In the past, in order to make full use of white LED beams, LED manufacturers have developed large-sized LED chips to try to achieve the desired goal in this way. However, when the applied power of white LEDs continues to exceed 1W or more, the 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 that of the traditional LED, and the power consumption characteristics hope to surpass the fluorescent lamp, it is necessary to overcome the following four major issues, including, suppressing temperature rise, ensuring service life, and improving luminescence. Equalization of efficiency and luminescence characteristics.
The specific method of temperature rise is to reduce the thermal resistance of the package; the specific method to maintain the service life of the LED is to improve the chip shape and use small chips; the specific method to improve the luminous efficiency of the LED is to improve the chip structure and use small chips; as for the uniform light emission characteristics The specific method is to improve LED packaging methods, and 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 rapidly below 10K / W, foreign companies have developed high-temperature white LEDs to try to improve the above problems. However, the heat output of high-power LEDs is actually higher than that of low-power LEDs. More than ten times, and the temperature rise will also make the luminous efficiency drop significantly. Even if the packaging technology allows high heat, the bonding temperature of the LED chip may exceed the allowable value. 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 light-emitting spectrum of white LEDs 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 large chips, but the result will be luminous efficiency. Not as good as the dilemma 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 try 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 smoothness of the heat dissipation of the chip.
In order to reduce the thermal impedance, many foreign LED manufacturers install LED chips on the surface of heat sinks made of copper and ceramic materials, and then use soldering to connect the heat dissipation wires on the printed circuit board to the cooling fan. On the forced air-cooled heat dissipation fins, according to the German OSRAM Opto Semiconductors Gmb experiment results, the thermal impedance of the LED chip with the above structure to the solder joint can be reduced by 9K / W, which is about 1/6 of the traditional LED, and the packaged LED is applied with 2W When the power is used, the bonding temperature of the LED chip is 18K higher than the soldering point. Even if the temperature of the printed circuit board rises to 500C, the bonding temperature is at most about 700C; compared with the past, if the thermal resistance decreases, the bonding temperature of the LED chip will be Affected by the temperature of the printed circuit board, it is necessary to try to reduce the temperature of the LED chip, in other words to reduce the thermal resistance of the LED chip to the solder joint, which can effectively reduce the burden of the LED chip cooling operation. Conversely, even if the white light LED has a structure that suppresses thermal resistance, if heat cannot be conducted from the package to the printed circuit board, the luminous efficiency will drop sharply as a result of the increase in the temperature of the LED. Therefore, Matsushita Electric has developed an integrated technology of printed circuit board and package. The company encapsulates a 1mm square blue LED on a ceramic substrate with a flip chip, and then pastes the ceramic substrate on the surface of a copper printed circuit board. According to Panasonic, the thermal resistance of the entire module including the printed circuit board is about 15K / W about.
Various companies demonstrate the power of heat dissipation design
Because the adhesion between the heat dissipation 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 this, American Lumileds and Japanese CITIZEN and other lighting equipment and LED packaging manufacturers have successively developed high-power LEDs. Using simple heat dissipation technology, CITIZEN started to ship white LED packages in 2004. It does not require special bonding technology and can directly dissipate the heat of the heat dissipation fins with a thickness of about 2 to 3 mm. According to the CITIZEN, although the LED chips are bonded The 30K / W thermal impedance of the point-to-heat dissipation fins is larger than that of OSRAM, and the room temperature will increase the thermal impedance by about 1W under normal conditions. However, even if the traditional printed circuit board has no cooling fan and forced air cooling, the white light The LED module can also be used continuously.
Lumileds started shipping samples of high-power LED chips in 2005. The bonding allowable temperature is up to + 1850C, which is 600C higher than that of other companies' similar products. When using traditional RF4 printed circuit board packaging, the ambient temperature range of 400C can be input equivalent to 1.5W electric current (about 400mA). Therefore, Lumileds and CITIZEN have adopted to increase the allowable temperature of the junction. The German OSRAM company has set the LED chip on the surface of the heat dissipation fin to achieve a 9K / W ultra-low thermal resistance record, which is 40% lower than the thermal resistance of the same class 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 and the thermal fin are bonded, the light emitting layer closest to the LED chip is selected as the bonding surface, thereby making the light emitting layer 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, without cooling fans and other special heat dissipation components, and tried to produce an LED module with a beam of 300lm. Toshiba Lighting has a wealth of Trial work experience, so the company said that due to the advancement of analog analysis technology, after 2006, white LEDs with more than 60lm / W can easily use lamps and frames to improve thermal conductivity, or use cooling fans to force air cooling to design lighting equipment. For heat dissipation, white LEDs can also be used for module structures that do not require special heat dissipation technology.
Change the packaging material to suppress the deterioration of the material and the speed of the decrease of 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 to disperse the fluorescent materials in the packaging materials, especially the silicon packaging materials are better than the traditional blue and near ultraviolet LED chip epoxy resin packaging materials. More effectively suppress the speed of material degradation and light penetration rate reduction. Since the percentage of epoxy resin absorbing light with a wavelength of 400-450nm is as high as 45%, the silicon encapsulation material is less than 1%, and the time for the brightness to be halved 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 LED does not need to be replaced during the use of the lighting equipment. However, silicone resin is a high-elasticity soft material, and it is necessary to use a manufacturing technique that does not scratch the surface of the silicone resin during processing. In addition, the silicone resin is very easy to adhere to the dust during the manufacturing process, so it is necessary to develop technologies that can improve the surface characteristics in the future.
Although silicon encapsulation materials can ensure the lifespan of 40,000 hours of LEDs, lighting equipment manufacturers have different opinions. The main argument is that the service life of traditional incandescent lamps and fluorescent lamps is defined as "the brightness falls below 30%." If the LED is halved for 40,000 hours, if the brightness is reduced to less than 30%, there are only about 20,000 hours left. At present, there are two countermeasures to extend the service life of components, namely, suppressing the overall temperature rise of white LEDs, 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 30% brightness of 40,000 hours 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 100C and the life will be extended by 2 times. Stopping the use of resin encapsulation can completely eliminate the deterioration factors, because the light generated by the LED is reflected in the encapsulation resin. If you use a resin reflector that can change the direction of light travel on the side of the chip, the reflector will absorb the light, so the amount of light taken out will be rapid. The sharp reduction is also the main reason why LED manufacturers have consistently adopted ceramic and metal-based packaging materials.
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