White LED life

Have developed large-size LED chip, in order to obtain the full white LED beam, trying in this way to achieve the desired objectives. Actually applied on the white LED electric power continues to be more than 1W beam but will decrease, luminous efficiency is relatively reduced by 20% to 30%, the problem LED High Bay Light must be overcome to improve the white LED input power and the light emitting efficiency: Suppression of temperature rise; ensure life; improve the luminous efficiency; the luminescence properties equalization.
Increased power white LED package thermal impedance is decreased to below 10K / W and abroad have developed a high temperature white LED, an attempt to improve the problem of temperature rise. High Power White LED heat than low-power white LED number more than ten times, even if the white LED package allows high-calorie, but the white LED chip allows the temperature is constant. Suppress the temperature rise of the specific method is to reduce the thermal impedance of the package.
The specific method to improve white LED service life is to improve the chip shape, a small chip. Because of the white LED luminous spectrum contains wavelengths below 450nm short wavelength light, the traditional epoxy sealing material can be easily short-wavelength light damage, high power white LED light quantity but also accelerated the deterioration of the sealing material. Switch to siliceous sealing material and ceramic packaging materials, white LED life can improve a few.
White LED luminous efficiency improvements is to improve the LED High Bay Lights IP65chip package structure, to achieve the same level with the low-power white LED, the main reason is the current density increased by 2-fold or more, not only is not easy to remove the light from the large chip, but the results will be cause the light emitting efficiency than the low-power white LED, if the improvement of the electrode structure of the chip, in theory, can solve the above-mentioned light extraction. Achieve uniform luminescent characteristics of the specific method is to improve the white LED encapsulation method, is generally believed that as long as the improvement of white LED phosphor material concentration uniformity and phosphor production technology can overcome these troubled.
Reduce the thermal impedance, improve the heat dissipation problem are:
(1) reduce the thermal impedance of the chip to the package.
(2) inhibition of the thermal impedance of the package to the printed circuit board.
(3) improve the chip thermal smoothness.
In order to reduce the thermal impedance, many foreign LED manufacturers LED Bulb Lightwill set the LED chip surface of the cooling fins in copper with a ceramic material, as shown in Figure 1, by welding the radiating conductor of the printed circuit board connected to the cooling fan forced air-cooled heat sink. The German OSRAM Opto Semiconductors the Gmb experimental results confirmed the structure of the LED chip to the thermal impedance of the soldering point can be reduced 9K / W, is about 1/6 of the conventional LED. After the LED package 2W electric power is applied, the LED chip temperature higher than the soldering point of 18 ° C, even if the printed circuit board temperature rises to 500 ° C, the temperature of the LED chip is only 700 ° C or so. Once the thermal impedance is reduced, the temperature of the LED chip will be the influence of the temperature of the printed circuit board, and this must be reduced to the LED chip to the heat resistance of the soldering point. Conversely, even if the white LED has to suppress the thermal impedance of the structure, if the heat from the LED package can not be conducted to the printed circuit board, LED temperature rise will be decreased so that the light emitting efficiency, Panasonic Corporation developed a printed circuit board and package integration technology, the company side length of 1 mm square blue LED package to cover the chip way on the ceramic substrate, ceramic substrate can then paste in the copper surface of the printed circuit board contains the module, including the printed circuit board The overall thermal impedance is about is 15K / W.
(A) OSRAM LED package (B) CITIZEN LED package Figure 1 LED heat dissipation structure Longevity problems for the white LED, the LED manufacturers to takeETL T5 LED Tube countermeasures is to change the sealing material, while the fluorescent material is dispersed in the sealing material, and a reduced speed of deterioration of the material and the light transmittance can be suppressed more effectively.
Since the epoxy absorption wavelength of 400 ~ 450nm light percentage up to 45% siliceous sealing material is less than 1% epoxy luminance half-time of less than 1 million hours, siliceous sealing material may be extended to 4 million hours (Figure 2) is almost identical with the design life of the lighting device, this means that the lighting device during use without replacement of the white LED. However siliceous sealing material is a soft, flexible material, the processing must be used on the production technology will not scratch the surface of the siliceous sealing material, in addition to the process on the siliceous sealing material easily attached to Fenxie of the future must be developed to improve the surface properties of technology.
Figure 2 siliceous sealing material with epoxy resin LED optical characteristics Although the siliceous sealing material white LED can ensure that the service life of 40,000 hours, lighting industry, however, have different views on the major issue is the life of traditional incandescent and fluorescent lamps is defined as "brightness to 30% brightness The half-time of 40,000 hours of white LED, if converted into a luminance below 30 per cent, only about 20,000 hours. There are two types of countermeasures to extend the service life of components, namely: ① suppression of white LED overall temperature rise.
(2) stop using the resin package.
The above two countermeasures can reach a service life of 40,000 hours to 30% brightness. Inhibition of white LED temperature rise can be used the method of the printed circuit board of the cooling white LED package, mainly due to the encapsulating resin under high temperature, together with the glare quickly degraded, with the Porto Vilnius rule, the lower the temperature to 100 ° C when the life will be extended two times.
Stop can completely eliminate the deterioration factors, because the white LED light is reflected in the encapsulating resin using a resin package, use can be changed if the chip side of the reflective plate of the light traveling direction of the resin material, due to the reflecting plate will absorb light, so light removing sharp drop, which is the main reason for using a ceramic and metal packaging materials. LED encapsulation substrate resin structure shown in Figure 3.
3 LED package substrate resin structure There are two methods to improve the luminous efficiency of the white LED chips:LED Candle Lamp one is the use of an area 10 times larger than the small chip (1mm2) large LED chip; another using a plurality of small high light-emitting efficiency of LED chips into a single composition body module. Although the large LED chip for large beam, but increase the chip area have a negative impact, such as light-emitting layer chip uneven, light-emitting area has been limited, the chip internally generated light radiation to an external severely attenuated. Against the above problems, by the improvement of the electrode structure on the white light LED, using a cover-chip package, while integrating the chip surface signing technology, the luminous efficiency of 50lm / W has been reached. The large white LED package is shown in Figure 4. On the whole chip uniformity of the light emitting layer, since the comb with a grid-like P-type electrode, the electrode development towards Optimizer.
Figure 4 large LED package Follow-chip package, the light-emitting layer close to the package end vulnerable to heat emissions, coupled with light emitted to the outside of the light-emitting layer electrode shield troubled, United States Lumileds Toyota cooperation has been officially cover of chip package The chip surface processing can prevent light emitted from the chip toward the outside of the chip, the reflection occurs at the interface, if the structure of the concavo-convex shape is provided on the light to remove the parts of the sapphire substrate, the the chip external light extraction efficiency can be improved about 30%. The improved LED chip package entity can make the light emitted by the side of the chip road, high efficiency reflector plate toward the top of the package to remove the chip light package size is about 7mm × 7mm. The last large LED package shown in Figure 5.
Figure 5 the last large LED package The small LED chips emitting efficiency improvement appears to be more effective than large LED chip module. For example, Japan CITIZEN combination of eight small LED chip to achieve high luminous efficiency of 60lm / W. Made with Nichia 0.3mm x 0.3mm small LED chip, a package module using up to 12 chips, each LED chip using the traditional gold wire bonding package, applied power is 2W. For luminance and color temperature of white LED uneven, must be screened in the use of the optical properties similar to white light LED. This fact reduce the unevenness of the white LED light-emitting characteristics of the LED chip emitting characteristics and the implementation of the uniform distribution of the concentration of phosphor material management is very important. Light-emitting properties of the LED chip, the manufacturers are very active in microarray light-emitting properties of the equalization process to reduce uneven LED light-emitting characteristics, such as Matsushita Electric Corporation has been reached by the screening of the chip the same characteristics of the target. The company uses a follow-chip, 64 LED chip package on a substrate, and finally cover the phosphor. LED chip processing package the luminescence properties in the sub-substrate test, followed by the light-emitting characteristics consistent chip migration package on the main base. 8 LED chip is packaged on a single substrate, even if the uneven light emission characteristics of the LED chip, the total of eight LED chip emitting characteristics in the package between the unevenness will become very small. Using a plurality of small LED chip composition to improve the uniformity of the light emitting wavelength of effect shown in Figure 6.
Figure 6 using the composition to improve the uniformity of the emission wavelength of multiple small LED chips The white LED is usually directly coated with a sealing resin containing fluorescent material LED chip, deviation may occur in the concentration of the phosphor material of the sealing resin, and finally resulting in an uneven distribution of color temperature of white LED. Accordingly, the resin sheet of the phosphor-containing material combined with the LED chip, the strict management due to the thickness of the sheet and the concentration of the phosphor material, so the color temperature of white LED uneven distribution of degree 4/5 less than the traditional way. The variation of the industry that the use of the phosphor sheet, with the light emission characteristics of the LED chip, changing the concentration of phosphor and the thickness of the sheet, you can make the color temperature of the white light LED is controlled within the expected range.
Although white LED luminous efficiency step-by-step, the possibility of the white LED lighting field is also growing, but it is clear, single white LED luminous flux is low, the current package is unlikely to achieve a single white LED lighting lumens. In response to this problem, the solution can be broadly divided into two categories: one more traditional to more than just LED light source module to which each white LED drive power and general use of the same (20 to 30mA); another type of method is to use a large area of ​​the chip, 0.3mm2 size of the chip is no longer in use at this time, while the use of a chip size of 0.6 ~ 1mm2, and use a high driving current to drive so light-emitting components (typically 150 to 350mA, up to 500mA or more). But no matter which method you use will be very high heat must be dealt with in a very small LED package, the components trapped heat, in addition to a variety of packaging materials due to the expansion coefficient different from each other and product reliability the problem, the luminous efficiency of the chip as the temperature rises and drops significantly, and causing the service life is significantly shortened. Therefore, how to disperse heat from the component, to become an important subject of the white LED packaging technology. For the purposes of the white LED, the most important is the output of luminous flux and light color, so the white LED must not end blackout coated with the epoxy material and the need to use the effect of high transparency. However, the epoxy almost all the thermally non-conductive material, so for current white LED packaging technology, mainly using white LED chip below the metal foot to disperse the heat emitted by the component. Opinion on the current trend, the metal foot material is mainly based on the high thermal conductivity of the material mainly composed of, e.g., aluminum, copper or even ceramic material, etc., but a great difference in coefficient of thermal expansion between these materials and chip, if its direct contact, it may also be because stress generated between the material at elevated temperatures and cause reliability problems, so usually in the material with an intermediate material having the appropriate conductivity and coefficient of expansion as the interval. Panasonic will only white light LED light source module made in metal material and the metal-based composite material made of a multilayer circuit board module to form, the effect of the high thermal conductivity of the light source substrate, the light source output is still in use for a long time to maintain stability. Lumileds white light produced by the LED substrate material having a high conductivity copper, and then connect it to the a special metal circuit plate, and can take into account the circuit conduction and increase the thermal conductivity effect.
High Power White LED chip manufacturing technology, packaging technology seems to have become the mainstream technology of high-brightness white LED associated with the largest chip manufacturing technology and packaging technology, however, is not just the chip area bigger, if you want the white LED used in high The brightness of the lighting field, the technology still has to be further studied.

White LED used in the field of general lighting, there are many problems to be solved, first white LED efficiency, there is still a lot of development, such as the GaInN Department of green, blue and near-UV LED efficiency margin. In addition, the internal quantum efficiency of the energy efficiency improvement is most important, the internal quantum efficiency from the active layer of non-light emitting recombination percentage of the light-emitting recombination percentage of focus can therefore be locked in a non-light emitting recombination in this part, and trying to reduce crystal defects. And to reduce UV LED indexable density indeed can significantly improve the internal quantum efficiency must further reduce its translocation density for UV LED. However, this countermeasure green, blue LED and no significant effect.
Green and blue LED when in the low current density (about 1A/cm2) having a maximum quantum efficiency of the quantum efficiency at high current density but will decrease, as shown in Figure 7. Hope LED can be driven at high current density as possible while increasing the output power of the assembly, so as soon as possible to unlock the mechanism and causes of decreased quantum efficiency in the green and blue LED of high current density, not only from a cost viewpoint Materials Physics exploration needs of the characteristics, the study also has a key role for the future application. The present study shows violet LED (wavelength of 382nm) and the quantum efficiency does not decrease even if the applied high current density (50A/cm2).

FIG 7 the GaInN Department LED quantum efficiency and current density relationship Conventional white LED is the side length of 200 ~ 350μm square chip is packaged into a round column shape, are arranged in a matrix shape after in order to obtain the desired beam lighting a plurality of white LED components, and then encapsulated. Simple high output power for the purpose and in particular the development and the area ratio of a conventional chip 6 to 10 times, dimensions up to 500μm ~ 1mm white LED Package Earn hundred milliwatts of output power (tens of lumens) However, to increase the external dimensions of the chip, but the white LED internal light absorption ratio increased external light extraction rate. AlGaInP LED chip Dimensions increase from 0.22mm × 0.22mm to 0.50mm × 0.50mm, external light extraction rates have reduced by about 20%. If the switch TIP structure, the internal multiple reflection so that the internal light absorption rate decreased, the external light extraction efficiency is significantly improved. The GaInN LED also has the same effect. How to improve the LED chip external light extraction efficiency is the key to the LED used in the field of general lighting. In addition, the high thermal impedance (150 to 200K / W) high-brightness output negative. LED internal quantum efficiency of the active layer temperature dependence great, so in addition to the low thermal impedance packaging technology, the use of a heat sink to exclude active layer heat flow become a hot research and development in the future.Undo editsAlpha
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