In the past, in order to obtain sufficient white LED beams, LED manufacturers have developed large-size LED chips in an attempt to achieve the desired goal in this way. However, in fact, when the applied power of the white LED continues to exceed 1W or more, the light beam will decline, and the luminous efficiency will be relatively reduced by 20 ~ 30%. In other words, if the brightness of the white LED is several times greater than the traditional LED, the power consumption characteristics exceed the fluorescent In the case of a light lamp, it is necessary to overcome the following four major issues: suppressing temperature rise, ensuring service life, improving luminous efficiency, and equalizing luminous characteristics.
The solution to the temperature rise problem is to reduce the thermal resistance of the package; the method to maintain the service life of the LED is to improve the chip shape and use small chips; the method to improve the luminous efficiency of the LED is to improve the chip structure and use small chips; as for the uniformity of the light-emitting characteristics The method is to improve LED packaging methods, these methods have been developed one after another.
Solving the heat dissipation problem of the package is the fundamental method
Since the increase in power will cause the thermal resistance of the package to drop sharply below 10K / W, foreign companies have developed high-temperature white LEDs to try to improve the above problems. However, in fact, the heat output of high-power LEDs is several dozen times higher than that of low-power LEDs, and the temperature rise will also cause the luminous efficiency to drop significantly. Even if the packaging technology allows high heat, the bonding temperature of the LED chip may exceed the allowable value, and finally the industry finally realizes that solving the heat dissipation problem of the package is the fundamental method.
Regarding the service life of LEDs, for example, the use of silicon packaging materials and ceramic packaging materials can improve the service life of LEDs by one digit, especially the white LED light spectrum contains short-wavelength light below 450nm, traditional epoxy resin packaging The material is easily damaged by short-wavelength light, and the large light volume of high-power white LEDs accelerates the deterioration of packaging materials. According to the test results of the industry, the brightness of the high-power white LEDs has been reduced by more than half, which is unsatisfactory. Basic requirements for long life of lighting sources.
Regarding the luminous efficiency of LEDs, the improvement of chip structure and packaging structure can reach the same level as low-power white LEDs. The main reason is that when the current density is increased by more than 2 times, it is not only difficult to take out light from a large chip, but the result is that the luminous efficiency is not as good as that of low-power white LEDs. If the electrode structure of the chip is improved, the above-mentioned light extraction problem can be solved theoretically.
Try to reduce thermal impedance and improve heat dissipation
Regarding the uniformity of the luminescence characteristics, it is generally believed that as long as the uniformity of the phosphor material concentration of the white LED and the manufacturing technology of the phosphor are improved, the above-mentioned problems should be overcome. As described above, while increasing the applied power, it is necessary to find ways to reduce thermal impedance and improve heat dissipation. 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 smoothness of the heat dissipation of the chip.
In order to reduce the thermal impedance, many foreign LED manufacturers set the LED chip on the surface of a heat sink made of copper and ceramic materials, and then use soldering to connect the heat dissipation wires of the printed circuit board to the forced air cooling using a cooling fan On the radiator. According to the experimental results of the German OSRAM Opto Semi conductors Gmb, the thermal impedance of the above-mentioned LED chip to the solder joint can be reduced by 9K / W, which is about 1/6 of the traditional LED. The bonding temperature is 18K higher than the solder joint. Even if the temperature of the printed circuit board rises to 50 ° C, the bonding temperature is at most about 70 ° C; in contrast, once the thermal resistance is reduced, the bonding temperature of the LED chip will be affected by the printed circuit board The effect of temperature. Therefore, it is necessary to try to reduce the temperature of the LED chip, in other words, to reduce the thermal impedance of the LED chip to the soldering point, which can effectively reduce the burden of the cooling effect of the LED chip. Conversely, even if the white LED has a structure that suppresses thermal resistance, if heat cannot be conducted from the package to the printed circuit board, the result of the LED temperature rise will still cause a sharp drop in luminous efficiency. Therefore, Matsushita Electric developed integrated technology for printed circuit boards and packaging. The company packaged 1mm square blue LEDs on a ceramic substrate with a flip chip, and then attached the ceramic substrate to the surface of the copper printed circuit board. According to Panasonic reports, The thermal resistance of the entire internal module of the printed circuit board is about 15K / W.
Various companies demonstrate the power of heat dissipation design
Because the compactness between the heat sink and the printed circuit board directly affects the heat conduction effect, the design of the printed circuit board becomes very complicated. In view of this, American Lumileds and Japanese CITIZEN and other lighting equipment and LED packaging manufacturers have successively developed simple heat dissipation technologies for high-power LEDs. CITIZEN began to manufacture white LED sample packages in 2004. It does not require special bonding technology and can be about 2 ~ thick. The heat of the 3mm radiator is directly discharged to the outside. According to the CITIZEN report, although the thermal impedance of the 30K / W junction of the LED chip to the radiator is larger than the 9K / W of OSRAM, and the room temperature will increase the thermal impedance by about 1W under normal conditions. Even if the traditional printed circuit board has no cooling fan and forced air cooling, the white LED module can be used continuously.
Lumileds started manufacturing high-power LED chips in 2005. The bonding allowable temperature is up to + 185 ° C, which is 60 ° C higher than that of other companies' products. When using traditional RF 4 printed circuit board packaging, the ambient temperature can be input within 40 ° C The current is equivalent to 1.5W power (about 400mA). Therefore, Lumileds and CITIZEN are to increase the allowable temperature of the junction, and the German OSRAM company is to place the LED chip on the surface of the heat sink to achieve a 9K / W ultra-low thermal resistance record, which is 40% lower than the thermal resistance of the same level products developed by OSRAM in the past. It is worth mentioning that the LED module is packaged using the same flip chip method as the traditional method, but when the LED module is connected to the heat sink, the light emitting layer closest to the LED chip is selected as the bonding surface, thereby making the light emitting layer The heat can be conducted and discharged in the shortest distance.
In 2003, Toshiba Lighting used a low-impedance white LED with a luminous efficiency of 60lm / W on a 400mm square aluminum alloy surface, and without trial cooling components such as cooling fans, trial-produced an LED module with a beam of 300lm. Because Toshiba Lighting has extensive trial production experience, the company said that due to the advancement of analog analysis technology, after 2006, white LEDs over 60lm / W can easily use lamps and frames to improve thermal conductivity, or use cooling fans to force air cooling The design of heat dissipation of lighting equipment can use white LEDs without the need for special heat dissipation technology.
Change the packaging material to suppress the deterioration of the material and the speed of the decrease of the light transmittance
Regarding the longevity of LEDs, the current countermeasures adopted by LED manufacturers are to change the packaging materials, and at the same time disperse the fluorescent materials in the packaging materials, especially the silicon packaging materials are better than the traditional blue, near ultraviolet LED chip epoxy resin packaging materials, It can more effectively suppress the speed of material degradation and light penetration reduction. Since the percentage of epoxy resin absorbing light with a wavelength of 400 ~ 450nm is as high as 45%, and the silicon encapsulation material is less than 1%, the brightness halving time is less than 10,000 hours. The silicon encapsulation material can be extended to About 40,000 hours, almost the same as the design life of the lighting equipment, which means that the white light LED does not need to be replaced during the use of the lighting equipment. However, the silicone resin is a highly elastic and soft material, which must be used during processing without scratching the surface of the silicone resin Manufacturing technology, in addition, silicon resin is very easy to adhere to dust during processing, so in the future, it is necessary to develop technologies that can improve surface characteristics.
Although silicon packaging materials can ensure the life of 40,000 hours of LEDs, lighting equipment manufacturers have different views. The main argument is that the life of traditional incandescent lamps and fluorescent lamps is defined as "the brightness falls below 30%." . The LED with the brightness halved for 40,000 hours, if converted to a brightness below 30%, only about 20,000 hours remain. At present, there are two countermeasures to extend the service life of components, namely, suppressing the temperature rise of the whole white LED, and stopping the use of resin encapsulation.
It is generally believed that if the above two life extension measures are thoroughly implemented, the requirement of 40,000 hours at 30% brightness can be achieved. To suppress the temperature rise of the white LED, the method of cooling the LED package printed circuit board can be used. The main reason is that the high temperature of the packaging resin, combined with strong light irradiation, will quickly deteriorate. According to the Arrhenius rule, the temperature will be reduced by 10 ° C and the life will be doubled. Stopping the use of resin encapsulation can completely eliminate the deterioration factor, because the light generated by the LED is reflected in the encapsulation resin. If you use a resin reflective plate that can change the direction of light traveling on the side of the chip, the reflective plate will absorb the light and make the amount of light extraction sharp. Less. This is also the main reason why LED manufacturers consistently use ceramic and metal-based packaging materials.
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