In order to obtain sufficient white LED beams, large-size LED chips have been developed in an attempt to achieve the desired goal in this way. In fact, when the electric power applied to the white LED continues to exceed 1W or more, the beam will decline, and the luminous efficiency will be relatively reduced by 20% to 30%. The problems that must be overcome to improve the input power and luminous efficiency of the white LED are: suppress the temperature rise; Service life; improve luminous efficiency; equalize luminous characteristics. Increasing the power will reduce the thermal resistance of the white LED package to below 10K / W. Therefore, foreign countries have developed high-temperature white LEDs to try to improve the temperature rise problem. Because the heat output of the high-power white LED is several dozen times higher than that of the low-power white LED, even if the white LED package allows high heat, the allowable temperature of the white LED chip is fixed. The specific method to suppress temperature rise is to reduce the thermal resistance of the package. The specific method to improve the service life of white LED is to improve the chip shape and use small chips. Because the light-emitting spectrum of white LEDs contains short-wavelength light with a wavelength below 450 nm, traditional epoxy resin sealing materials are easily damaged by short-wavelength light, and the large amount of high-power white LEDs accelerates the deterioration of the sealing material. Switching to silicon-based sealing materials and ceramic packaging materials can increase the service life of white LEDs by one digit. The specific method of improving the luminous efficiency of white LEDs is to improve the chip structure and packaging structure to 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 As a result, 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. The specific method to achieve the uniformity of the luminescence characteristics is to improve the packaging method of the white LED. 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 can be improved, the above-mentioned problems can be overcome. The specific contents of reducing thermal impedance and improving heat dissipation are: ①Reduce the thermal resistance from chip to package. ② Suppress the thermal resistance of the package to the printed circuit board. ③ Improve 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 the heat dissipation fins made of copper and ceramic materials. Air-cooled cooling fins. The German OSRAM Opto Semiconductors Gmb experiment results confirmed that 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. When 2W of electric power is applied to the packaged LED, the temperature of the LED chip is 18 ° C higher than the solder joint. Even if the temperature of the printed circuit board rises to 500 ° C, the temperature of the LED chip is only about 700 ° C. Once the thermal impedance is reduced, the temperature of the LED chip will be affected by the temperature of the printed circuit board. For this reason, the thermal impedance of the LED chip to the solder joint must be reduced. Conversely, even if the white LED has a structure that suppresses thermal impedance, if heat cannot be conducted from the LED package to the printed circuit board, the rise in LED temperature will cause its luminous efficiency to decrease, so Panasonic has developed a printed circuit board and Integrated packaging technology, the company encapsulates a square blue LED with a length of 1mm on a ceramic substrate by chip-on-chip packaging, and then pastes the ceramic substrate on the surface of a copper printed circuit board, including the printed circuit board module The overall thermal impedance is about 15K / W. (A) OSRAM LED packaging method (B) CITIZEN LED packaging method Figure 1 LED heat dissipation structure In response to the longevity of white LEDs, the current countermeasures adopted by LED manufacturers are to change the sealing material, and at the same time disperse the fluorescent material in the sealing material, which can more effectively suppress the deterioration of the material and the rate of decrease in light transmission rate. Since epoxy resin absorbs up to 45% of the light with a wavelength of 400 ~ 450nm, the silicon sealing material is less than 1%, the epoxy resin halves the brightness in less than 10,000 hours, and the silicon sealing material can be extended to 4 Ten thousand hours or so (as shown in Figure 2), it is almost the same as the design life of the lighting equipment, which means that the lighting equipment does not need to replace the white LED during use. However, the silicon sealing material is a highly elastic and soft material, which must be used in processing The manufacturing technology that will scratch the surface of the silicon sealing material. In addition, the silicon sealing material is easy to adhere to the dust during the process. Therefore, in the future, it is necessary to develop technologies that can improve the surface characteristics. Figure 2 The influence of silicon sealing material and epoxy resin on LED optical properties Although the silicon sealing material can ensure that the white LED has a service life of 40,000 hours, the lighting equipment industry has different views. The main argument is that the service life of traditional incandescent lamps and fluorescent lamps is defined as "brightness falls below 30%". The white LED with the halving time of 40,000 hours, if converted to a brightness below 30%, only about 20,000 hours remain. There are currently two countermeasures to extend the service life of components: ①Suppress the overall temperature rise of white LED. ② Stop using resin encapsulation. The above two countermeasures can achieve the requirement of 40,000 hours of service life when the brightness drops to 30%. To suppress the temperature rise of the white LED, the method of cooling the white LED package printed circuit board can be used. The main reason is that the encapsulating resin will deteriorate rapidly under high temperature and strong light irradiation. According to the Arrhenius rule, the life is reduced when the temperature is reduced by 100 ℃ Will extend 2 times. Stopping the use of resin encapsulation can completely eliminate the deterioration factor, because the light generated by the white LED is reflected in the encapsulation resin. If you use a resin reflector that can change the direction of the light on the side of the chip, the reflector will absorb the light, so the amount of light taken out will be The sharp reduction is also the main reason for using ceramic and metal-based packaging materials. The resin-free structure of the LED package substrate is shown in Figure 3. Figure 3 LED package substrate without resin structure External slim,intrinsic majestic,Xunda Tablet gas hob is 24mmthick,with ultimate slim body,and the thickness is 1mm thinnerthan a coin,and it is beautiful and well rounded Tablet gas hob, tablet gas cooker, tablet range hood Xunda Science & Technology Group Co.ltd , https://www.xundatec.com
