LED has become one of the most promising high-tech areas in the past century. LED is an injection electroluminescent device. Made of III-IV compounds, such as gallium phosphide (GaP), gallium arsenide (GaAsP) and other semiconductors under the action of ~I-DN electric field. The electrolysis of electrons and holes combines to convert a portion of the energy into light energy. That is, the quantum effect, while the lattice vibration generated by the non-radiative compounding converts the rest of the energy into heat. At present, high-brightness white LEDs have reached the level of 1001m/W in the laboratory, and 501m/w high-power white LEDs have also been commercialized. Single LED devices have also jumped from the first few milliwatts to 1,5kW. . For high-power LEDs larger than 1W, the current electro-optical conversion efficiency is about 15%, and the remaining 85% is converted into thermal energy. The chip size is only 1mm × 1mm ~ 2. 5mm ~ 2. 5mm. This means that the power density of the chip is very different from that of the conventional lighting device. The white LED does not contain the infrared portion in the luminescence spectrum. Therefore, its thermal energy cannot be relied on by radiation. Therefore, how to improve the heat dissipation capability is one of the key technical problems that need to be solved in the industrialization of high-power LEDs.
2The effect of thermal effect on high power LED
For a single LED. If the heat is concentrated in a chip of a small size, it cannot be effectively dissipated. This will cause the temperature of the chip to rise. The non-symmetric distribution causing thermal stress, the luminous efficiency of the chip, and the lasing efficiency of the phosphor are lowered. Studies have shown that when the temperature exceeds a certain value. The device's failure rate will climb exponentially. For every 2 °C rise in component temperature, reliability will drop by 10%. In order to ensure the lifetime of the device, the junction temperature of the pn junction is generally required to be below 110 °C. Follow the temperature rise of the pn junction. The white light LED device's illuminating wavelength will be red-shifted according to the count data. At a temperature of 100 ° C. The wavelength can be red shifted by 4 to 9 nm. As a result, the absorption rate of the YAG phosphor is decreased, the total luminescence intensity is reduced, and the white chromaticity is deteriorated. At room temperature, the temperature increases by 1 °C. The luminous intensity of the LED will be reduced by about 1%. When the device rises from ambient temperature to l20 °C. The brightness drops by as much as 35%. When multiple LEDs are densely arranged to form a white light illumination system. The dissipation of heat requires more serious questions from others. Therefore, solving the problem of heat dissipation that needs to be answered by others has become a prerequisite for power LED applications.
3 Research progress at home and abroad
A solution to the heat dissipation problem of high power LEDs. Device presetters and manufacturers at home and abroad have optimized the thermal system of the device in terms of layout, materials and processes. E.g. In the package layout, accept large-area chip flip layout, metal circuit board layout, thermal bath layout, micro-flow array layout, etc.; in the selection of materials, select the appropriate substrate material and adhesive material, replace the epoxy with silicone resin Resin.
3, 1 package layout
In order to meet the problem of packaging heat dissipation of high-power LEDs, various layouts have been developed internationally, including:
(1) Silicon-based flip chip (FCLED) layout
Conventional LEDs accept a formal layout that is usually coated with a layer of epoxy. The sapphire is accepted as a substrate below. Due to the poor thermal conductivity of epoxy resins. Sapphire is also a bad conductor of heat. The heat can only be dissipated by the pins under the chip, so it is difficult to dissipate heat in both front and back. Affects the performance and reliability of the device.
2001. LumiLeds has developed the A1GaInN power flip chip layout. Figure 1 shows the package layout and flip-chip layout of the chip . The LED chip is flip-chip bonded to the silicon substrate. such. The heat generated by the high-power LED does not have to pass through the sapphire substrate of the chip. Instead, it is directly transferred to a silicon or ceramic substrate with a higher thermal conductivity and then to a metal base because its active heating region is closer to the heat sink. Therefore, the internal heat sink thermal resistance can be reduced [21. The thermal resistance of this type of layout can be calculated up to 1.34K/W. Actually, it has been 6~8K/W, and the light extraction rate has also increased by about 60%. However, the thermal resistance is proportional to the thickness of the heat sink. Therefore, it is limited by the mechanical strength and thermal conductivity of the silicon wafer. It is difficult to further reduce the thermal resistance of the internal heat sink by thinning the silicon wafer, which limits the further improvement of the heat transfer performance.
(2) Metal circuit board layout
Metal circuit board layout utilizes metals such as aluminum to have excellent thermal conductivity. The chip is packaged on a PCB board covered with a copper electrode of a few millimeters thick, or the chip is packaged on a PCB of a metal core. Then, it is packaged on the heat sink to solve the problem that the LED needs to be solved by the heat caused by the increase of power. Accepting this layout can achieve good heat dissipation characteristics and greatly increase the input power of the LED.
Norlux series LEDs from UOE, USA. The packaged product is assembled on a metal core PCB with an aluminum interlayer. The PCB board is used as an electrode connection wiring for the LED device. The aluminum core interlayer acts as a heat sink to dissipate heat. Figure 2 shows the layout of a metal circuit board. The drawback is that the PCB in the interlayer is a poor conductor of heat. It blocks the conduction of heat. According to research, OSRAM's GoldenDragon series white LED chip LWW5SG is flipped over a 3ram~3mm. On the horizontally placed metal circuit board, 1898In-Sil-8 thermal interface material is applied between the LED device and the metal circuit board, and the system thermal resistance is about 66.12K/Wt".
(3) Weipu layout
In 2006, ShengLiu et al. solved the problem that the heat dissipation of LED needs to be solved by installing a micro-pull system on the heat sink, and invented that its heat dissipation performance is superior to that of the heat pipe and the heat sink. In the shutdown system, the water enters the small groove of the LED under the action of the micro-pull, and then returns to the small water container, and then absorbs heat through the fan. Figure 3 shows such a micro-pull layout. It reduces the external thermal resistance to zero. 192K/W. And can be packaged [41. Such micro-layouts have better refrigeration. However, as in the first two layouts, if the internal interface has a large thermal resistance, its heat transfer will be greatly reduced. And the layout is too complicated.
3, 2 packaging materials
After determining the package layout. The thermal resistance of the system can be further reduced by selecting different materials to improve the thermal conductivity of the system. At present, domestic and foreign materials are often preferred for substrate materials, adhesive materials and packaging materials.
(1) Substrate material
For high-power LEDs, it is necessary to solve the problem that the electrode leads are disconnected due to the thermal expansion mismatch between the chip material and the heat-dissipating material. Alloys such as porcelain ceramics, Cu/Mo plates and Cu/W plates can be used as heat-dissipating materials, but the production cost of these alloys is too high, which is not conducive to large-scale, low-cost production. The use of aluminum plates and copper plates with good thermal conductivity as the heat sink substrate is one of the current research priorities.
(2) Adhesive material
A suitable chip substrate adhesion material is selected. And in the mass production process to ensure that the adhesion thickness is as small as possible. This is important to ensure the thermal conductivity of the device. Three kinds of materials, such as thermal conductive glue, conductive silver paste and tin paste, are usually used for adhesion. Thermal pastes have a lower hardening temperature LED device packaging material. It has excellent electrical insulation properties, adhesion and dielectric properties, but epoxy resin is hygroscopic, easy to age, poor heat resistance, easy to change color under high temperature and short-wave illumination, and has certain toxicity before curing, so LED devices The life span has an impact. Many LED packaging companies have switched to silicone and porcelain ceramics instead of epoxy resin as packaging materials to improve the life of LEDs.
3, 3 summary
In general. A new package layout with low thermal resistance, good heat dissipation and low mechanical stress is the key to the package. Different layouts and materials need to solve the problem that the heat dissipation from the chip junction to the epitaxial layer, the epitaxial layer to the package substrate, the package substrate to the cooling device needs to be answered by others. The solid-state illumination source heat conduction channel composed of these three links. Any weakness in this will cause the LED light source to be destroyed. There are three types of conduction, convection and radiation from the junction to the surrounding environment. This means that the heat dissipation performance and reliability of the power LED should be maximized. All three materials must accept materials with high thermal conductivity.
4 development trends
now. Many power LEDs can drive up to 70mA, 100mA or even 1A*. With the increase of the working current, solving the problem of solving the need for heat dissipation has become a prerequisite for the industrialization of high-power LEDs. According to the above heat dissipation of the LED device. The heat dissipation performance of high power LEDs has been studied from the following aspects.
(1) The amount of thermal energy generated by LEDs depends on the internal quantum effect. Improve material layout during the growth of GaN materials. Optimize the growth parameters, obtain high-quality epitaxial wafers, improve the quantum efficiency of the device, reduce the occurrence of thermal energy from the substrate, and accelerate the heat conduction from the chip junction to the epitaxial layer.
(2) A material based on aluminum-based metal core printed circuit board (MC-PCB), porcelain ceramics, DBC, composite metal substrate, etc., is selected as a substrate to accelerate the dissipation of thermal energy from the epitaxial layer to the heat dissipation substrate. By optimizing the thermal preset of the MCPCB board. Or the porcelain ceramics can be directly bonded to the metal substrate to form a metal-based low-temperature sintered porcelain ceramic (LTCC-M) substrate to obtain good thermal conductivity. A substrate having a small coefficient of thermal expansion is heated.
(3) In order to make the thermal energy on the substrate more quickly stretched to the surrounding environment. A metal material having good thermal conductivity such as aluminum or copper is usually used as the heat sink. Forced cooling such as fans and loop heat pipes. From the perspective of cost and appearance. LED lighting should not accept external cooling devices. Therefore, according to the law of conservation of energy, the use of piezoelectric ceramics as a heat sink to convert thermal energy into a vibrating body pattern directly consumes heat energy will become one of the focuses of future research.
(4) For high-power LED devices, the total thermal resistance is the sum of the thermal resistances of several heat sinks from the pn junction to the external environment. This includes the internal heat sink resistance of the LED itself and the internal heat sink to the PCB. The thermal resistance of the thermal paste between the plates, the thermal resistance of the thermal paste between the PCB and the external heat sink, the thermal resistance of the external heat sink, etc., each heat sink in the heat transfer circuit will hinder the heat transfer. Therefore, it is considered by long-term research. The number of internal heat sinks is reduced, and the thin film process is used to construct the indispensable interface of the electric heat sink and the insulating layer directly on the metal heat sink. It is possible to greatly reduce the total thermal resistance. This type of technology is likely to become the mainstream of high-power LED thermal packaging.
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