There is no doubt that the world needs high brightness light emitting diode (HB LED), not only high brightness white LED (HB WLED), but also high brightness color LED. From now on, we will make active efforts and need ultra high brightness LED (UHD led for short).
Replacing the original El backlight and CCFL backlight of handheld devices with LED backlight not only makes the circuit design simpler and easier, but also has high external force resistance. Replacing the original CCFL backlight of LCD TV with LED backlight is not only more environmentally friendly, but also more realistic and bright. Using LED lighting instead of white light lamp, halogen lamp and other lighting can not only save more light and power, but also be used for a longer time, and the lighting response is faster. When used for brake lamp, it can reduce the rear collision rate.
Therefore, LED can only be used in the status indicator of electronic devices in the past, has progressed to become the backlight of liquid crystal display, and then expanded to electronic lighting and public display, such as vehicle lights, traffic lights, Kanban message running lights, large film and television walls, and even lighting in projectors.
More importantly, just like Moore’s law, the brightness efficiency of LEDs doubles every 24 months. In the past, it was considered that white LEDs can only be used to replace incandescent lamps and halogen lamps that consume too much power, that is, the level of luminous efficiency within 10 ∼ 30lm / W. however, after white LEDs break through 60lm / W or even reach 100lm / W, even fluorescent lamps High pressure gas discharge lamps are also beginning to feel threatened.
Although LEDs continue to enhance brightness and luminous efficiency, in addition to the core patented technologies such as fluorescence quality and light mixing, it will also be an increasing challenge for packaging, which is a double challenge. On the one hand, packaging must allow LEDs to have the maximum light extraction rate and the highest luminous flux to minimize light loss, and pay attention to light divergence angle, light uniformity Compatibility with 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). The current value in and out of the LED continues to increase. If the heat dissipation is not good, it will not only weaken the brightness of the LED, but also shorten the service life of the LED.
Therefore, in the continuous pursuit of high brightness LED, if the packaging technology used does not have corresponding enhancement and improvement, the high brightness performance will also be reduced. Therefore, this paper will discuss more about the packaging technology of HB led, including light communication and thermal conductivity.
Note: it is called “light emitting diode” in mainland China.
Note: Generally speaking, Hb led refers to luminous efficiency above 8lm / w (8 lumens per watt).
Note: Generally speaking, HP led mostly refers to the power consumption of more than 1W (W), and the power consumption watts are the direct conduction voltage multiplied by the direct conduction current (VF) × If, f = forward).
■ bare crystal layer: “quantum well, multi quantum well” to improve “light conversion efficiency”
Although this paper mainly talks about the enhancement of luminous flux by LED packaging, we have to explain the bare crystal part of the deeper core first. After all, the improvement of bare crystal structure can also greatly improve the luminous flux.
The first is to strengthen the light conversion efficiency, which is also the root cause. Only 15% – 20% of the current LED power consumption per watt is converted into light energy, and the rest is converted into heat energy and dissipated (waste heat). The key to improve this conversion efficiency is the p-n junction, which is the main luminous and heating position of the LED, The conversion efficiency can be improved by changing the structural design of p-n interface.
In this regard, at present, quantum well (QW) is mostly drilled on the p-n interface to improve the proportion of power conversion into light energy. Further efforts will be made towards more drilling numbers, that is, multiple quantum well (MQW) technology.
■ bare crystal layer: “refueling modification, light transmittance folding” to improve “light output efficiency”
If the light conversion efficiency is difficult to meet the requirements, we must further start from the level of light output efficiency. There are quite a lot of methods at this level, which are different according to different composite materials. At present, the two commonly used composite materials for HB LED are AlGaInP and GaN / InGaN. The former is used to produce bright orange, orange, yellow and green light, and the latter Gan is used to produce green, emerald and blue light, And using InGaN to produce near ultraviolet, blue-green and blue light.
What are the methods? This includes changing the solid geometry (from transverse to vertical), changing the material of substrate (also known as substrate), adding new material layer, changing the bonding mode of material layer, different material surface treatment, etc. However, no matter what the change, there are two main principles: first, reduce shielding and increase light transmittance. 2、 Enhance the utilization of light refraction and reflection.
For example, in the past, the substrate of AlGaInP led was made of GaAs, but the GaAs on the black surface blocked and absorbed half of the light emitted from the p-n interface, resulting in a waste of light energy. Therefore, transparent gap material was used as the substrate. As another example, Nichia made the p-type electrode part into mesh pattern in GaN LED, so as to increase the transparency of p-electrode, reduce light obstruction and improve light transmittance.
As for increasing catadioptric reflection, a DBR (distributed Bragg reflector) reflection layer is added to the structure of AlGaInP to fold the light source on the other side to the same side. In Gan, the substrate material is replaced with sapphire (Al2O3, Al2O3) to increase the reflection, and the substrate surface is designed into concave convex texture, so as to increase the scattering angle after light reflection, so as to improve the light extraction rate. Or, for example, OSRAM of Germany uses SiC substrate and designs the substrate as an inclined plane, which also helps to increase reflection, or adds silver and aluminum metal mirror layers.
▲ the LED with improved brightness has been applied in public places. This is the traffic direction indicator light outside the domestic construction site, which is composed of HB led. (Photo by Guo Changyou)
Note: AlGaInP (aluminum gallium indium phosphide), also known as “quaternary luminescent material”, is composed of Al, GA, in and P.
Note: in the general explanation of graphic structure, the p-n interface is also called “emitting layer, emitting layer or active layer, active region”.
Note: in addition to reducing light shielding and increasing reflection, sometimes different technologies are used to avoid patents applied by other manufacturers.
▲ for the luminous efficiency enhancement method of various AlGaInP LEDs, from left to right is the difference of technical progress first, the leftmost is the most basic standard led geometry, then the DBR (distributed Bragg reflector) reflector is added, then the current blocking technology is added after DBR, and the omni directional mirror adhesion (OMA) of wafer optoelectronics is on the right Omnidirectional mirror bonding technology, which also changes the substrate material from GaAs to Si. (photo source: wafer Optoelectronics)
▲ for LEDs made of GaN and InGaN composite materials, it also has its own set of process structure luminous flux enhancement method. For German OSRAM, the standard structure was still used in 1999, the Aton structure was developed in 2002, the better nota structure was replaced in 2003, and the thingan structure was used in 2005. (photo source: wafer Optoelectronics)
Package layer: anti aging yellow light, light transmittance defense
After trying to increase the brightness from the bare crystal level, it will officially take over from the packaging level to ensure that the light flux is maintained at the highest and the light attenuation is minimized.
To have a high lumen retention (transmittance, penetration, expressed in percentage units), the first step is the packaging material. In the past, epoxy resin was the most commonly used for LEDs. However, the epoxy resin will gradually turn yellow after aging (due to the “benzene ring” composition), which will affect the bright color. Especially, the lower the wavelength, the faster the aging. Especially, some wleds use near ultraviolet light Compared with other visible light, near ultraviolet has lower wavelength and faster aging.
The new proposal is to replace epoxy resin with silicone. For example, the Luxeon series LED of Lumileds company in the United States is to adopt silicon sealing adhesive.
It is not only Lumileds Luxeon that uses silica gel, but also other manufacturers have silica gel solutions, such as invisisi1 of Ge Toshiba and Sr 7010 of Dow coring Toray.
Silica gel not only has better resistance to low wavelengths and is less prone to aging, but also protects human health by blocking near ultraviolet rays. In addition, silica gel has ideal light transmittance, refractive index and heat resistance. Invisisi1 of Ge Toshiba has a refractive index of 1.5 ∼ 1.53, and the light transmittance in the wavelength range of 350nm ∼ 800nm is 95%, When the wavelength is as low as 300nm, there is still 75% – 80% light transmittance, or the refractive index is reduced to 1.41 by making a trade-off with the refractive index, so that 95% light transmittance can be maintained even at 300nm wavelength. Similarly, the light transmittance of SR 7010 of Dow coring Toray is 99% above 405nm wavelength, and the refractive index after hardening is also 1.51. In addition, the heat resistance can reach the level of 180 ℃ – 200 ℃. We won’t discuss the problem of heat here for the time being.
In addition, some people have proposed the so-called non resin encapsulation, that is, using glass as a coat protection, or as a ceramic package proposed by KYOCERA, Japan (Kyocera), for the purpose of anti-aging, and ceramics also have better heat resistance.
▲ cross sectional view of Luxeon series LED (InGaN) of Lumileds lighting company. It can be seen from the figure that Luxeon uses silicon packaging for bare crystal protection instead of traditional epoxy resin. (photo source: Lumileds. Com)
▲ with the increase of service time, the luminous flux of LED will gradually decrease. In the figure, the life luminous flux curves of two LEDs are compared. The blue line below is a general 5mm WLED indicator, and the red line above is a high-power LED lamp. (photo source: Lumileds. Com)
Note: another factor that accelerates the aging and yellowing of epoxy resin comes from temperature. High temperature will accelerate the aging.
■ packaging layer: transmission of lens, reflection and refraction of reflection cup
The aforementioned packaging is mainly to protect the LED bare crystal and make the light and heat transfer outward faithfully as much as possible. Next, it is still at the packaging level, but it is no longer the resin part of the inner cover, but the lens part of the outer cover.
After sealing with glue, there will be various connection methods according to different uses of LEDs, such as making independent packaging components one by one, which was the most typical single LED indicator in the past. The other is to combine multiple LEDs into an integral component, such as seven segment display, dot matrix display, etc. In addition, there are two differences in welding foot position, namely, through hole technology (THT) and surface mount technology (SMT).
Let’s not talk about the cluster seven segment display and dot matrix display for the moment, but for the independent, separated and discrete packaging one by one, it should also have different packaging appearance according to different applications. If it is the same as the LED in the past, it is used as a perforated welding status indicator, as long as it adopts the lamp type packaging (commonly known as “shell type” Today), Even if it is determined to be this type, there are differences in lens types, such as typical lamp, oval oval oval oval, super oval oval, flat flat, etc. For the surface adhesive type, there are also top view, side view, dome, etc.
Why are there different lens shapes? In fact, they also have their own application requirements. Generally speaking, lamp is used as indicator light signal, oval is used for outdoor signs or signs, top view is used as straight down backlight, flat and side view cooperate with light guide plate (LGP) to do side entry backlight, dome is used as small lighting bulb, small flash, etc.
With different shapes and applications, the view angle of light emission will be different. This part will test the packaging design again. Different light emission angles, light intensity and luminous flux can be obtained by using different design methods. There are four common methods in this regard: axial lens, flat lens, reflective cup Reflective cup by island.
The general lamp uses the central axis lens method. Dome and oval / super oval are similar, but the brightness of oval / super oval is more concentrated in a small axial angle than lamp. Flat uses the flat lens method, which has the advantage that the light angle is larger than the central axis lens method, but the disadvantage is that the luminous flux is reduced and the light intensity is weakened. As for top view and side view, reflection cups or island reflection cups are mostly used. This method is to add mirrors in the package to reflect and refract the beam with partial divergence angle, so as to balance the angle and light intensity.
▲ for the 5mm white LED of Nichia, it can be seen in the figure that the lamp type package also uses a bowl shaped reflective cup design to enhance the illumination angle and intensity. (photo source: ledstyles. De)
In terms of technical difficulty and ease, the axial lens and flat lens with only the upper lens are really simple. As long as the transmission and beam divergence are considered, the reflective cup is different. The original transmission and divergence should be considered as well as the reflection, refraction and beam convergence, which is indeed more complex.
In addition, we haven’t discussed the material yet. In addition to the original rubber coated material, the lens can also be made of other materials, because the lens has paid more attention to light transmission rather than bare crystal protection. In this way, plastic, acrylic, glass, polycarbonate, etc. can also be adopted, as mentioned earlier, The light transmittance is related to the wavelength. The light transmittance of different wavelengths is different. In addition, there are different materials to choose. Even the lens should be colored to increase the contrast of light color, or depending on the decorative effect of the application (toys, Christmas tree), as well as the geometric design of the front lens and reflection cup. All these constitute the fourth topic on LED light communication.
Note: some LEDs today also use reflective cup technology in lamp package.
Finally, Hb LED is emphasized as “green lighting”, which implies that “environmental protection” is its great appeal point. Therefore, it should not only be lead-free packaging, but also comply with the laws and regulations of RoHS (restriction of Hazardous Substances Directive) in Europe today. No matter the packaging and LED can contain mercury, cadmium and hexavalent chromium (hexavalent chromium), polybrominated biphenyls (PBB), polybrominated diphenyl ether (PBDE) and other environmental hazards. In addition, WEEE (waste electrical and Electronic Equipment Directive) and other relevant regulations must also be observed.
Source; International LED network