In the past, in order to profit from the full white LED beam, the LED industry has developed large-size LED chips to try to achieve the desired goal. However, when the power applied by the white LED continues to exceed 1W, the beam will decrease and the luminous efficiency will decrease. ~30%, in other words, if the brightness of white LEDs is several times larger than that of traditional LEDs, and the power consumption characteristics are expected to exceed fluorescent lamps, it is necessary to overcome the following four major problems, including suppressing temperature rise, ensuring service life, and improving illumination. Efficiency, and luminescence characteristics are equalized.
1 Solving the heat dissipation problem of the package is the fundamental method
As the increase of power will cause the thermal impedance of the package to drop to below 10K/W, foreign companies have developed high-temperature resistant white LEDs to try to improve the above problems. However, the heat of high-power LEDs is actually higher than that of low-power LEDs. More than ten times, and the temperature rise will also cause the luminous efficiency to drop sharply. Even if the packaging technology allows high heat, the bonding temperature of the LED chip may exceed the allowable value. Finally, the operator finally realized that solving the heat dissipation problem of the package is the fundamental method.
The service life of LEDs, such as the use of silicon packaging materials and ceramic packaging materials, can increase the lifetime of LEDs by a single digit, especially the white LED's emission spectrum contains short wavelengths of light below 450nm, traditional epoxy packaging The material is easily destroyed by short-wavelength light, and the large amount of high-power white LED accelerates the deterioration of the packaging material. According to the test results of the industry, the continuous lighting is less than 10,000 hours, and the brightness of the high-power white LED has been reduced by more than half. The basic requirements for long life of lighting sources.
Regarding the luminous efficiency of the LED, the chip structure and the package structure can be improved to the same level as the low-power white LED. The main reason is that when the current density is increased by more than 2 times, it is not easy to extract light from a large chip, but the result is luminous efficiency. It is not as good as the low-power white LED. If the electrode structure of the chip is improved, the above-mentioned light extraction problem can be solved theoretically.
2 Try to reduce thermal impedance and improve heat dissipation
Regarding the uniformity of the luminescent characteristics, it is generally considered that the above-mentioned problems should be overcome as long as the uniformity of the phosphor material concentration of the white LED and the fabrication technique of the phosphor are improved. While increasing the applied power as described above, it is necessary to reduce the thermal impedance and improve the heat dissipation problem. The specific contents are: reducing the thermal impedance of the chip to the package, suppressing the thermal impedance of the package to the printed circuit board, and improving the heat dissipation of the chip.
In order to reduce the thermal impedance, many foreign LED manufacturers set the LED chips on the surface of the heatsink made of copper and ceramic materials, and then connect the heat-dissipating wires on the printed circuit board to the cooling fan. On the air-cooled fins, according to the results of the German OSRAMOptoSemiconductorsGmb experiment, the thermal impedance of the LED chip to the solder joint of the above structure can be reduced by 9K/W, which is about 1/6 of that of the conventional LED, and when the packaged LED applies 2W of power. The bonding temperature of the LED chip is 18K higher than the solder joint. Even if the temperature of the printed circuit board rises to 500C, the bonding temperature is only about 700C at most. When the thermal impedance is lowered, the bonding temperature of the LED chip is affected by the printed circuit board. The influence of temperature, in this way, must try to reduce the temperature of the LED chip, in other words, reduce the thermal impedance of the LED chip to the solder joint, which can effectively reduce the burden of cooling the LED chip. Conversely, even if the white LED has a structure that suppresses the thermal impedance, if the heat cannot be conducted from the package to the printed circuit board, the luminous efficiency of the LED will rise sharply as a result of the rise in the temperature of the LED. Therefore, Matsushita Electric develops a printed circuit board and package integration technology. The company encapsulates a 1mm square blue LED in a flipchip package on a ceramic substrate, and then attaches the ceramic substrate to the surface of the copper printed circuit board. According to Panasonic, the thermal impedance of the module including the printed circuit board is about 15K/W. .
3 Various manufacturers show the heat dissipation design skills
Because the adhesion between the heat sink fins and the printed circuit board directly affects the heat conduction effect, the design of the printed circuit board becomes very complicated. In view of the lighting equipment and LED package manufacturers such as Lumileds and Japan CITIZEN, high power LEDs are successively developed. With the simple heat-dissipation technology, CITIZEN began to sample the white LED package in 2004. It can directly discharge the heat of the heat-dissipating fins of about 2?3mm to the outside without special bonding technology. According to the CITIZEN, although the LED chips are bonded, The 30K/W thermal impedance of the heat sink fin is larger than the OSK's 9K/W, and the room temperature increases the thermal impedance by about 1W in the general environment, but even if the conventional printed circuit board has no cooling fan forced air cooling, the white light The LED module can also be used continuously for lighting. Lumileds began sampling high-power LED chips in 2005, with a junction temperature of up to +1850C, which is 600C higher than other companies' same-class products. When using the traditional RF4 printed circuit board package, the ambient temperature can be input within 400C. The current of 1.5W power (approximately 400mA). Therefore, Lumileds and CITIZEN made it possible to increase the allowable temperature of the joint. The German OSRAM company set the LED chip on the surface of the heat sink fin to achieve a 9K/W ultra-low thermal impedance record, which is less than the thermal impedance of the OSRAM developed in the past. %, it is worth mentioning that when the LED module is packaged, the same flipchip method as the conventional method is adopted, but when the LED module is bonded with the heat fin, the light emitting layer closest to the LED chip is selected as the bonding surface, thereby making the heat of the light emitting layer It is capable of conducting emissions at the shortest distance. In 2003, Toshiba Lighting used to lay a luminous efficiency of 60lm/W low thermal impedance white LED on a 400mm square aluminum alloy surface. Without a special cooling component such as a cooling fan, the test beam is a 300lm LED module, which is rich in Toshiba Lighting. Trying to experience, so the company said that due to advances in analog analysis technology, white LEDs exceeding 60 lm/W after 2006 can easily use the lamps and frames to improve thermal conductivity, or use cooling fans to force air cooling to design lighting equipment. The heat dissipation, module structure that does not require special heat dissipation technology can also use white LEDs.
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