Since the birth of light-emitting diodes ( LEDs ), full color and high brightness have been realized, and white LEDs have been developed on the basis of blue LEDs and violet LEDs. It brings a rush to the history of people's total lighting. Compared with self-igniting lamps and fluorescent lamps, LEDs have been widely used in automotive lighting, finishing lighting, telephone flash, and large and medium sizes due to their small size, all solid state, long life, environmental protection, and power saving. NB and LCD. TV and other display light source modules.

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".

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