1 Introduction
LED is a kind of light-emitting device that can directly convert electric energy into visible light and radiant energy. It has low working voltage, low power consumption, high luminous efficiency, short response time, pure light color, firm structure, impact resistance and resistance. With a series of characteristics such as vibration, stable and reliable performance, light weight, small size and low cost, its development has been leaps and bounds, and it has been able to mass produce high-brightness and high-performance products of various colors in the visible spectrum. Domestically produced red, green, orange and yellow LEDs account for about 12% of the world's total. The industry goal during the 10th Five-Year Plan period is to achieve an annual production capacity of 30 billion, and to achieve ultra-high brightness AlGsInP LED epitaxial wafers and chips. Mass production, with an annual output of more than 1 billion red, orange and yellow ultra-high brightness LED dies, breaking through the key technologies of GaN materials, enabling medium-to-large production of blue, green and white LEDs.
In the LED industry link, upstream is the production of LED substrate chips and substrates. The industrialization of the midstream is LED chip design and manufacturing, downstream LED packaging and testing, research and development of low thermal resistance, excellent optical characteristics, and highly reliable packaging technology. The new LED is going to be practical, going to the market, and the only way to achieve industrialization is, in a sense, the link between the industry and the market. Only the packaged LED can become a terminal product, and it can be put into the market for practical application. Providing services to make the industry chain interlocking and seamless.
2, the particularity of LED packaging
LED packaging technology is mostly developed from the discrete device packaging technology, but it also has a lot of special features. In general, the die of the discrete device is sealed within the package, and the package functions primarily to protect the die and complete the electrical interconnection. The LED package is to complete the output electrical signal, protect the normal operation of the die, output: visible light function, both electrical parameters, optical design and technical requirements, can not simply use the discrete device package for LED.
The core light-emitting portion of the LED is a PN junction die composed of a p-type and an n-type semiconductor. When a minority carrier injected into the PN junction is combined with a majority carrier, visible light, ultraviolet light or near-infrared light is emitted. However, the photons emitted by the PN junction region are non-directional, that is, the same probability is emitted in all directions. Therefore, not all the light generated by the die can be released, depending on the quality of the semiconductor material, the structure of the die, and the geometry. The internal structure of the package and the encapsulation material are required to improve the internal and external quantum efficiency of the LED. Conventional Φ5mm LED package is to bond or sinter a square die with a side length of 0.25mm on the lead frame. The positive electrode of the die is bonded to the gold wire through the ball contact point as an inner lead and connected to one pin. The reflector cup is connected to the other leg of the lead frame and then the top is encapsulated with epoxy. The function of the reflector cup is to collect the light emitted from the side of the die and the interface and emit it in the desired direction. The top encapsulated epoxy resin is shaped to protect the die from external attack; to use different shapes and material properties (with or without colorants), with lenses or diffusing lenses Function, control the divergence angle of light; the refractive index of the die is too much related to the refractive index of the air, resulting in a smaller critical angle of total reflection inside the die, and only a small part of the light generated by the active layer is taken out, most of which remains in the tube The inside of the core is absorbed by multiple reflections, which is prone to total reflection and causes excessive light loss. The epoxy resin with corresponding refractive index should be used as a transition to improve the light-emitting efficiency of the die. The epoxy resin used to form the envelope must have moisture resistance, insulation, and mechanical strength, and has a high refractive index and transmittance for light emitted from the die. The choice of packaging materials with different refractive indices and different package geometries has different effects on photon emission efficiency. The angular distribution of luminous intensity is also related to the structure of the die, the way of light output, and the material and shape of the package lens. If a pointed resin lens is used, the light can be concentrated in the axial direction of the LED, and the corresponding viewing angle is small; if the resin lens at the top is circular or planar, the corresponding viewing angle will increase.
Under normal circumstances, the LED's emission wavelength changes with temperature as 0.2~0.3nm/°C, and the spectral width increases, which affects the color freshness. In addition, when the forward current flows through the PN junction, the heat generation loss causes the junction region to have a temperature rise. At around room temperature, for every 1 °C increase in temperature, the luminous intensity of the LED is reduced by about 1%. It is very important to maintain color purity and luminous intensity when the package is cooled. In the past, the method of reducing the driving current was used to reduce the junction temperature. The driving current of most LEDs is limited to about 20 mA. However, the light output of LEDs increases with the increase of current. At present, many power LEDs can drive currents up to 70mA, 100mA or even 1A. This requires improved package structure, new LED package design concept and low thermal resistance. Package structure and technology, improve thermal characteristics. For example, a large-area chip flip-chip structure is used, and a silver paste with good thermal conductivity is used to increase the surface area of ​​the metal support, and the silicon carrier of the solder bump is directly mounted on the heat sink. In addition, in the application design, the thermal design and thermal conductivity of the PCB circuit board are also very important.
After entering the 21st century, LED's high efficiency, ultra-high brightness, full colorization continue to develop and innovate. Red and orange LED light effects have reached 100lm/W, green LEDs are 501m/W, and the luminous flux of single LEDs has reached dozens. Lm. LED chips and packages no longer follow the traditional design concept and manufacturing mode. In terms of increasing the light output of the chip, research and development is not limited to changing the amount of impurities in the material, lattice defects and dislocations to improve internal efficiency, and how to improve the tube. Core and package internal structure, enhance the probability of photon emission inside the LED, improve light efficiency, solve heat dissipation, light extraction and heat sink optimization design, improve optical performance, accelerate surface mount SMD process is the mainstream direction of industry research and development.
3, product packaging structure type
Since the 1990s, LED chip and material fabrication technology has made many breakthroughs in research and development, transparent substrate trapezoidal structure, textured surface structure, chip flip structure, commercial ultra-high brightness (above 1cd) red, orange, yellow Green and blue LED products have been asked one after another. In 2000, they began to be used in special lighting with low and medium light flux. The upper and middle reaches of LED industry have received unprecedented attention, further promoting the downstream packaging technology and industrial development. Different types of package structures and sizes, different luminescent color dies and their two-color or three-color combination can produce a variety of series. , varieties, specifications of the product.
The types of LED product packaging structures are classified according to the characteristics of the color of the light, the material of the chip, the brightness of the light, the size, and the like. A single die generally constitutes a point source, and a plurality of die assemblies generally constitute a surface source and a line source for information, status indication, and display. The illuminating display also uses a plurality of dies, through appropriate connections of the dies (including series and Parallel) combined with a suitable optical structure to form the illumination segment and the illumination point of the illuminated display. Surface mount LEDs can gradually replace pin-type LEDs, and the application design is more flexible. It has already occupied a certain share in the LED display market and has a tendency to accelerate development. Some of the solid-state lighting sources have been launched, which will become the future direction of LED development in the future.
4, leaded package
The LED lead package uses lead frames as pins for various package types. It is the first package structure successfully developed on the market. The variety of products is high, the technology maturity is high, and the structure and reflective layer inside the package are still being improved. Standard LEDs are considered by most customers to be the most convenient and economical solution in the display industry. Typical LEDs are housed in an enclosure that can withstand 0.1W of input power. 90% of the heat is from the negative leadframe. Dissipated to the PCB and then released into the air. How to reduce the temperature rise of the PN junction during operation is a must for packaging and application. The encapsulating material is mostly made of high-temperature curing epoxy resin, which has excellent optical rotation performance, good process adaptability, high product reliability, and can be made into colored transparent or colorless transparent and colored scattering or colorless scattering lens package, different lens shapes. A variety of shapes and sizes are formed. For example, the circular shape is divided into several types according to the diameter: Φ2 mm, Φ3 mm, Φ4.4 mm, Φ5 mm, Φ7 mm, etc., and different components of the epoxy resin can produce different illuminating effects. The color point light source has a variety of different package structures: the ceramic base epoxy resin package has better working temperature performance, the lead can be bent into a desired shape, and the volume is small; the metal base plastic reflective cover type package is an energy-saving indicator light. Suitable for power indication; flashing CMOS oscillating circuit chip and LED die package, can produce self-generated flashing light with strong visual impact; two-color type consists of two different luminescent color dies, packaged in the same epoxy In the resin lens, a third mixed color can be obtained in addition to the two colors, which is widely used in a large-screen display system, and can be packaged to form a two-color display device; the voltage type is a combination of a constant current source chip and an LED die. Can directly replace 5~24V various voltage indicators. The surface light source is formed by bonding a plurality of LED dies to a predetermined position of the micro PCB board, and is formed by using a plastic reflective frame cover and potting the epoxy resin. The different designs of the PCB board determine the arrangement and connection manner of the outer leads, and the double row is straight. Insert and single-row inline and other structural forms. Point and surface light sources have been developed in hundreds of package shapes and sizes for the market and customers to choose.
The LED light-emitting display can be composed of a digital tube or a meter tube, a symbol tube, and a rectangular tube to form various products, and is designed into various shapes and structures according to actual needs. Take the digital tube as an example, there are three kinds of package structures, such as a reflector cover, a single-chip integrated type, and a single seven-segment type. The connection mode includes a common anode and a common cathode. One is a commonly known digital tube, and two or more. Generally referred to as a display. The reflector type has the characteristics of large font, material saving, and flexible assembly. It is generally made of white plastic into a seven-section housing with a reflective cavity, and a single LED die is bonded to the seven reflective chambers of the reflector. On the PCB board that is aligned with each other, the center position of the bottom of each reflection cavity is the light-emitting area formed by the die, and the lead wire is bonded by a pressure welding method, and epoxy resin is dropped into the reflection cover, and the PCB board of the die is bonded. Bonded in place and then cured. Reflective cover type is divided into air seal and solid seal. The former uses epoxy resin with scattering agent and dye, and is mostly used for unit and double-position devices; the latter is covered with color filter and uniform film, and is in the die and The bottom plate is coated with transparent insulating glue to improve the light-emitting efficiency, and is generally used for digital display of four or more digits. The monolithic integrated system is to fabricate a large number of seven-segment digital display graphic dies on the luminescent material chip, and then dicing into a single-chip graphic die, bonding, pressure-welding, and encapsulating a lens with a lens (commonly known as a "fisheye" lens). . A single seven-segment type of large-area LED chip that has been fabricated into a light-emitting strip containing one or more dies, so that the same seven pieces are bonded to the digital-shaped slash frame, and are pressure-welded and epoxy. Resin package construction. The monolithic and single strip features are miniaturized and can be used in dual in-line packages, mostly for specialty products. The LED light column display houses 101 cores (up to 201 dies) on a 106mm length circuit board. It is a high-density package that uses optical refraction principles to image the point source through 13-15 rasters of the transparent cover. The dot-to-line display of each die is completed, and the packaging technology is more complicated.
The electroluminescence mechanism of the semiconductor PN junction determines that it is impossible for LED to produce white light with continuous spectrum, and it is impossible for a single LED to produce more than two kinds of high-brightness monochromatic light, which can only be used in packaging by means of fluorescent substances, blue or ultraviolet LEDs. The phosphor is coated on the die to indirectly generate a broadband spectrum, and the white light is synthesized; or a plurality of (two or three or more) dice of different color lights are packaged in one component casing, and the white LED is formed by mixing the color lights. . Both methods have been put into practical use. In 2000, Japan produced 100 million white LEDs, which developed into a stable white light-emitting product. The design of multiple white LEDs was not required to meet the luminous flux requirements. Lord, the pursuit of a new electric light source.
5, surface mount package
In 2002, surface mount packaged LEDs (SMD LEDs) were gradually accepted by the market and gained a certain market share. From lead package to SMD, it is in line with the development trend of the whole electronics industry. Many manufacturers have launched such products one after another. .
Most of the early SMD LEDs were modified with SOT-23 with a transparent plastic body. The outer dimensions were 3.04×1.11mm, and the reel-type container was taped. Based on SOT-23, SLM-125 series and SLM-245 series LEDs with high-intensity SMD with lens are developed. The former is monochromatic illumination, and the latter is two-color or three-color illumination. In recent years, SMD LED has become a development hotspot, which solves the problems of brightness, viewing angle, flatness, reliability, consistency, etc., using a lighter PCB board and reflective layer material to display the ring that needs to be filled in the reflective layer. Less oxygen resin, and remove the heavier carbon steel material pins, can reduce the weight of the product by half, reduce the weight of the product, and finally make the application more perfect, especially suitable for indoor, semi-outdoor full color display application.
The pad is an important channel for SMD LED heat dissipation . The SMD LED data provided by the manufacturer is based on a 4.0mm × 4.0mm pad. Reflow soldering can be used to design the pad to be equal to the pin. Ultra-high-brightness LED products can be packaged in PLCC (plastic package with lead chip carrier)-2, the external dimensions are 3.0mm×2.8mm, and the high-brightness die is assembled by a unique method. The thermal resistance of the product is 400°C/W, which can be CECC. In the way of soldering, the luminous intensity reaches 1250mcd at a driving current of 50mA. The seven-segment one, two, three and four digital SMD LED display devices have a character height of 5.08 to 12.7 mm and a wide display size range. The PLCC package avoids the manual insertion and pin alignment processes required for the seven-segment digital display of the pin, and meets the production requirements of the automatic pick-and-place device. The application design space is flexible and the display is clear and clear. The multi-color PLCC package features an external reflector that can be easily combined with an LED or light guide to replace the current transmissive optical design with a reflective version to provide uniform illumination over a wide area, developed at 3.5V, 1A drive conditions. The working power SMD LED package offers advantages.
6, power type package
The LED chip and package are developed in the direction of high power. Under the high current, the luminous flux is 10~20 times larger than that of the Φ5mm LED. The effective heat dissipation and non-degrading packaging materials must be used to solve the light decay problem. Therefore, the package and the package are also the key. Technology, LED packages that can withstand several watts of power have emerged. 5W series white, green, blue green, blue power LEDs have been available since the beginning of 2003. The white LED light output is 1871m, the light efficiency is 44.31m/W, and the light attenuation problem has been developed to develop a large area of ​​LED that can withstand 10W power. The tube has a size of 2.5mm×2.5mm, can work under 5A current, and has a light output of 2001m. It has great development space as a solid illumination source.
The Luxeon series power LEDs are flip-chip soldered to a silicon carrier with solder bumps, and then the flip-chip soldered silicon carrier is placed in the heat sink and the package, and the bonding leads are packaged. . This package is optimized for light extraction efficiency, heat dissipation performance, and increased operating current density. Its main features: low thermal resistance, generally only 14 ° C / W, only 1 / 10 of conventional LED; high reliability, flexible gel filled inside the package, in the range of -40 ~ 120 ° C, not due to temperature The internal stress generated by the sudden change breaks the gold wire from the lead frame and prevents the epoxy lens from turning yellow, and the lead frame is not stained by oxidation; the optimal design of the reflective cup and lens makes the radiation pattern controllable and optical The most efficient. In addition, its output optical power, external quantum efficiency and other performance is excellent, and the LED solid light source has developed to a new level.
N orlux series power LED package structure is a multi-chip combination of hexagonal aluminum plate as base (making it non-conductive), the base diameter is 31.75mm, the light-emitting area is at the center, the diameter is about 0.375×25.4mm, and it can accommodate 40 LED tubes. The core and the aluminum plate serve as heat sinks at the same time. The bonding wires of the die are connected to the positive and negative electrodes through two contact points made on the base, and the number of aligned dies on the base is determined according to the required output optical power, and the packaged ultra-high brightness AlGaInN and AlGaInP can be combined. The die, which emits light in a single color, color, or synthetic white, is finally encapsulated in an optically designed shape with a high refractive index material. This package uses a conventional high-density package with high density, low light efficiency, low thermal resistance, good protection of the die and bond wires, high light output power at high current, and also a development. LED solid light source for the foreground.
In the application, the packaged product can be assembled on a metal core PCB board with aluminum interlayer to form a power density LED. The PCB board is used as the wiring for connecting the device electrodes, and the aluminum core interlayer can be used as a heat sink. Higher luminous flux and photoelectric conversion efficiency. In addition, the packaged SMD LEDs are small in size and can be flexibly combined to form a variety of illumination sources such as module type, light guide type, concentrating type, and reflective type.
The thermal characteristics of power LEDs directly affect the operating temperature, luminous efficiency, wavelength of illumination, and lifetime of LEDs. Therefore, the packaging design and manufacturing technology of power LED chips are particularly important.
7, LED development and application prospects
In recent years, the luminous efficiency of LEDs has increased by 100 times, and the cost has dropped to one-tenth of the original. It is widely used in large-area graphic display full-color display, status indication, sign illumination, signal display, backlight of liquid crystal display, and automobile combination. In terms of taillights and interior lighting, its development prospects have attracted global lighting manufacturers to join its light source and market development. The development and application prospects are white LEDs, which are economical for solid lighting devices and environmentally friendly. They are gradually replacing traditional incandescent lamps, with an annual growth rate of over 20% in the world, China, the United States, Japan and Europe. The country has launched a semiconductor lighting program. At present, ordinary white LEDs have a luminous efficiency of 25 lm/W. The excellent heat dissipation characteristics and optical characteristics of power LEDs are more suitable for the general lighting field, and are considered by the academic community and the industry to be the only way for LEDs to enter the lighting market. In order to replace fluorescent lamps, white LEDs must have a luminous efficacy of 150~200lm/W, and the price per lm should be significantly lower than 0.015$/lm (current price is about 0.25$/lm, red LED is 0.065$/lm). To achieve this There are still many technical issues that need to be addressed, but overcoming them is not a long way off. According to the principle of solid-state luminescence, the luminous efficiency of LED can be approximately 100%. Therefore, LED is known as "a new light source in the 21st century" and is expected to become the fourth generation light source after incandescent lamps, fluorescent lamps and high-intensity discharge lamps.
8. Conclusion
There are more than 20 upstream and midstream research and production units and more than 150 packaging companies in the domestic LED industry. High-end packaging products have not yet been introduced to the market. At present, the GaN-based blue-green LED mid-stream technology industrialization research has been completed, and the performance index of the products has reached the level of similar products in foreign countries in the short term, striving to achieve the monthly production capacity of 10 kk in a short period of time, and develop white light. New products such as power LED chips for lighting sources. The Ministry of Science and Technology will invest 80 million yuan to start the national semiconductor lighting project, pay attention to the terminal products, start with special products, take the automobile and urban landscape lighting as the market breakthrough, and put the high-power and high-brightness LEDs in the prominent position. The results will serve the Beijing Olympics and the Shanghai World Expo. Undoubtedly, the substrate, epitaxy, chip, package and application in the industrial chain need to be developed together, and multi-party interactive cultivation. Packaging is the part of the industry chain, which needs attention and attention.
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