LED lighting is the general term of LED lamps. With the further maturity of LED technology, LED will achieve more and better development in the field of lighting design and development. The lighting design in the 21st century will take LED lighting design as the mainstream, fully reflect the development trend of energy conservation, health, artistry and humanization, and become the leading of lighting culture. In the new century, LED lighting lamps will bring about a great change in the development and design of lamps
A high-quality LED outdoor lighting lamp is the crystallization of high technology. From materials, base products, chips, packaging, electronic circuit design, shell radiator design, machining to overall assembly, it is the synthesis of many disciplines, such as physics, chemistry, optics, structure, electronics, electronics, environmental climatology, heat conduction, new materials, machining, metal inorganic gas precipitation, aesthetics, human vision and so on, At the same time, it is also the result of the combination of various disciplines. It condenses the latest scientific achievements. It not only saves energy, environmental protection, long life, soft light, makes people’s eyes comfortable, but also enjoys beauty. It should be modular design, which is convenient for assembly, disassembly and maintenance. Therefore, any part of LED lamps has been carefully designed, processed and assembled. The lack of any link and inadequate cooperation can not produce good LED lamps.
As a new energy-saving and environment-friendly solid light source, LED provides designers with greater imagination space and innovation ability due to the diversity of colors. It is more convenient to design personalized products, enrich and beautify people’s life. In the design, the characteristics of LED light source can be fully played to make LED closer to people’s life. The main components of LED lighting lamps are: LED chip, driving circuit board and shell. These three parts constitute the main body of LED lighting lamps and basically determine the quality and performance of LED lighting lamps.
1、 LED chip
(1) LED chip classification. LED chips can be divided into low-power chips, medium power chips and high-power chips according to power. The specific size of the chip is determined according to the actual production level of different chip manufacturers, and there are no specific requirements. As long as the technology passes the customs, the small chip can improve the unit output and reduce the cost, the photoelectric performance will not change fundamentally, and the unit current density of the chip is basically the same. If the working current of 10mil chip is 20mA, the working current of 40mil chip can be increased by 16 times, that is, 320mA However, considering the heat dissipation under high current, the luminous efficiency of large chip is lower than that of small chip. In addition, due to the increase of area, the body resistance of the chip will be reduced, so the forward conduction voltage will be reduced
The luminous brightness of LED light source is determined by the brightness and package outline of LED bare chip, and the working life is determined by the working environment and quality of LED chip. The luminous brightness of chips of different sizes can be the same, but their working life is completely different, because the greater the current density of chips per unit area, the shorter their working life, which is also the reason why the prices of LEDs with the same brightness are different.
The rated working current of high-power LED has several levels, such as 35050070010001500ma. Under the condition of adopting the same technology, the greater the power of a single LED, the lower the light efficiency, but it can reduce the number of LEDs in the lamp, which is conducive to saving investment; The smaller the power of a single lamp, the greater the light efficiency. However, the number of LEDs required in each lamp increases, and the size of the lamp body increases, which increases the design difficulty of the optical lens and has an adverse impact on the light distribution curve.
The main wavelength of high-power LED is the main parameter that determines the color of LED. Whether it is selected correctly or not will affect the lighting effect The half wavelength width and color purity of led actually represent the same optical performance of LED.
The higher the allowable working junction temperature of high-power LED chip, the better the product performance and the more expensive the price. The maximum working shell temperature of high-power LED is a parameter required for packaging technology. The lower the maximum working shell temperature is, the higher the requirements for packaging heat dissipation are, indicating that the better the packaging heat dissipation technology is. The thermal resistance parameters of high-power LED light source directly reflect the level of packaging technology. The better the heat dissipation technology, the lower the thermal resistance and the smaller the light attenuation. The longer the service life of LED lamps The theoretical life of high-power LED is 100 000 h, but now it can actually reach 50000 h, that is, after 50 000 h of use, the light attenuation is no more than 30%, and the luminous flux can still maintain 70% of the initial value
As far as the current situation is concerned, the power of LED single chip in the market is usually about 1 ~ 5W, and the light output is only a few hundred lumens. In order to make LED really be applied in public places such as road lighting on a large scale, the luminous flux of LED light source must reach thousands or even tens of thousands of lumens. Such a high light output cannot be realized by single chip. In order to meet such high light output requirements, at present, most of the light sources outside China use the combination of multiple LED chips (usually 1W) in one lamp to meet the requirements of high brightness lighting. This method solves the problem of insufficient brightness of single chip light source to a certain extent. However, this process has the following problems
1) The manufacturing process of lamps is cumbersome, with low production efficiency and low reliability.
2) The design of lamps is limited by the arrangement amount and arrangement mode of a single LED, and the produced lamps are difficult to give consideration to both beautiful appearance and guaranteed performance.
3) The secondary light distribution design of lamps is complex, which is difficult to meet the requirements of various lighting designs, and will reduce the light efficiency of lamps.
4) Dozens of single chip LEDs are arranged in the same lamp. The photoelectric performance parameters of each single LED chip must be consistent, otherwise the photoelectric performance and service performance of the lamp will be greatly reduced.
5) In the process of use, blind spots caused by local faults are easy to appear, resulting in dark spots and increasing maintenance costs.
In order to solve the above technical problems, the application technology of multi chip packaging high-power LED lighting is proposed. At present, most of the LED lamps produced in China are assembled by multiple applications and parallel connection of 1W LED chips. The thermal resistance of this method is higher than that of products using advanced packaging technology. It is not easy to produce high-quality lamps. The required power can be achieved by assembling with 30, 50W or even larger modules. However, the packaging materials of LED modules are encapsulated with epoxy resin and silica gel. The difference between the two is that epoxy resin packaging has poor temperature resistance and is easy to aging over time. Silica gel packaging has good temperature resistance, so attention should be paid to selection when using.
Using multi chip large-scale, high current and high light efficiency packaging to improve the luminous efficiency of LED can achieve the goal of high brightness. It is better to use multi chip and radiator as a whole, or use aluminum substrate multi chip packaging, and then connect with the radiator through phase change material or heat dissipation silicone grease. The thermal resistance of the product is lower than that of the product assembled with LED devices, which is more conducive to heat dissipation. For lamps using LED module, the module substrate is generally copper substrate. Good phase change material or good heat dissipation silicone grease shall be used for connection with the external radiator to ensure that the heat on the copper substrate can be transmitted to the external radiator in time. If it is not handled well, it is easy to accumulate heat, resulting in too high temperature rise of the module chip and affecting the normal operation of the LED chip. Multi chip packaging is suitable for manufacturing ordinary lighting lamps, and module packaging is suitable for manufacturing compact LED lamps in places with limited space.
(2) Chip cooling. If the heat dissipation problem of LED chip is not solved, the heat of LED cannot be eliminated, and then the working temperature of LED will rise, which will weaken the luminous brightness and decay the service life. For example, when the PN junction temperature of the LED is 25 ℃ (typical operating temperature), the brightness is 100, while when the temperature rises to 75 ℃, the brightness decreases to 80, to 125 ℃, to 60, and to 175 ℃, there is only 40. Obviously, the relationship between PN junction temperature and luminous brightness is inversely linear. The higher the PN junction temperature rise, the darker the LED brightness.
The influence of temperature on brightness is linear, but the influence on service life is exponential. Similarly, the PN junction temperature shall prevail. If it is kept below 50 ℃, the LED will have a service life of nearly 20000 hours, 75 ℃ will have only 10000 hours, 100 ℃ will have 5000 hours, 125 ℃ will have 2000 hours, 150 ℃ will have 1000 hours, the PN junction temperature will change from 50 ℃ to 100 ℃, and the service life will be reduced from 20000 hours to 5000 hours. Temperature has a great impact on the service life of LED
The solution to the temperature rise of PN junction is to convert electric energy into light energy as much as possible, and less into heat energy, that is, when light energy is increased, heat energy is reduced, so as to reduce heating. Electro optic conversion efficiency is the internal quantization efficiency of LED. Generally speaking, it has reached the level of 70% ~ 90%. The real reason lies in the low external quantization efficiency of LED.
The junction temperature (T) of Luxeon series LED of lumiledslighting company is 25 ℃, and the forward driving current is 350mA. For InGaN led, with different wavelength (light color), its efficiency is about 5% ~ 27%. The higher the wavelength, the lower the efficiency (grass green is only 5%, blue can be up to 27%), and allingap led also changes with the wavelength, but the higher the wavelength, the higher the efficiency, The efficiency is generally between 8% ~ 40% (light yellow is low, orange is the highest).
Luxeon series LEDs use silicon sealing glue to fix the LED bare crystal and the fluorescent substance on the bare crystal (if the fluorescent substance is useful), and then the lens is formed after sealing the glue. The lower part of the bare crystal is connected with the silicon inlaid chip by welding (or heat transfer paste). This chip can also strengthen ESD electrostatic protection, and then connect the heat dissipation substrate downward. Some LEDs also directly connect the bottom of the bare crystal with the heat dissipation substrate.
The bare crystal of Luxeon series LED adopts the cladding inlay method, so its sapphire substrate becomes at the upper end, and a layer of silver is added as the light reflection layer to increase the light extraction. In addition, two zener diodes are made in the silicon secondary adhesive base, which can make the LED obtain the voltage stabilizing effect and make its work more stable. Increasing the light extraction rate (also known as light absorption efficiency and light extraction efficiency) is equal to reducing the heat divergence rate. Increasing heat dissipation at the bare crystal level and changing the material and geometric structure have once again become necessary means. At present, the two most commonly used methods are:
1) Improve the material of substrate (also known as base plate and substrate).
2) Tree crystal is inlaid by covering crystal (also known as inverted crystal)
The improved substrate material must match with the bare crystal material. The existing common substrate materials of aigalnp are GaAs, Si, InGaN are SiC and sapphire (and ain is used as the buffer layer).
In order to strengthen the heat dissipation of LED, the past FR and printed circuit board can not be met. Therefore, a printed circuit board with metal core is proposed, which is called MCPCB. The meter uses metals with better thermal conductivity such as aluminum or steel based on FCB to accelerate the dare to heat, but its heat conduction is limited due to the characteristics of rope edge layer.
For light, the substrate should be transparent so that it will not hinder light, that is, add a layer of reflective material layer between the light-emitting layer and the substrate to avoid light being blocked and absorbed by the substrate and waste. For example, GaAs substrate is opaque, so add a layer of DBR (distributed Bragg reflector) reflective layer to reflect light. The sapphire substrate can reflect light directly, and the transparent gap substrate can transmit light.
In addition, the substrate material must also have good thermal conductivity, which is responsible for rapidly guiding the heat released by the crystal to the radiator at the lower layer. However, the substrate and the radiator must also use mediators with good heat conduction, such as solder or heat transfer paste. At the same time, the epoxy resin or silicone resin (i.e. sealing layer) above the bare crystal must also have certain heat resistance to conduct the temperature from the PN junction to the bare crystal surface.
In addition to strengthening the substrate, another method is cladding inlay, which turns the bare crystal electrode at the top to the bottom, and the electrode is directly connected with the wire foil at the bottom. In this way, the heat can also be transmitted to the bottom faster and guide the heat to the lower MCPCB. In the past, copper foil printed circuit board (PCB) was directly used to dissipate heat, that is, the most common fr, printed circuit substrate. However, with the higher and higher heating of LED, FR, Printed circuit boards have become increasingly unbearable due to insufficient thermal conductivity [only 0.36w / (m.k)]
In order to improve the heat dissipation at the circuit board level, metal core PCB (MCPCB) is adopted, that is, the original printed circuit board is attached to another metal with better heat conduction effect (such as aluminum and copper), so as to strengthen the heat dissipation effect. This metal is located in the printed circuit board, so it is called MCPCB. The heat conduction efficiency of MCPCB is higher than that of traditional FR and PCB, and the heat conductivity reaches 1 ~ 2.2W / (m.k)
However, when the circuit system works, the temperature of MCPCB shall not exceed 140 ℃, which is mainly limited by the characteristics of dielectric layer (also known as insulated layer). In addition, it shall not exceed 250 ~ 300 ℃ in the manufacturing process
Although the heat dissipation effect of MCPCB is better than that of FRA PCB, the dielectric layer of MCPCB does not have very good thermal conductivity, which is roughly the same as that of FR and PCB, and the thermal conductivity is only 0.3w / (m.k), which has become the conduction bottleneck between radiator and MCPCB. In order to improve this situation, the improvement method of IMS (insulated metal substrate) is proposed. The polymer insulating layer and copper foil circuit are directly bonded with aluminum and copper plates in the form of epoxy, and then the LED is configured on the insulating substrate. The thermal conductivity of the insulating substrate is relatively high, which can reach 11.2w / (m.k). If the insulating layer is still considered to have poor thermal conductivity, there is also a radiator at the bottom of the LED, Through the through hole on the printed circuit board, it is directly in contact with the core metal of the MCPCB, so as to accelerate the heat dissipation.
In addition to MCPCB and MCPCB + IMS method, some people also proposed to use ceramic substrate, or the so-called direct copper bonded substrate (DBC), or metal composite substrate. Both ceramic substrates and direct copper bonded substrates have high conductivity of 24 ~ 170W / (m.k). Among them, the direct copper bonded substrate allows the temperature of the manufacturing process to reach more than 800 ℃, but these technologies need to be further improved.
The heat dissipation path of LED chip is mainly heat conduction and heat convection. The heat dissipation structure of LED lamp includes substrate, heat sink and radiator. The substrate conducts the heat energy of the grain, which can conduct heat but not conduct electricity. The heat sink diffuses the heat to prevent the heat from accumulating at the LED light source, and can improve the efficiency of the radiator, which effectively radiates the heat into the air. If the thermal conductivity of the substrate material is very low, it is easy to increase the thermal resistance of the device, resulting in serious self heating effect, which has a devastating impact on the performance and reliability of the device.
The soaking plate can realize ultra-high heat flux and heat transfer, and can solve the hot spot problem of high-power LED. The soaking plate is a vacuum cavity with microstructure on the inner wall. When the heat is transmitted from the heat source to the evaporation zone, the working medium in the cavity will begin to produce liquid-phase gasification in the environment of low vacuum. At this time, the medium absorbs heat energy and expands rapidly, and the gas-phase medium will soon fill the whole cavity. When the gas-phase medium contacts a relatively cold area, condensation will occur, and the heat accumulated during evaporation will be released by the condensation phenomenon. The condensed liquid-phase medium will return to the evaporation heat source from the capillary phenomenon of microstructure. This work will be carried out repeatedly in the cavity, which is the working mode of soaking plate. Because the microstructure of the working medium can produce capillary force during evaporation, the work of the soaking plate can not be affected by gravity.
The common ways of high-power LED chips are: large-scale chip, improving luminous efficiency, packaging with high light absorption efficiency, and high current. Although the luminous amount of such practices will increase in proportion, the calorific value will also increase. Therefore, it is necessary to use high heat conduction ceramic or metal resin packaging structure to solve the heat dissipation problem and strengthen the original electrical, optical and thermal characteristics.
For high-power LED packaging technology, the problem of heat dissipation has caused a certain degree of trouble. In this context, the cost-effective metal substrate technology has become another new technology after high-efficiency led.
In the past, due to the small output power of LED, the use of glass epoxy resin packaging substrate such as traditional FRA will not cause too much heat dissipation problem. However, although the chip area of high-power LED applied to lighting is quite small and the overall power consumption is not high, the heat generation per unit area is very large. Generally speaking, the use of resin substrate can only support the heat dissipation of LEDs below 0.5W. LEDs above 0.5W are mostly encapsulated with metal or ceramic high heat dissipation substrate. The main reason is that the heat dissipation of the substrate directly affects the service life and performance of LEDs. Therefore, the packaging substrate has become the development focus of designing high-power LED products.
With the development of LED technology, its application in the field of lighting is more and more, especially the emergence of white LED, which promotes the development of semiconductor solid-state lighting technology. However, LED chip and packaging technology need to be further improved. In terms of chip, we should develop towards high power, high luminous efficiency and reducing thermal resistance. Increasing the power means that the working current of the chip increases. The most direct solution is to increase the chip size. Now the common high-power chips are 1mm? Mm, and the working current is 350mA. Due to the increase of the working current, the heat dissipation problem has become a prominent problem. Now this problem has been basically solved by the method of chip flip.
There are many packaging forms of LED chips. There are various packaging forms for different use requirements and different photoelectric characteristics. The requirements of LED lamps for high-power LED packaging are specifically reflected in:
1) Modularization. Through the interconnection of multiple LEDs (or modules), good lumen output superposition can be realized to meet the requirements of high brightness lighting. Through modular technology, multiple point light sources or LED modules can be combined according to arbitrary shapes to meet the lighting requirements of different fields.
2) Maximize system efficiency. In order to improve the luminous efficiency of LED lamps, in addition to the need for appropriate LED driving power supply, we must also adopt efficient heat dissipation structure and technology, and optimize the internal and external optical design to improve the efficiency of the whole system.
3) Low cost. In order to go to the market, LED lamps must have competitive advantages in cost (mainly referring to the initial installation cost), and packaging accounts for a large part of the production cost of LED lamps. Therefore, the key to realize the commercialization of LED lamps is to adopt new packaging structure and technology and improve the light efficiency cost ratio
4) Easy to replace and maintain. As the LED light source has long service life and low maintenance cost, it puts forward higher requirements for the packaging reliability of LED lamps. It is required that the design of LED lamps can be easily improved to meet the packaging requirements of LED chips with higher efficiency in the future, and the interchangeability of LED chips is required to be good, so that the lamp manufacturers can choose which chip to use.
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