Lumileds of the United States has developed the Luxeon series of high-power LED single-chip package structures. This power-type single-chip LED package structure is completely different from the conventional Φ5mm LED package structure. It is to directly solder the front-emitting LED chips to the thermal lining. Or the LED chip with the back light is first 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. High Power LED 0.5W High Power LED 3W High Power LED 5W High Power LED 10W High Power LED 50W
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.
Multi-chip package for 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. LED Energy Saving Lamp LED Lamp LED Street Light CREE LED CREE Agent
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. LED light fixture
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. LED light fixture
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. LED light 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.
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