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.