The heat dissipation problem of the package is the key to the brightness of white LEDs

In the past, in order to obtain a sufficient white LED beam, the LED industry has developed large-size LED chips in an attempt to achieve the desired goal. However, 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 the white LED is several times larger than that of the conventional LED, the power consumption characteristic exceeds the fluorescent lamp. In this case, it is necessary to overcome the following four major problems: suppressing temperature rise, ensuring service life, improving luminous efficiency, and equalizing luminescence characteristics.

The solution to the temperature rise problem is to reduce the thermal impedance of the package; the method of maintaining the service life of the LED is to improve the chip shape and use a small chip; the method for improving the luminous efficiency of the LED is to improve the chip structure and use a small chip; The method is to improve the packaging method of LEDs, and these methods have been developed one after another.

Heat problem solving package is the fundamental method

As the increase in power will cause the thermal impedance of the package to drop sharply below 10K/W, foreign companies have developed high-temperature resistant white LEDs to try to improve the above problems. However, in fact, high-power LED heat than low-power LED high number of times or more, and the luminous efficiency will rise sharply. Even though the packaging technology allows high heat, the junction temperature of the LED chip may exceed the allowable value. Finally, the operator finally realized that solving the heat dissipation problem of the package is the fundamental method.

The service life of LEDs, such as the use of silicon packaging materials and ceramic packaging materials, can increase the lifetime of LEDs by a single digit, especially the white LED's emission spectrum contains short wavelengths of light below 450nm, traditional epoxy packaging The material is easily destroyed by short-wavelength light, and the large amount of high-power white LED accelerates the deterioration of the packaging material. According to the test results of the industry, the continuous lighting is less than 10,000 hours, and the brightness of the high-power white LED has been reduced by more than half. The basic requirements for long life of lighting sources.

The LED's luminous efficiency, improved chip structure and package 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 easy to take out light from a large chip, but the result is that the luminous efficiency is not as good as that of a low-power white LED. 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 luminescent characteristics, it is generally considered that the above-mentioned problems should be overcome as long as the uniformity of the phosphor material concentration of the white LED and the fabrication technique of the phosphor are improved. 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: 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 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 heat sink made of copper and ceramic materials, and then connect the heat dissipation wires of the printed circuit board to the air cooling by the cooling fan. On the radiator. According to the results of the German OSRAM Opto Semi conductors Gmb 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. When the packaged LED applies 2W of power, the LED chip The bonding temperature is 18K higher than the solder joint. Even if the temperature of the printed circuit board rises to 50 °C, the bonding temperature is only about 70 °C. In contrast, once the thermal impedance is lowered, the bonding temperature of the LED chip is affected by the printed circuit board. The effect of temperature. Therefore, it is necessary to try to reduce the temperature of the LED chip, in other words, reducing the thermal impedance of the LED chip to the solder joint 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 LED temperature rises as a result of which the luminous efficiency drops sharply. Therefore, Matsushita Electric developed the integrated technology of printed circuit board and package. The company packaged a 1mm square blue LED on a ceramic substrate in a flip chip manner, and then attached the ceramic substrate to the surface of the copper printed circuit board, according to the Panasonic report. The thermal impedance of the printed circuit board as a whole is approximately 15K/W.

Various manufacturers show the heat dissipation design skills

Since the compactness between the heat sink 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 fact that Lulumineds and Japanese CITIZEN lighting equipment and LED packaging manufacturers have successively developed simple heat dissipation technology for high-power LEDs, CITIZEN began to manufacture white LED sample packages in 2004, which can be thicker without special bonding technology. The heat of the 3mm heat sink is directly discharged to the outside. According to the CITIZEN report, although the thermal impedance of the LED chip's junction to the heat sink is 30K/W larger than the OSRAM's 9K/W, and the room temperature increases the thermal impedance by about 1W. The white LED module can be continuously used for lighting even when the conventional printed circuit board is not cooled by the cooling fan.

Lumileds started manufacturing high-power LED chips in 2005 with a junction temperature of +185°C, 60°C higher than other companies in the same class. When using a conventional RF 4 printed circuit board package, the ambient temperature can be input within 40°C. A current equivalent to 1.5W of power (approximately 400mA). Therefore, Lumileds and CITIZEN are used to increase the allowable temperature of the junction. The German OSRAM company sets the LED chip on the surface of the heat sink to achieve an ultra-low thermal impedance record of 9K/W, which is 40% less than the thermal impedance of the OSRAM developed in the past. It is worth mentioning that when the LED module is packaged, the same flip chip method as the conventional method is adopted, but when the LED module is bonded to the heat sink, the light emitting layer closest to the LED chip is selected as the bonding surface, thereby making the heat of the light emitting layer It is capable of conducting emissions at the shortest distance.

In 2003, Toshiba Lighting used to lay a 40mm LED module with 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. Because Toshiba Lighting has extensive experience in trial production, the company said that due to advances in analog analysis technology, white LEDs exceeding 60 lm/W after 2006 can easily use lamps and frames to improve thermal conductivity, or use air cooling fans to force air cooling. The way to design the heat dissipation of the lighting device, the module structure without special heat dissipation technology can also use the white LED.

Change the packaging material to suppress the deterioration of the material and the speed of light transmittance reduction

Regarding the longevity of LEDs, the current countermeasures adopted by LED manufacturers are to change the packaging materials and disperse the fluorescent materials in the packaging materials, especially the silicon encapsulating materials than the conventional 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 encapsulation material is less than 1%, and the halving of the luminance is less than 10,000 hours. The siliceous packaging 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 need not be replaced during the use of the lighting equipment. However, the silicone resin is a highly elastic soft material, which must be processed without scratching the surface of the silicone resin. In the production technology, in addition, the siliceous resin is extremely easy to adhere to the powder during processing, so it is necessary to develop a technology that can improve the surface characteristics in the future.

Although the silicon packaging material can ensure the life of the LED 40,000 hours, the lighting equipment industry has different views. The main argument is the service life of traditional incandescent lamps and fluorescent lamps, which is defined as "the brightness is reduced to less than 30%". 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%. At present, there are two countermeasures for extending the service life of the components, namely, suppressing the overall temperature rise of the white LED and stopping the use of the resin packaging method.

It is generally believed that if the above two life extension measures are fully implemented, the requirement of 40,000 hours when the brightness is 30% 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 law, the temperature will be extended by 10 times and the life will be extended by 2 times. Discontinuation of the resin package can completely eliminate the degradation factor, because the light generated by the LED is reflected in the encapsulation resin. If a resin-based reflector that changes the direction of light travel on the side of the chip is used, the reflector absorbs light and sharply extracts the light. Less. This is also the main reason why LED manufacturers consistently use ceramic and metal packaging materials.

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