Analysis of the principle technology of "packaged light communication" of high brightness LED
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 now more active efforts and needs Ultra high 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 the LCD TV with LED backlight is not only more environmentally friendly but also more realistic and bright. Replacing white lights, halogen lights and other lights with LED lighting is not only brighter and power-saving, but also more effective in use, and the lighting response is faster. When used for brake lights, it can reduce the collision rate of rear cars. Therefore, LEDs can only be used as the status indicator of electronic devices in the past, and have progressed to become the backlight of liquid crystal displays, and then expanded to electronic lighting and public displays, such as car lights, traffic lights, information boards, large video walls, and even The application of lighting in the projector continues to expand. More importantly, the brightness efficiency of LEDs is like Moore's Law, which doubles every 24 months. In the past, white LEDs could only be used to replace incandescent and halogen lamps that consume too much power. That is, the luminous efficiency is within the level of 10 to 30 lm / W. However, after the white LED breaks through 60 lm / W or even reaches 100 lm / W, even fluorescent lamps and high-pressure gas discharge lamps have begun to feel threatened.
Although LEDs continue to enhance the brightness and luminous efficiency, in addition to the core phosphor quality, light mixing and other patented technologies, it will also be a growing challenge for packaging, and a dual challenge. On the one hand, packaging must allow LED has the largest light extraction rate and the highest luminous flux, which minimizes the optical deflection. At the same time, it also needs to pay 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 allow the LED to have the best heat dissipation, especially HB (high brightness) almost means HP (high power, high power consumption), and the current value into and out of the LED continues to increase. If the heat dissipation is not good, It will not only reduce the brightness of the LED, but also shorten the life of the LED. Therefore, if you continue to pursue high-brightness LEDs, if the packaging technology used does not have corresponding enhancements, then the high-brightness performance will be discounted. Therefore, this article will discuss more about HB LED packaging technology, including light communication. , Including discussions on thermal conductivity.
Bare crystal layer: "quantum wells, multi-quantum wells" enhance "light conversion efficiency"
Although this article is mainly talking about the enhancement of luminous flux by LED packaging, it is necessary to explain the die part of the deeper core first. After all, the improvement of the die structure can also greatly increase the luminous flux. The first is to enhance the efficiency of light conversion, which is also the most root cause. Only 15% to 2% of the electricity per watt of existing LEDs 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 the pn junction (pn juncTIon). The pn junction is the main light emitting and heating position of the LED. The conversion efficiency can be improved by changing the structural design of the pn junction. At present, quantum wells (Quantum Well; QW) are mostly drilled on the pn junction to increase the proportion of electricity converted into light energy, and further efforts will be made towards more excavations, that is, multi-quantum wells (MulTIple Quantum Well; MQW) technology.
"Refueling and restructuring, light transmission" increase the "light efficiency".
If the light conversion efficiency is difficult to demand, further must start from the level of light output efficiency. There are quite a lot of methods at this level, which are also different according to different compound materials. Currently, the two compound materials that HB LEDs commonly use are AlGaInP and GaN / InGaN, the former is used to generate high-brightness orange, orange, yellow, and green light, the latter GaN is used to generate green, emerald green, and blue light, and InGaN is used to generate near ultraviolet, blue-green, and blue light. Methods include changing the solid geometry (transverse to vertical), changing the material of the substrate (substrate, also called: substrate), adding a new material layer, changing the bonding method of the material layer, and surface treatment of different materials. However, no matter how it changes, there are basically two principles: First, reduce shadowing and increase light transmittance. 2. Strengthen the utilization of light refraction and reflection. For example, in the past AlGaInP LEDs, the material used for the substrate was GaAs, but GaAs on the black surface caused half of the light emitted by the pn junction to be blocked and absorbed, resulting in a waste of light energy, so the transparent GaP material was used as the substrate. Another example is Japan's Nichia Chemical Industry (Nichia), where the p-type electrode (p-type) part is made into a mesh pattern (Mesh Pattern) to increase the transparency of the p-electrode, reduce light obstruction and increase the amount of light transmission. As for increasing the refractive reflection, a layer of DBR (Distributed Bragg Reflector) reflection is added to the structure of AlGaInP to fold the light source on the other side to the same side. For GaN, the substrate material is replaced with sapphire (aluminum oxide) to increase reflection. At the same time, the surface of the substrate is designed into a concave-convex pattern, thereby increasing the scattering angle after light reflection, thereby improving the light extraction rate. Or, for example, the German OSRAM (OSRAM) uses a SiC material substrate, and the substrate is designed as a slope, which also helps to increase reflection, or add a silver, aluminum metal mirror layer.
Encapsulation layer: The anti-aging yellow light and the transmittance defense battle strive to increase the brightness from the die level, and then formally take over from the encapsulation level to ensure that the luminous flux is maintained at the highest and the light attenuation is minimized.
To have a high lumen retention (Transmittance), the first step is to encapsulate the material. In the past, the most commonly used LED is epoxy, but the epoxy will gradually turn yellow after aging, which will affect the bright color, especially the lower the wavelength, the faster the aging, especially some WLEDs use near ultraviolet Compared with other visible light, its wavelength is lower and aging is faster. The new proposal is to use silicone resin (silicone), for example, the US Lumileds company Luxeon series LED is to adopt silicone sealant. Not only Lumileds Luxeon, other companies also have silicone solutions, such as General Electric. Toshiba's InvisiSi1, Toray. Dow Corning's SR 7010, etc. are also LED silicone packaging solutions.
In addition to having better resistance to low wavelengths and less aging, silicone rubber is also a kind of protection for human health by blocking near ultraviolet rays so that it does not leak out. In addition, the light transmittance, refractive index and resistance of silicone rubber The heat is ideal. GE Toshiba's InvisiSi1 has a refractive index as high as 1.5 to 1.53, the light transmittance in the wavelength range of 350nm to 800nm ​​reaches 95%, and the light transmission is still 75% to 80% when the wavelength is as low as 300nm, reducing the refractive index 1.41, even at a wavelength of 300nm can maintain 95% optical transparency. Dow Coring Toray's SR 7010 has a light transmittance of 99% when the wavelength is above 405nm, and the refractive index after curing is 1.51. In addition, it can also reach the level of 180 ° C to 200 ° C in heat resistance. In addition, some manufacturers have proposed so-called resin-free encapsulation, which uses glass as a protective cover. For example, the ceramic encapsulation proposed by Kyocera in Japan is proposed for anti-aging, and ceramics also have better heat resistance.
Encapsulation layer: The reflection and refraction of the lens' transflective cup after encapsulation with glue, there will be a variety of different connection methods according to the different uses of the LED. This is the case with the indicator light. The other is to combine multiple LEDs into a whole component, such as a seven-segment display, dot matrix display, etc. In addition, there are two distinctions in welding foot position, namely through-hole technology (Through-Hole Technology) and surface adhesion technology (Surface-Mount Technology).
In terms of one-by-one independent, separated and discrete packages, they also have different package appearances according to different applications. If it is used as a status indicator for perforated welding, as long as it adopts a lamp type package (commonly known as "cannonball type"), even there is a difference in lens type (Lens Type), such as typical Lamp, Oval Oval, Super Oval, Flat Flat, etc. If it is a surface-adhesive type, there are also Top View, Side View, Dome, etc.
Why are there different lens shapes? In general, Lamp is used as an indicator, Oval is used for outdoor signs or signs, Top View is used as a direct-type backlight, Flat and Side View are combined with a Guide Plate as a side-light type The backlight, Dome as a small lighting bulb, small flash, etc. Different shapes and different applications, the viewing angle (View Angle) of light is also different, this part will once again test the packaging design. Using different design methods, you can get different luminous angles, light intensity, and luminous flux. There are four common practices in this regard: Axial lens, flat lens, flat lens, ReflecTIve cup, ReflecTIve cup by island block ReflecTIve cup by island. The general Lamp uses the mid-axis lens method. Dome and Oval / Super Oval are also similar, but the brightness of Oval / Super Oval is more concentrated in the axial small angle than Lamp. Flat uses the straight lens method. The advantage is that the light viewing angle is larger than the central axis lens method, but the disadvantage is that the light flux is reduced and the light intensity is weakened. As for Top View, Side View, etc., reflector cups or island reflector cups are mostly used. This method is to add a reflector in the package to reflect and refract the beams with partial divergence angles to converge, so that the angle and light intensity can be balanced.
As far as the technical difficulty is concerned, the Axial lens and Flat lens that use only the lens are relatively simple. As long as the transmission and beam divergence are considered, the relative reflection cup is different. The original transmission and divergence must be considered and considered. Reflection, refraction, and beam convergence are more complicated.
In terms of material, the lens part can be replaced with other materials in addition to the original rubber-covered material, because the lens has been more focused on light transmission and not on the protection of bare crystal, so plastic (plastic) and acrylic (Acrylic), glass (Glass), polycarbonate (Polycarbonate), etc., and as mentioned before, the light transmittance is related to the wavelength, the light transmittance at different wavelengths is different, plus there are different materials to choose from, and even to The lens is colored to increase the contrast of the light color, or depending on the decorative effect of the application (toy, Christmas tree), as well as the geometric design of the front lens, reflective cup, etc., all of these constitute the fourth problem on LED light .
Conclusion
HB LED is emphasized as "green lighting", the meaning of the word "environmental protection" is its big demand point, so not only must it be lead-free (Pb Free) package, but also comply with today's European RoHS (Restriction of Hazardous Materials Directive) According to the statutes and regulations, no package and LED as a whole can contain mercury, cadmium, hexavalent chromium (PolyBrominated Biphenyls; PBB), polybrominated biphenyl ether (PolyBrominated Diphenyl Ether; PBDE) and other environmental hazardous substances, in addition to WEEE (Waste Electronic and Electrical Equipment Directive) and other relevant regulations must also be followed.
Earlier we also briefly mentioned that the encapsulant must be able to block and resist low wavelengths and ultraviolet light, and it must have a certain hardness to resist mechanical forces and heat resistance. In addition, insulation, antistatic and moisture resistance must also be note. More importantly, no matter whether it is high brightness or not, the light must be exported as much as possible, because if the light energy cannot be faithfully exported, the light energy is absorbed in the package layer, which will be converted into heat energy, which adds to the heat dissipation problem on the package. A subject, if the heat of LED cannot be smoothly resolved and reduced, it becomes a heat load, which in turn will damage the LED body, including brightness, will be affected. Therefore, to achieve the best and most ideal luminous flux, package design is indispensable Pay attention to the subject! (Text / Guo Changyou)
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