If the brightness of white LEDs is several times larger than that of conventional LEDs, and the power consumption characteristics are expected to exceed fluorescent lamps, it is necessary to overcome the following four major problems: a. suppress temperature rise; b. ensure service life; c. improve luminous efficiency; The luminescence characteristics are equalized.
The specific method for temperature rise is to reduce the thermal impedance of the package; the specific method for maintaining the service life of the LED is to improve the shape of the chip and use a small chip; the specific method for improving the luminous efficiency of the LED is to improve the chip structure and use a small chip; The specific method is to improve the packaging method of LED. It is generally believed that white light LEDs are expected to adopt the above countermeasures from 2005 to 2006.
The service life of LEDs, such as the use of silicon sealing materials and ceramic packaging materials, can increase the service life of LEDs by 10%, especially the light-emitting spectrum of white LEDs contains short-wavelength light with wavelengths below 450nm, traditional epoxy sealing materials. It is easily destroyed by short-wavelength light. The large amount of high-power white LEDs accelerates the deterioration of the sealing material. According to the test results of the industry, the continuous lighting is less than 10,000 hours, and the brightness of high-power white LEDs has been reduced by more than half. The basic requirements for long life of the light source.
Regarding the luminous efficiency of LEDs and improving the chip structure and package structure, the same level as low-power white LEDs can be achieved. The main reason is that when the current density is increased by more than 2 times, it is not easy to extract light from a large chip, but the result is luminous efficiency. It is not as good as the low-power white LED. If the electrode structure of the chip is improved, the above-mentioned light extraction problem can be solved theoretically.
Regarding the uniformity of the luminescent characteristics, it is generally considered that as long as the uniformity of the phosphor material concentration of the white LED is improved, the fabrication technique of the phosphor should overcome the above problems.
While increasing the applied power as described above, it is necessary to try to reduce the thermal impedance and improve the heat dissipation problem. The specific contents are as follows:
1 reduce the thermal impedance of the chip to the package
2 suppress the thermal impedance 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 chips on the surface of the heatsink made of copper and ceramic materials, and then connect the heat-dissipating wires on the printed circuit board to the cooling fan. On the air-cooled fins, according to the results of the German OSRAMOptoSemiconductorsGmb experiment, the thermal impedance of the LED chip to the solder joint of the above structure can be reduced by 9K/W, which is about 1/6 of that of the conventional LED, and when the packaged LED applies 2W of power. The bonding temperature of the LED chip is 18K higher than the solder joint. Even if the temperature of the printed circuit board rises to 500C, the bonding temperature is only about 700C at most. When the thermal impedance is lowered, the bonding temperature of the LED chip is affected by the printed circuit board. The influence of temperature, in this way, must try to reduce the temperature of the LED chip, in other words, reduce the thermal impedance of 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 the thermal impedance, if the heat cannot be conducted from the package to the printed circuit board, the luminous efficiency of the LED will rise sharply as a result of the rise in the temperature of the LED. Therefore, Matsushita Electric develops a printed circuit board and package integration technology. The company encapsulates a 1mm square blue LED in a flipchip package on a ceramic substrate, and then attaches the ceramic substrate to the surface of the copper printed circuit board. According to Panasonic, the thermal impedance of the module including the printed circuit board is about 15K/W. .
Because the adhesion between the heat sink 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 the lighting equipment and LED package manufacturers such as Lumileds and Japan CITIZEN, high power LEDs are successively developed. With the simple heat-dissipation technology, CITIZEN began to sample the white LED package in 2004. It can discharge the heat of the heat-dissipating fins of about 2~3mm directly to the outside without special bonding technology. According to the company, although the LED chip is bonded The 30K/W thermal impedance of the heat sink fin is larger than the OSK's 9K/W, and the room temperature increases the thermal impedance by about 1W in the general environment, but even if the conventional printed circuit board has no cooling fan forced air cooling, the white light The LED module can also be used continuously for lighting.
Lumileds began sampling high-power LED chips in 2005. The junction allowable temperature is +1850C, which is 600C higher than other companies' same-class products. When using the traditional RF4 printed circuit board package, the ambient temperature can be input within 400C. The current of 1.5W power (approximately 400mA).
As mentioned above, Lumileds and CITIZEN have adopted an increase in the allowable temperature of the joint. In Germany, OSRAM has placed the LED chip on the surface of the heat sink fin to achieve a 9K/W ultra-low thermal impedance record, which is a thermal impedance of the same grade as OSRAM. 40% reduction, it is worth mentioning that the LED module is packaged in the same flipchip method as the conventional method. However, when the LED module is bonded to the hot fin, the light emitting layer closest to the LED chip is selected as the bonding surface, thereby making the light emitting layer The heat can conduct emissions in the shortest distance.
In 2003, Toshiba Lighting Company used to lay a luminous efficiency of 60lm/W low thermal impedance white LED on a 400mm square aluminum alloy surface. Without a special cooling component such as a cooling fan, the test beam is a 300lm LED module, which is owned by Toshiba Lighting. With rich experience in trial production, the company said that due to advances in simulation analysis technology, white LEDs exceeding 60 lm/W after 2006 can easily use lamps and frames to improve thermal conductivity, or use cooling fans to force air cooling to design lighting. The heat dissipation of the device can also use white LEDs in a module structure that does not require special heat dissipation technology.
Regarding the longevity of LEDs, the current LED manufacturers are taking measures to change the sealing materials and disperse the fluorescent materials in the sealing materials. In particular, the silicon sealing materials are epoxy resin sealing materials above the traditional blue and near-ultraviolet LED chips. It is more effective in suppressing the deterioration of the material and the decrease in the light transmittance.
Since the epoxy resin absorbs up to 45% of the wavelength of 400-450 nm, the siliceous sealing material is less than 1%, and the halving of the brightness is less than 10,000 hours. The silicon sealing 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 LEDs do not need to be replaced during the use of the lighting equipment. However, the siliceous resin is a highly elastic and soft material, and it is necessary to use a manufacturing technique that does not scratch the surface of the siliceous resin, and in addition, the siliceous resin is highly liable to adhere to the powder, so it is necessary to develop a technique for improving the surface characteristics in the future.
Although the silicon sealing material can ensure the service life of the LED for 40,000 hours, the lighting equipment industry has different opinions. The main argument is the service life of the traditional incandescent lamp and the fluorescent lamp, which is defined as "the brightness is reduced to below 30%", and the brightness is defined. The LED that has been halved for 40,000 hours is about 20,000 hours if it is converted to a brightness of less than 30%. There are currently two countermeasures to extend the life of the components:
1. Suppress the overall temperature rise of white LEDs;
2. Stop using the resin packaging method.
It is generally believed that if the above two life extension measures are fully implemented, a requirement of 30% and 40,000 hours of brightness can be achieved. The method of cooling the LED light-emitting LED can be used to cool the LED package printed circuit board. The main reason is that the high temperature state of the package resin and the strong light irradiation will quickly deteriorate. According to the Arrhenius rule, the temperature will decrease by 100 times.
Discontinuation of the resin package can completely eliminate the deterioration factor, because the light generated by the LED is reflected in the encapsulation resin. If a resin-based reflector that changes the direction of the light on the side of the chip is used, since the reflector absorbs light, the amount of light taken out will be impatient. Sharp reduction, this is also 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 that is 10 times larger than a small chip (about 1mm2); the other is to use multiple small high-efficiency LED chips to form one. Single module. Although large LED chips can obtain large beams, there is a disadvantage in increasing the chip area. For example, the unevenness of the electrical boundaries of the luminescent layer in the chip, the limitation of the illuminating portion, and the emission of light generated inside the chip to the external process are severely attenuated. In response to the above problems, LED manufacturers have achieved a luminous efficiency of 50 lm/W through the improvement of the electrode structure, the use of flipchip packaging, and the integration of chip surface processing techniques.
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