White light LED longevity, high power and low power consumption technology
In the past, in order to make a full profit of the beam, the industry has developed a large size and tried to achieve the desired goal by this method, but in fact, when the applied power of the white LED continues to exceed 1W, the beam will decrease, and the luminous efficiency will be relatively reduced by 20~30%. In other words, if the brightness of white LEDs is several times larger than that of traditional LEDs and the power consumption characteristics are to surpass those of fluorescent lamps, the following four major issues must be overcome first: a. suppressing temperature rise; b. ensuring service life; c. improving luminous efficiency d. Equalization of luminous properties.
The specific method for the temperature rise problem is to reduce the thermal impedance of the package; the specific method to maintain the service life of the LED is to improve the shape of the chip and use a small chip; the specific method to improve the luminous efficiency of the LED is to improve the chip structure and use a small chip; as for the uniform luminous characteristics The specific method is to improve the packaging method of LED. It is generally believed that white LEDs are expected to adopt the above-mentioned measures in 2005~2006.
The development of Jingwei to increase the power will cause the thermal impedance of the package to drop sharply below 10K/W. Therefore, foreign companies have developed high temperature resistant white LEDs to try to improve the above problems. However, the actual calorific value is dozens of times higher than that of low-power LEDs. The above, and the temperature rise will also greatly 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, the use of silicon sealing materials and ceramic packaging materials can increase the service life of LEDs by 10%, especially the luminous spectrum of white LEDs contains short-wavelength light with wavelengths below 450nm, traditional epoxy resin sealing materials It is very easy to be damaged by short-wavelength light. The large amount of light of high-power white LEDs accelerates the deterioration of sealing materials. According to the test results of the industry, the brightness of high-power white LEDs has been reduced by more than half for less than 10,000 hours of continuous lighting, which cannot satisfy the light source. Basic requirements for long life.
Regarding the luminous efficiency of LEDs, improving the 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 extract light from large chips, but it will lead to luminous efficiency. It is 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 theoretically be solved.
Regarding the uniformity of luminous characteristics, it is generally believed that as long as the uniformity of the concentration of the phosphor material of the white LED is improved, the manufacturing technology of the phosphor should be able to overcome the above problems.
As mentioned 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:
①Reduce thermal resistance from chip to package
②Suppress the thermal impedance from the package to the printed circuit
③Improve the smoothness of heat dissipation of the chip
In order to reduce thermal impedance, many foreign LED manufacturers place LED chips on the surface of heat sinks made of copper and ceramic materials, and then use soldering methods to connect the heat dissipation wires on the printed circuit board to the use of cooling fans. On the cooling fins with forced air cooling, according to the experimental results of OSRAM Opto Semiconductors Gmb in Germany, the thermal impedance from the LED chip to the solder joint of the above structure can be reduced by 9K/W, which is about 1/6 of the traditional LED, and the packaged LED applies 2W When the power is high, the bonding temperature of the LED chip is 18K higher than that of the solder joint. Even if the temperature of the printed circuit board rises to 500C, the bonding temperature is only about 700C at most. In contrast, once the thermal impedance is reduced, the bonding temperature of the LED chip will be higher. 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, reducing the thermal resistance from the LED chip to the solder joint, which can effectively reduce the burden of cooling the LED chip. Conversely, even if the white LED has a structure that suppresses thermal resistance, if the heat cannot be conducted from the package to the printed circuit board, the luminous efficiency of the LED will drop sharply as a result of the increase in the temperature of the LED. The company encapsulates the 1mm square blue LED on the ceramic substrate in the form of flip chip, and then pastes the ceramic substrate on the surface of the copper printed circuit board. According to Panasonic, the thermal impedance of the whole module including the printed circuit board is about 15K/W. about.
Because the adhesion between the heat dissipation fin 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 manufacturers such as Lumi in the United States and CITIZEN in Japan have successively developed high-power LEDs. Using simple heat dissipation technology, the white LED package that CITIZEN started to sample in 2004 can directly discharge the heat of heat dissipation fins with a thickness of about 2~3mm to the outside without special bonding technology. According to the company, although the bonding of LED chips The thermal impedance of 30K/W from the point to the cooling fin is larger than the 9K/W of OSRAM, and the room temperature will increase the thermal impedance by about 1W in a normal environment, but even if the traditional printed circuit board has no cooling fan for forced air cooling, the white light It can also be used for continuous lighting.
The high-power LED chip that Lumileds started to sample in 2005 has a higher bonding temperature of +1850C, which is 600C higher than that of other companies' products at the same level. When using the traditional RF4 printed circuit board package, the ambient temperature can be input within the range of 400C equivalent to 1.5W power current (about 400mA).
As mentioned above, Lumileds and CITIZEN have adopted to increase the allowable temperature of the junction, while OSRAM of Germany has set the LED chip on the surface of the heat dissipation fin to achieve an ultra-low thermal impedance record of 9K/W, which is higher than the thermal impedance of OSRAM's previous development of similar products. 40% reduction. 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 bonded to the thermal fin, the light-emitting layer closest to the LED chip is selected as the bonding surface, so as to make the light emitting The heat of the layer can be dissipated by conduction over the shortest distance.
In 2003, Toshiba Lighting Co., Ltd. once laid a white LED with a luminous efficiency of 60lm/W low thermal impedance on the aluminum alloy surface of 400mm square, without special heat dissipation components such as cooling fans, and tried to make an LED module with a beam of 300lm. Because Toshiba Lighting Co., Ltd. has With rich experience in trial production, the company said that due to the advancement of simulation analysis technology, white LEDs exceeding 60lm/W after 2006 can be easily used, the thermal conductivity of the frame can be improved, or the lighting equipment can be designed with forced air cooling by cooling fans. The module structure that does not require special cooling technology can also use white LEDs.
Regarding the longevity of LEDs, the current countermeasures taken by LED manufacturers are to change the sealing material, and at the same time disperse the fluorescent material in the sealing material, especially the silicon sealing material is better than the epoxy resin sealing material above the traditional blue and near-ultraviolet LED chips. It is more effective to suppress the speed of material deterioration and light transmittance reduction.
Since the percentage of epoxy resin absorbing light with a wavelength of 400~450nm is as high as 45%, the silicon sealing material is less than 1%, and the time for halving the brightness of the epoxy resin is less than 10,000 hours, and the silicon sealing material can be extended to About 40,000 hours, which is almost the same as the design life of the lighting equipment, which means that the white LEDs do not need to be replaced during the use of the lighting equipment. However, silicone resin is a highly elastic and soft material, and a manufacturing technology that does not scratch the surface of the silicone resin must be used during processing. In addition, the silicone resin is easily attached to dust during the process. Therefore, it is necessary to develop technologies that can improve the surface characteristics in the future.
Although the silicon sealing material can ensure the service life of LEDs for 40,000 hours, the lighting equipment industry has different views. The main debate is that the service life of traditional incandescent lamps and fluorescent lamps is defined as "brightness reduced to 30% or less". If the halving time of LEDs is 40,000 hours, if the brightness is reduced to less than 30%, there are only about 20,000 hours left. There are currently two countermeasures to prolong the service life of components, namely:
1. Suppress the overall temperature rise of white LEDs;
2. Stop using resin encapsulation.
It is generally believed that if the above two life extension measures are thoroughly implemented, the requirement of 30% brightness for 40,000 hours can be achieved. To suppress the temperature rise of white LEDs, the method of cooling the LED packaging printed circuit board can be used. The main reason is that the packaging resin will deteriorate rapidly under the high temperature state and strong light irradiation. According to the Arrhenius law, the life will be extended by 2 times if the temperature is reduced by 100C.
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 reflector that can change the direction of light on the side of the chip, the reflector will absorb the light, so the amount of light taken out will be sharp. This is the main reason why LED manufacturers consistently use ceramic and metal packaging materials.
There are two ways to improve the luminous efficiency of white LED chips. One is to use a large LED chip with an area that is 10 times larger than that of a small chip (about 1mm2); Single module. Although a large LED chip can obtain a large beam, increasing the chip area will have disadvantages, such as uneven electrical boundary of the light-emitting layer in the chip, limited light-emitting parts, and serious attenuation of the light generated inside the chip when it is radiated to the outside. In response to the above problems, LED manufacturers have achieved a luminous efficiency of 50lm/W by improving the electrode structure, adopting the flip chip packaging method, and integrating the chip surface processing skills.
Regarding the electrical equality of the entire chip, since the appearance of comb-shaped and mesh-shaped (mesh) p-type electrodes two or three years ago, the number of manufacturers using this method has continued to increase, and the electrodes are also developing in the direction of optimization.
Regarding the flip chip packaging method, because the light-emitting layer is close to the package end, it is easy to emit heat, and the light from the light-emitting layer is radiated to the outside without the trouble of being shielded by electrodes. Therefore, the US Lumileds and Japan Toyoda Gosei have officially adopted the flip chip packaging method. In 2005 Matsushita Electric, Matsushita Electric Works and Toshiba, which started mass production of large-scale LEDs, also followed suit. Nichia, which used wire bonding packaging in the past, and 50lm/W customer-specific LEDs released in 2004 also used flip chip packaging.
Regarding the surface processing of the chip, it can prevent the light from being reflected from the inside of the chip to the outside of the chip from being reflected at the interface. According to a Japanese LED manufacturer, when flip chip packaging, if a concave-convex structure is set on the sapphire substrate at the light extraction part, the extraction of the outside of the chip will not occur. Beam can be increased by 30%.
