In the past, in order to obtain sufficient white light LED beams, LED manufacturers have developed large-size LED chips in an attempt to achieve the expected goals. However, in fact, when the applied power of a white LED continues to exceed 1W, the light beam will decrease, and the luminous efficiency will be reduced by 20 to 30%. In other words, if the brightness of the white LED is several times greater than that of traditional LEDs and the power consumption characteristics exceed those of fluorescent lamps, the following four major issues must be overcome: 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 impedance of the package; the way to maintain the service life of the LED is to improve the chip shape and use small chips; the way to improve the luminous efficiency of the LED is to improve the chip structure and use small chips; as for the way to uniformize the luminous characteristics, it is to improve the packaging method of the LED, and these methods have been developed one after another.
Solving the heat dissipation problem of the package is the fundamental solution
Since increasing power will cause the thermal impedance of the package to drop sharply below 10K/W, foreign manufacturers have developed high-temperature-resistant white LEDs in an attempt to improve the above problems. However, in fact, the heat generated by high-power LEDs is dozens of times higher than that of low-power LEDs, and the temperature rise will significantly reduce the luminous efficiency. Even if the packaging technology allows high heat, the bonding temperature of the LED chip may exceed the allowable value. Finally, the industry finally realized that solving the heat dissipation problem of the packaging is the fundamental solution.
Regarding the service life of LEDs, for example, switching to silicon packaging materials and ceramic packaging materials can increase the service life of LEDs by one digit. In particular, the luminous spectrum of white LEDs contains short-wavelength light with wavelengths below 450nm. Traditional epoxy resin packaging materials are easily damaged by short-wavelength light. The large amount of light from high-power white LEDs accelerates the degradation of packaging materials. According to industry test results, After less than 10,000 hours of continuous lighting, the brightness of high-power white LEDs has been reduced by more than half, which cannot meet the basic requirements for long life of lighting sources.
Regarding the luminous efficiency of LEDs, improving the chip structure and packaging structure can reach the same level as low-power white light LEDs. The main reason is that when the current density is increased by more than 2 times, not only is it difficult to extract 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 light extraction problem can theoretically be solved.
Try to reduce thermal impedance and improve heat dissipation issues
Regarding the uniformity of luminous characteristics, it is generally believed that as long as the uniformity of phosphor material concentration and phosphor manufacturing technology of white LEDs are improved, the above problems should be overcome. While increasing the applied power as mentioned above, it is necessary to find ways to reduce thermal impedance and improve heat dissipation problems. The specific contents are: reducing the thermal impedance from the chip to the package, suppressing the thermal impedance from the package to the printed circuit substrate, and improving the smoothness of the chip's heat dissipation.
In order to reduce thermal impedance, many foreign LED manufacturers place LED chips on the surface of a heat sink made of copper and ceramic materials, and then use welding to connect the heat dissipation wires of the printed circuit board to the heat sink that uses a cooling fan to force air cooling. According to the experimental results of OSRAM Opto Semi conductors Gmb in Germany, the thermal impedance from the LED chip to the soldering point of the above structure can be reduced by 9K/W, which is about 1/6 of the traditional LED. When the packaged LED applies 2W of power, the joining temperature of the LED chip is 18K higher than the soldering point. Even if the printed circuit board temperature rises to 50°C, the joining temperature is only about 70°C at most. In contrast, once the thermal impedance is reduced in the past, the joining temperature of the LED chip will be affected by the printed circuit board temperature. Therefore, it is necessary to find ways to reduce the temperature of the LED chip. In other words, reducing the thermal impedance from the LED chip to the soldering point can effectively reduce the burden of the cooling effect of the LED chip. On the other hand, even if the white LED has a structure to suppress thermal impedance, if the heat cannot be conducted from the package to the printed circuit board If so, the rise in LED temperature will still cause a sharp drop in luminous efficiency. Therefore, Panasonic Electric Works developed printed circuit board and packaging integrated technology. The company packaged 1mm square blue LEDs on a ceramic substrate in a flip chip manner, and then pasted the ceramic substrate on the surface of the copper printed circuit board. According to Panasonic reports, the thermal impedance of the entire module including the printed circuit board is about 15K/W.
Various industries demonstrate their thermal design skills
Since the density between the radiator 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, lighting equipment and LED packaging such as Lumileds of the United States and CITIZEN of Japan Manufacturers have successively developed simple heat dissipation technologies for high-power LEDs. CITIZEN began manufacturing white LED sample packages in 2004. It can directly discharge the heat of a 2~3mm thick radiator to the outside without special joining technology. According to the CITIZEN report, although The 30K/W thermal impedance from the junction point of the LED chip to the radiator is larger than the 9K/W of OSRAM, and under normal circumstances, room temperature will increase the thermal impedance by about 1W. Even if the traditional printed circuit board is forced to air-cool without a cooling fan, the white LED module can be continuously lit.
The high-power LED chips manufactured by Lumileds in 2005 have an allowable bonding temperature of up to +185°C, which is 60°C higher than similar products from other companies. When packaged using a traditional RF 4 printed circuit board, a current equivalent to 1.5W of power (approximately 400mA) can be input within the ambient temperature range of 40°C. Therefore, Lumileds and CITIZEN increased the allowable temperature of the joint point. The German OSRAM company placed the LED chip on the surface of the radiator to achieve an ultra-low thermal impedance record of 9K/W. This record is 40% lower than the thermal impedance of similar products developed by OSRAM in the past. It is worth mentioning that the LED module is packaged using the same flip method as the traditional method. Chip method, however, when the LED module is connected to the heat sink, the light-emitting layer closest to the LED chip is selected as the joint surface, so that the heat of the light-emitting layer can be conducted and discharged over the shortest distance.
In 2003, Toshiba Lighting used to lay a low thermal impedance white LED with a luminous efficiency of 60lm/W on a 400mm square aluminum alloy surface, without special heat dissipation components such as a cooling fan, and trial-produced an led module with a light beam of 300lm. Due to Toshiba Lighting's rich experience in trial production, the company said that due to the advancement of analog analysis technology, white LEDs with more than 60lm/W after 2006 can easily use lamps and frames to improve thermal conductivity, or use cooling fans to force air-cooling to design heat dissipation of lighting equipment, and white LEDs can be used without special heat dissipation technology in the module structure.
Change the encapsulation material to inhibit material deterioration and the speed of light transmission reduction
Regarding the longevity of LEDs, the current countermeasures taken by LED manufacturers are to change the packaging materials and disperse fluorescent materials within the packaging materials. In particular, silicone packaging materials can more effectively suppress material deterioration and light transmittance reduction than traditional blue and near-ultraviolet LED chip epoxy resin packaging materials. Since the epoxy resin absorbs up to 45% of light with a wavelength of 400~450nm, the silicon encapsulation material is less than 1%. The time for the epoxy resin to halve the brightness is less than 10,000 hours, while the silicon encapsulation material can be extended to about 40,000 hours, which is almost the same as the design life of the lighting equipment. This means that the white LED does not need to be replaced during the use of the lighting equipment. However, silicone resin is a highly elastic and soft material. During processing, it is necessary to use manufacturing technology that will not scratch the surface of the silicone resin. In addition, silicone resin easily adheres to dust during processing, so technology that can improve surface properties must be developed in the future.
Although silicone packaging materials can ensure the service life of LEDs for 40,000 hours, lighting equipment manufacturers have different views. The main argument is that the service life of traditional incandescent lamps and fluorescent lamps is defined as "brightness drops below 30%." LEDs with a brightness reduction time of 40,000 hours have only about 20,000 hours left if the brightness drops below 30%. There are currently two strategies to extend the service life of components. They are to suppress the overall temperature rise of white LEDs and to stop using resin packaging.
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 white LEDs, you can use the method of cooling the LED packaging printed circuit board. The main reason is that the packaging resin will deteriorate rapidly under high temperature and strong light exposure. According to Arrhenius's law, the life span will be extended by 2 times if the temperature is reduced by 10°C. Stopping the use of resin packaging can completely eliminate the deterioration factors, because the light generated by the LED is reflected in the packaging resin. If a resin reflective plate is used that can change the direction of the light on the side of the chip, the reflective plate will absorb the light, causing the amount of light taken out to decrease sharply. This is also the main reason why LED manufacturers consistently use ceramic and metal packaging materials.
ANNA