In the past, LED practitioners have developed large-size LED chips to achieve the desired goals in order to obtain sufficient white LED beams.
In fact, when the applied power of the white LED continues to exceed 1 W or more, the light beam will decrease, and the luminous efficiency is relatively reduced by 20 to 30%.
In other words, if the brightness of a white LED is several times larger than that of a conventional LED and the power consumption characteristics exceed the fluorescent lamp, it is necessary to overcome the following four major problems: suppressing temperature rise, ensuring the service life, improving the luminous efficiency, and equalizing the luminous characteristics. .
The solution to the temperature rise problem is to reduce the thermal impedance of the package;
The way to maintain the life of the LED is to improve the chip shape and use a small chip;
The method for improving the luminous efficiency of LEDs is to improve the chip structure and adopt a small chip;
As for the method of uniformizing the luminescent characteristics, the method of encapsulating the LEDs has been improved, and these methods have been successively developed.
Solving the heat dissipation problem of the package is the fundamental method
As the power of the LED lamp is increased, the thermal impedance of the package is drastically reduced to less than 10K/W. In response to this situation, foreign companies have developed high-temperature resistant white LEDs in an attempt to improve the above problems.
However, in fact, the heat of a high-power LED is several times higher than that of a low-power LED, and the temperature rise also causes the luminous efficiency to drop drastically.
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 tantalum 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 light of the high-power white LED accelerates the degradation of the packaging material.
According to the test of the industry, the results show that 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, which cannot meet the basic requirements of the long life of the illumination source.
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 luminescence characteristics in research and development, 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, and even if the temperature of the printed circuit board rises to 50 ° C, the bonding temperature is only about 70 ° C at most.
In contrast, once the thermal impedance is lowered, the junction temperature of the LED chip is affected by the temperature of the printed circuit board.
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 about 15K/W.
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