The life of the relevant LEDs, such as the use of silicon packaging materials and porcelain ceramic packaging materials, can increase the number of years of operation of LEDs by a single digit, especially the flash spectrum of white LEDs includes short-wavelength light with wavelengths below 450 nm. The traditional epoxy gas natural resin encapsulation material is easily damaged by short-wavelength light. The large amount of high-power white-light LED accelerates the deterioration of the encapsulation material. According to the final test results of the manufacturer, it is less than 10,000 hours for the high-power white LED. The brightness has been reduced by more than one-half, and there is no way to satisfy the basic requirements of the long life of the illumination source. Up to now, there have been two measures to extend the life of the components. The difference is that the temperature rise of the white LED group is restricted, and the natural resin package is used.
However, in fact, the high- power LED is more than ten times higher than the low-power LED, and the temperature rise will cause the flash rate to drop significantly. The specific internal substantive significance is to reduce the thermal impedance of the chip to the package, to limit the thermal impedance of the package to the printed circuit board, and to improve the smooth and smooth heat dissipation of the chip.
Find ways to reduce thermal impedance and improve heat dissipation
The flash rate of the associated LEDs, improved chip structure and package structure, can be achieved at the same level as low power white LEDs. In view of the lighting fixtures and LED packaging manufacturers such as Lumileds and Toyo CITIZEN, and a simple heat dissipation technology for developing high-power LEDs, CITIZEN started to make white LED sample packages in 2004, which can be thicker without special technology. The calorie of the ~3mm Heat Sink is directly discharged to the outside. According to the CITIZEN report, the 30K/W thermal impedance of the junction of the LED chip to the heat sink is larger than the 9K/W of the OSRAM, and the room temperature increases the thermal impedance by 1W at room temperature. Left and right, even if the traditional printed circuit board has no cooling fan forced air cooling, the white LED panel can also be used in conjunction with lighting.
The special properties of the relevant flash average, it is generally believed that as long as the liquid concentration average of the phosphor material of the white LED is improved and the manufacturing technique of the phosphor is used, it should be able to overcome the above-mentioned siege and disturb.
Because the increase in power will cause the thermal impedance of the package to drop rapidly below 10K/W, because this overseas industry has previously developed high-temperature resistant white LEDs, intending to improve the above-mentioned problems.
Although the silicon packaging material can guarantee the 40,000 hours of operation of the LED, but the lighting industry has revealed a different view. The main argument is that the life of the traditional light bulb and fluorescent lamp is defined as "brightness drop." Up to 30% or less." When the brightness is reduced by half, the LED is 40,000 hours. If the brightness is reduced to 30% or less, only about 20,000 hours remain.
It is generally felt that if the above two life extension measures are fully implemented, the requirement of 40% brightness and 40,000 hours can be achieved. Because of this, Matsushita Electric developed the integrated technology of printed circuit board and package. The company packaged 1mm square blue LED on the porcelain ceramic substrate in the form of flipchip, and then adhered the ceramic ceramic substrate to the appearance of the copper printed circuit board. The Panasonic report contains a printed circuit board Shunde LED display panel thermal resistance of about 15K / W. Therefore, Lumileds and CITIZEN are adopting the growth point to allow the temperature. The German OSRAM company sets the LED chip on the heat sink and achieves a 9K/W ultra-low thermal impedance record. This record is 40% less than the thermal impedance of the OSRAM. . When it comes to the LED panel package, it is considered appropriate to use the same flipchip form as the conventional method. However, when the LED panel and the heat sink are combined with each other, the flash layer closest to the LED chip is selected as the bonding surface, thereby making the flash The calories of the layer are capable of conducting emissions at the shortest distance.
In the past, in order to obtain a sufficient white LED beam, the LED industry has previously developed a large-size LED chip to achieve its intended purpose. At the same time as the increase in power is given, it is necessary to find ways to reduce the thermal impedance and improve the heat dissipation problem. However, in fact, when the white LED is given more than 1W, the light column will decrease, and the flash rate will be reduced by 20~30%. In other words, if the bright vacation of white LEDs is several times larger than that of traditional LEDs, and the special nature of power consumption exceeds the fluorescent lamps, it is necessary to overcome the four major issues listed below: restricting temperature rise, ensuring the life of survival, improving the flash rate, and The special nature of the flash is averaged. Conversely, even though white LEDs have a structure that restricts thermal impedance, if there is no way for calories to pass from the package to the printed circuit board, the final result of the LED temperature rise will make the flash rate drop rapidly.
Solving the heat dissipation problem of the package is the fundamental method
The solution to the temperature rise problem is to reduce the thermal impedance of the package; the way to maintain the life of the LED is to improve the chip shape and use a small-scale chip as appropriate; the way to improve the flash rate of the LED is to improve the chip structure and consider it appropriate. The use of small-scale chips; as for the special nature of flash averaging is to improve the packaging of LEDs, these methods have been developed. Because epoxy gas natural resin draws up to 45% of light with a wavelength of 400~450nm, silicon encapsulation material is less than 1%, and the time of halving the brightness of epoxy resin is less than 10,000 hours. Silicon package The material can be extended to about 40,000 hours, almost the same as the preset life of the lighting facilities, which means that the lighting facilities do not need to change the white LED. However, the natural resin of silicon is attributed to high elasticity and soft materials. When processing, it is indispensable to use the manufacturing technology that does not scratch the appearance of the natural silicone resin. This processing of silicon natural resin is easy to adhere to the powder, because this future is essential. Develop technologies that improve the specific properties of the exterior.
The longevity of related LEDs, until now, the countermeasures adopted by LED manufacturers are to change the packaging materials, and at the same time spread the fluorescent materials in the packaging materials, especially the silicon encapsulating materials than the traditional blue and near-ultraviolet LED chips. The packaging material can be used to control the deterioration of the material and the reduction of the light penetration rate.
Changing the packaging material limits the degradation of the material and the reduction of the light penetration rate.
In 2003, Toshiba Lighting used a 400mm square aluminum alloy surface to repair a 60lm/W low thermal impedance white LED. Without a special cooling component such as a cooling fan, try to make a 300lm LED panel. The main reason is that when the current density increases by more than 2 times, it is not only difficult to extract light from a large chip, but the final result will result in a flash rate that is not as good as that of a low-power white LED. According to the final results of the German OSRAMSemiconductorsGmb experiment, the thermal impedance of the LED chip to the solder joint of the above-mentioned structure can be reduced by 9K/W, which is about 1/6 of that of the conventional LED, and the LED after the package gives 2W of power. The bonding temperature of the chip is 18K higher than the soldering point. Even if the temperature of the printed circuit board rises to 50 °C, the combined temperature is only about 70 °C. When the thermal impedance is reduced in the past, the bonding temperature of the LED chip will suffer. The effect of printed circuit board temperature. Restricting the white LED temperature rise can be considered appropriate and using the method of cooling LED packaging printed circuit board, the main end is the high temperature condition of the packaged natural resin, plus the strong light mapping will quickly deteriorate, following the Arrhenius law to reduce the temperature by 10 ° C The number of years will be extended by 2 times.
Because the fine precision between the heat sink and the printed circuit board directly affects the heat conduction effect, the preset of the printed circuit board becomes very complicated.
In order to reduce the thermal impedance, many overseas LED manufacturers set the LED chips on the heatsink made of copper and porcelain ceramic materials, and continue to use the soldering form to connect the heat dissipation wires of the printed circuit board to the cooling fan. Air-Cooled Heat Sink. Because Toshiba Lighting has the experience of Haobo's trial system, because this company expresses the improvement of the simulation technology, and in 2006, more than 60lm/W white LED can easily use the lamp and the frame to increase the thermal conductivity. Or use a cooling fan to force the air cooling to preset the heat dissipation of the lighting device, and the white LED can also be used without the special heat dissipation technology.
Lumileds started producing high-power LED chips in 2005, combined with a temperature tolerance of +185 ° C, 60 ° C higher than other companies' same-class products. When using a traditional RF4 printed circuit board package, the surrounding ambient temperature can be input within 40 ° C. It is equivalent to 1.5W of electric current (about 400mA). This is also the main reason why LED manufacturers use ceramic pottery and metal-based packaging materials as they see fit. Even though the packaging technology allows high calories, the combined temperature of the LED chip may exceed the allowable value, and finally the operator finally realizes that solving the heat dissipation problem of the package is the fundamental method.
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