There is no doubt that the world needs high-brightness light-emitting diodes (HB LEDs ), not only high-brightness white LEDs (HB WLEDs), but also high-brightness LEDs of various colors, and the future is more active and demanding. Brightness LED (UHD LED).
Replacing the original EL backlight and CCFL backlight of the handheld device with LED backlight not only makes the circuit design simpler and easier, but also has higher resistance to external forces. Replacing the original CCFL backlight of LCD TVs with LED backlights is not only more environmentally friendly, but also displays more vivid and beautiful. LED lighting instead of white light, halogen light and other lighting, not only more bright and energy-saving, the use is also longer, and the lighting response is faster, can reduce the rear car collision rate when used in the vehicle lights. Therefore, LEDs can only be used in the state of the electronic device in the past, to become the backlight of the liquid crystal display, and then extended to electronic lighting and public display, such as car lights, traffic lights, information billboards, large video walls, and even It is the illumination inside the projector, etc., and its application continues to extend. More importantly, the brightness efficiency of LEDs is doubled every 24 months, just like Moore's Law. In the past, white LEDs could only be used to replace incandescent and halogen lamps that consume too much power. The luminous efficiency is in the range of 10 to 30 lm/W. However, after the white LED breaks through 60 lm/W or even 100 lm/W, even fluorescent lamps and high-pressure gas discharge lamps are beginning to feel threatened.
Although LEDs continue to enhance brightness and luminous efficiency, in addition to the core patented technologies such as phosphor and light mixing, packaging will be an increasingly challenging challenge and a double challenge. On the one hand, packaging must allow LEDs. It has the highest light extraction rate and the highest luminous flux, which minimizes the light loss, and also pays attention to the divergence angle of the light, the light uniformity, and the compatibility with the light guide plate. On the other hand, the package must have the best heat dissipation of the LED, especially HB (high brightness) almost means HP (high power, high power), the current value of the incoming and outgoing LEDs continues to increase, if not good heat dissipation, Not only will the brightness of the LED be reduced, but the life of the LED will also be shortened. Therefore, the continuous pursuit of high-brightness LEDs, if the package technology used does not have the corresponding enhancement, then the high-brightness performance will be discounted. Therefore, this article will discuss more about the packaging technology of HB LEDs, including the discussion of optical communication. Also includes discussion on thermal conductivity.
Bare layer: "quantum well, multi-quantum well" enhances "light conversion efficiency"
Although this article is mainly about the enhancement of the luminous flux of the LED package , it is necessary to first explain the bare part of the deeper core. After all, the improvement of the bare crystal structure can also greatly increase the luminous flux. The first is to enhance the efficiency of light conversion, which is also the most fundamental source. Only 15% to 2% of the electricity used per watt of LED is converted into light energy, and the rest are converted into heat energy and dissipated (waste heat). The key to improving this conversion efficiency is on the pn junction. The pn junction is the main illuminating heating position of the LED. The structural design change through the pn junction can improve the conversion efficiency. At present, quantum wells (Qumbo) are built on the pn junction to increase the proportion of electricity converted into light energy. Further, efforts will be made to more chiseling, that is, multiple quantum wells. (Multiple Quantum Well; MQW) technology.
"Refueling and remodeling, light and light folding" to raise the "lighting efficiency"
If the light conversion efficiency is difficult to re-require, further steps must be taken from the level of light extraction efficiency. There are quite a few different methods at this level. Different materials are different according to different materials. Currently, the two compounds commonly used for HB LEDs are AlGaInP and GaN/ InGaN, the former is used to produce high-brightness orange, orange, yellow, and green light, while the latter is used to produce green, emerald, and blue light, and to produce near-ultraviolet, blue-green, and blue light using InGaN. The method includes changing the physical geometry (transverse to vertical), changing the material of the substrate, adding a new material layer, changing the bonding mode of the material layer, and different surface treatment of the material. However, no matter how it changes, it basically does not deviate from two principles: First, reduce the shadow and increase the light transmittance. Second, strengthen the utilization of light refraction and reflection. For example, in the past, AlGaInP LEDs used GaAs as the substrate. However, GaAs on the black surface occludes half of the light emitted from the pn junction, causing waste of light energy. Therefore, a transparent GaP material is used as the substrate. Another example Nichia Chemical Industries (Nichia), the p-type electrode (p type) is made wavy portion (Mesh Pattern), in order to increase the transparency of the p-electrode, while reducing the lift amount of light hindering the light through. As for the increase of the fold reflection, a layer of DBR (Distributed Bragg Reflector) reflective layer is added to the structure of the AlGaInP, and the other side of the light source is folded to the same side. In GaN, the substrate material is replaced with sapphire (aluminum oxide) to increase the reflection, and the surface of the substrate is designed to be embossed, thereby increasing the scattering angle after light reflection, thereby increasing the light extraction rate. Or as Osram (OSRAM) using the SiC material substrate, and the substrate was designed as a ramp, but also help to increase reflection, or the addition of silver, a metal mirror layer of aluminum.
Encapsulation layer: anti-aging yellow light, light transmittance protection warfare <br> After trying to increase the brightness from the bare crystal level, it is officially taken over from the package level to ensure the highest light transmission and minimum light attenuation.
To have a high lumen retention (Transmittance), the first step is to package the material. In the past, the most commonly used LED was epoxy, but the epoxy resin will gradually turn yellow after aging, which will affect the bright color, especially the lower the wavelength, the faster the aging, especially the partial ultraviolet light of some WLEDs. Compared with other visible light, its wavelength is lower and aging is faster. The new proposal is to use silicone. For example, the Luxeon series of LEDs from Lumileds of the United States is a silicone sealant. Not only Lumileds Luxeon, but also other companies have silicone solutions, such as General Electric. Toshiba's InvisiSi1, Toray. Dow Corning's SR 7010 and so on are also LED silicone packaging solutions.
In addition to the low resistance to low wavelength, silica gel is less susceptible to aging. Silica gel blocks the near-ultraviolet light so that it does not leak. It is also a kind of protection for human health. In addition, the light transmittance, refractive index and heat resistance of silica gel are all Very ideal. GE Toshiba's InvisiSi1 has a refractive index of up to 1.5 to 1.53, a light transmittance of 95% between 350nm and 800nm, and 75% to 80% of light transmission at wavelengths as low as 300nm. 1.41, 95% light transmission is maintained even at a wavelength of 300 nm. Dow Coring Toray's SR 7010 has a light transmission rate of 99% above 405 nm and a refractive index of 1.51 after hardening, and a heat resistance of 180 ° C to 200 ° C. In addition, some manufacturers have proposed a so-called resin-free package, that is, glass is used as a jacket protection. For example, the ceramic package proposed by Kyocera of Japan is proposed for anti-aging, and ceramics also have better heat resistance.
Encapsulation layer: reflection and refraction of the transflective cup of the lens <br> After being encapsulated with glue, there are various connection methods depending on the different uses of the LED, for example, one by one, and the most typical package in the past. This is the case with a single LED indicator . The other is to combine multiple LEDs into one integral component, such as a seven-segment display, a dot matrix display, and the like. In addition, there are two distinctions between soldering feet, namely, Through-Hole Technology and Surface-Mount Technology.
For individual, separate, discrete packages, there are different package appearances depending on the application. If it is used as a status indicator for piercing welding, it is only necessary to adopt a Lamp type package (commonly known as "bullet type"), even if there is a difference in lens type (Lens Type), such as a typical Lamp. Egg oval Oval, super oval El Oval, straight Flat and so on. If it is a surface-adhesive type, there are also Top View, Side View, Dome and so on.
Why do you have a variety of different lens shapes? In general, Lamp is used for indicator numbers, Oval for outdoor signs or signs, Top View for direct-lit backlights, Flat and Side View with Guide Plate for side-lit light. Backlight, Dome as a small lighting bulb, compact flash, etc. The appearance is different, the application is different, and the viewing angle of the light is different. This part will test the package design again. Different illumination modes, light intensity, and luminous flux can be obtained by different design methods. There are four common methods in this aspect: Axial lens, flat lens, reflective cup, reflective cup, reflective cup by reflective cup. Island. The general Lamp uses the central axis lens method, and the Dome and Oval/Super Oval are similar, but the Oval/Super Oval is more concentrated than the Lamp in the small axial direction. Flat uses the straight lens method. The advantage is that the optical angle of view is larger than that of the central lens method, but the disadvantage is that the luminous flux is reduced and the light intensity is weakened. For Top View, Side View, etc., a reflective cup or an island reflector cup is often used. This method is to add a mirror to the package to reflect and refract a part of the divergent angle beam, so that the angle and the light intensity can be balanced.
As far as technical difficulty is concerned, the Axial lens and the Flat lens using only the upper lens are relatively simple. As long as the transmission and the beam divergence are considered, the opposite Reflective cup is different. The original transmission and divergence must be considered, and also consideration should be considered. Reflection, refraction, and beam convergence are more complicated.
In terms of materials, the lens part can be changed to other materials in addition to the original laminating material, because the lens has been more light-transparent and does not pay attention to the bare crystal protection, so it can also adopt plastic (plastic), acrylic (Acrylic), glass, polycarbonate, etc., and as mentioned before, the light transmittance is related to the wavelength, the light transmittance of different wavelengths is different, plus different materials can be selected, even Lens coloring, to increase the contrast of light color, or depending on the decorative effect of the application (toy, Christmas tree), as well as the geometric design of the front lens, reflector cup, etc., all of which constitute the fourth topic of LED light communication .
Conclusion HB LED is emphasized as "green lighting", the meaning of "environmental protection" is its great point of appeal, so not only lead (Pb Free) packaging, but also today's European RoHS (Restriction of Hazardous Substances Directive) The legal norms, whether package or LED as a whole, cannot contain environmental harmful substances such as mercury, cadmium, hexavalent chromium, polybrominated biphenyls (PBB), and polybrominated diphenyl Ether (PBDE), in addition to WEEE ( Other relevant regulations, such as the Waste Electrical and Electronic Equipment Directive, must also be observed.
We have also mentioned briefly that the package must be able to block and resist low wavelength, ultraviolet light, and have a certain hardness to resist mechanical external forces and heat resistance. In addition, insulation, antistatic and moisture resistance must also be note. More importantly, whether or not high brightness is required, the light must be exported as much as possible, because if the light energy cannot be faithfully derived, the light energy is absorbed in the encapsulation layer, which is converted into heat energy, which adds to the heat dissipation problem on the package. A subject, if the heat of the LED can not be smoothly solved and reduced, it becomes a heat load, which in turn will damage the LED body, including the brightness will also be affected. Therefore, to achieve the best and most ideal light, the package design is inevitable. Pay attention to the subject!
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