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Various forms of packaging structure and technical analysis of LED light-emitting diodes

Posted by: Yoyokuo 2022-08-14 Comments Off on Various forms of packaging structure and technical analysis of LED light-emitting diodes

LED is a type of light-emitting device that can directly convert electrical energy into visible light and radiant energy. It has low operating voltage, low power consumption, high luminous efficiency, extremely short luminous response time, pure light color, firm structure, impact resistance, and vibration resistance. , stable and reliable performance, light weight, small size, low cost and a series of characteristics, rapid development, can now mass produce high-brightness, high-performance products of various colors in the entire visible spectrum.The domestic production of red, green, orange, and yellow LEDs accounts for about 12% of the world’s total. The industrial goal during the “Tenth Five-Year Plan” period is to achieve an annual output of 30 billion pieces, and to achieve ultra-high brightness AiGslnP LED epitaxial wafers and chips. Dasheng

1 Introduction
LED is a type of light-emitting device that can directly convert electrical energy into visible light and radiant energy. It has low operating voltage, low power consumption, high luminous efficiency, extremely short luminous response time, pure light color, firm structure, impact resistance, and vibration resistance. , stable and reliable performance, light weight, small size, low cost and a series of characteristics, rapid development, can now mass produce high-brightness, high-performance products of various colors in the entire visible spectrum. The domestic production of red, green, orange, and yellow LEDs accounts for about 12% of the world’s total. The industrial goal during the “Tenth Five-Year Plan” period is to achieve an annual output of 30 billion pieces, and to achieve ultra-high brightness AiGslnP LED epitaxial wafers and chips. Large-scale production, with an annual output of more than 1 billion red, orange, and yellow ultra-high-brightness LED dies, breaking through the key technology of GaN materials, and realizing medium-volume production of blue, green, and white LEDs.

In the LED industry link, the upstream is LED substrate wafer and substrate production, the midstream industrialization is LED chip design and manufacturing, and the downstream is LED packaging and testing. The development of low thermal resistance, excellent optical properties, and high reliability packaging technology is The only way for new LEDs to become practical and market-oriented industrialization is, in a sense, the link between the industry and the market. Only after being packaged can it become a terminal product, put into practical application, and provide services to customers. The industrial chain is interlinked and seamless.
  

Various forms of packaging structure and technical analysis of LED light-emitting diodes

2 Particularities of LED Packages

LED packaging technology is mostly developed and evolved on the basis of discrete device packaging technology, but it has great particularity. Typically, the die of a discrete device is sealed in a package that protects the die and completes electrical interconnections. The LED package is to complete the output of electrical signals, protect the normal operation of the die, and output: the function of visible light, which has both electrical parameters and optical parameters. Design and technical requirements, it is impossible to simply use the packaging of discrete devices for LEDs.

The core light-emitting part of the LED is a pn junction die composed of p-type and n-type semiconductors. When the minority carriers injected into the pn junction recombine with the majority carriers, visible light, ultraviolet light or near-infrared light will be emitted. However, the photons emitted by the pn junction area are non-directional, that is, they have the same probability of being emitted in all directions. Therefore, not all the light generated by the die can be released, which mainly depends on the quality of the semiconductor material, die structure and geometry. , Encapsulation internal structure and encapsulation materials, application requirements to improve the internal and external quantum efficiency of LED. The 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 pole of the die is connected to the gold wire through the spherical contact point, and the inner lead is connected to a pin, and the negative pole is reflected by reflection. The cup is connected to the other pin of the lead frame, and the top of it is then encapsulated with epoxy. The function of the reflector cup is to collect the light emitted from the side and interface of the die and emit it in the desired direction angle. The epoxy resin encapsulated at the top is made into a certain shape, which has several functions: protect the die from external erosion; use different shapes and material properties (with or without dispersing agent) to act as a lens or diffuser lens Function, control the divergence angle of light; the refractive index of the die is too related to the refractive index of the air, so that the critical angle of total reflection inside the die is very small, and only a small part of the light generated by the active layer is taken out, and most of it is easy to be in the tube. The inside of the core is absorbed by multiple reflections, which is prone to total reflection and leads to excessive light loss. The epoxy resin with the corresponding refractive index is selected as the transition to improve the light output efficiency of the die. The epoxy resin used to form the tube shell must have moisture resistance, insulation, mechanical strength, and high refractive index and transmittance to the light emitted by the tube core. When packaging materials with different refractive indices are selected, the effect of packaging geometry on photon escape efficiency is different, and the angular distribution of luminous intensity is also related to the die structure, light output method, and the material and shape of the packaging 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 smaller; if the top resin lens is circular or flat, the corresponding viewing angle will be increased.

Under normal circumstances, the luminous wavelength of LED changes with temperature to 0.2-0.3nm/℃, and the spectral width increases accordingly, which affects the color vividness. In addition, when the forward current flows through the pn junction, the thermal loss causes a temperature rise in the junction area. Around room temperature, for every 1°C increase in temperature, the luminous intensity of the LED will correspondingly decrease by about 1%, and the package will dissipate heat; Purity and luminous intensity are very important. In the past, the method of reducing the driving current was used to reduce the junction temperature. The driving current of most LEDs was limited to about 20mA. However, the light output of LED will increase with the increase of current. At present, the driving current of many power LEDs can reach 70mA, 100mA or even 1A. It is necessary to improve the packaging structure, new LED packaging design concept and low thermal resistance packaging structure and technology to improve thermal characteristics. For example, a large-area chip flip-chip structure is used, silver paste with good thermal conductivity is used, the surface area of ​​the metal bracket is increased, 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, and full color have been continuously developed and innovated. The luminous efficacy of red and orange LEDs has reached 100Im/W, green LEDs are 501m/W, and the luminous flux of a single LED has also reached dozens. Im. LED chips and packages no longer follow Gong’s traditional design concepts and manufacturing production models. In terms of increasing the light output of chips, R&D is not limited to changing the amount of impurities, lattice defects and dislocations in materials to improve internal efficiency. At the same time, how to improve The internal structure of the die and package enhances the probability of photon emission inside the LED, improves light efficiency, solves heat dissipation, optimizes the design of light extraction and heat sink, improves optical performance, and accelerates the process of surface mount SMD, which is the mainstream direction of R&D in the industry. .

3 Product Package Structure Type

Since the 1990s, a number of breakthroughs have been made in the research and development of LED chip and material manufacturing technology, such as the trapezoidal structure of transparent substrates, textured surface structures, flip-chip structures, and commercialized ultra-high brightness (above 1cd) red, orange, Yellow, green and blue LED products have been on the market one after another. As shown in Table 1, they have been used in special lighting with low and medium luminous flux since 2000. The upstream and midstream industries of LED have received unprecedented attention, which further promotes the downstream packaging technology and industrial development. Using different packaging structures and sizes, dies with different luminous colors and their two-color or three-color combinations can produce a variety of series, varieties and specifications of products.

The type of LED product package structure is shown in Table 2, and it is also classified according to the characteristics of light emission color, chip material, light emission brightness, size and so on. A single die generally constitutes a point light source, and multiple die assemblies can generally constitute a surface light source and a line light source for information, status indication and Display. In parallel) combined with a suitable optical structure, it constitutes the light-emitting segment and light-emitting point of the light-emitting display. Surface-mount LEDs can gradually replace pin-type LEDs, and the application design is more flexible. It has occupied a certain share in the LED display market and has an accelerated development trend. Some solid-state lighting source products are listed, which will become the medium and long-term development direction of LED in the future.

4-Pin Package

The LED pin-type package uses lead frames as the pins of various package shapes. It is the first package structure that has been successfully developed and put on the market. There are many varieties and high technology maturity. The internal structure of the package and the reflective layer are still being continuously improved. Standard LEDs are considered by most customers to be the most convenient and economical solution in the display industry at present. A typical traditional LED is housed in a package that can withstand 0.1W input power, and 90% of its heat is generated by the negative pin. The frame is dissipated to the PCB board, and then dissipated into the air. How to reduce the temperature rise of the pn junction during operation must be considered in packaging and applications. The encapsulation material is mostly high temperature curing epoxy resin, which has excellent optical properties, good process adaptability and high product reliability. It can be made into colored transparent or colorless transparent and colored scattering or colorless scattering. It is composed of various shapes and sizes. For example, the circle is divided into Φ2mm, Φ3mm, Φ4.4mm, Φ5mm, Φ7mm and so on according to the diameter. Different components of epoxy resin can produce different luminous effects. There are many different package structures for the color point light source: the ceramic base epoxy resin package has better operating temperature performance, the pins can be bent into the required shape, and the volume is small; the metal base plastic reflector package is an energy-saving indicator light , suitable for power indication; the flickering type combines the CMOS oscillator circuit chip with the LED die, which can produce flickering light with strong visual impact; the two-color type is composed of two different luminous colors of the die, packaged in the same epoxy In the resin lens, in addition to the two-color, a third mixed color can be obtained, which is widely used in large-screen display systems and can be packaged to form a two-color display device; the voltage type combines the constant current source chip and the LED die to package, It can directly replace various voltage indicators of 5-24V. The surface light source is formed by bonding multiple LED dies on the specified position of the miniature PCB board, using a plastic reflective frame cover and potting epoxy resin. The different designs of the PCB board determine the arrangement and connection method of the outer leads. Plug and single in-line and other structural forms. Point and area light sources have now developed hundreds of package shapes and sizes for the market and customers.

LED light-emitting displays can be composed of digital tubes or meter tubes, symbol tubes, and rectangular tubes to form various multi-digit products, which are designed into various shapes and structures according to actual needs. Taking the digital tube as an example, there are three types of packaging structures: reflective cover type, single-chip integrated type, and single-segment seven-segment type. There are two types of connection methods: common anode and common cathode. Commonly referred to as a display. The reflector type has the characteristics of large font size, material saving and flexible assembly. It is generally made of white plastic into a seven-segment shell with a reflector cavity, and a single LED die is bonded to the seven reflector cavities of the reflector. On the PCB boards that are aligned with each other, the center of the bottom of each reflective cavity is the light-emitting area formed by the die. Use the pressure welding method to bond the leads, drop epoxy into the reflector, and glue the die to the PCB board. Glue in place and then cure. The reflector type is divided into two types: empty sealing and solid sealing. The former uses epoxy resin of scattering agent and dye, which is mostly used for unit and dual-position devices; The bottom plate is coated with transparent insulating glue to improve the light output efficiency, and is generally used for digital display with more than four digits. The monolithic integrated type is to make a large number of seven-segment digital display graphics die on the luminescent material wafer, and then dicing and dividing into a single graphics die, bonding, pressure welding, and packaging a shell with a lens (commonly known as a fisheye lens). A single seven-segment method cuts the fabricated large-area LED chips into light-emitting strips containing one or more dies. The same seven strips are bonded to the digital-shaped Kovar frame. After pressure welding, epoxy resin, etc. Resin encapsulated composition. Single-chip and single-stripe are characterized by micro-miniaturization and can be packaged in dual in-line packages, most of which are special-purpose products. The LED light bar display is placed on a circuit board with a length of 106mm, and 101 dies (up to 201 dies) are placed. It is a high-density package. Using the principle of optical refraction, the point light source is imaged through the 13-15 gratings of the transparent cover. To complete the point-to-line display of each die, the packaging technology is more complicated.

The electroluminescence mechanism of the semiconductor pn junction determines that it is impossible for the LED to produce white light with a continuous spectrum, and it is impossible for a single LED to produce more than two kinds of high-brightness monochromatic light. Coating phosphor powder on the die to indirectly generate a broadband spectrum to synthesize white light; or use several (two or three, multiple) dies that emit different colors to be packaged in a component shell, and form a white LED by mixing the color light. . Both of these methods have been put into practice. In 2000, Japan produced 100 million white LEDs, which have developed into a class of products that emit white light stably. The design and assembly of multiple white LEDs does not require high luminous flux. Lord, pursue trendy electric light sources.

5 Surface Mount Packages

In 2002, surface mount packaged LEDs (SMD LEDs) were gradually accepted by the market and gained a certain market share. The shift from lead package to SMD conformed to the general development trend of the entire electronics industry, and many manufacturers launched such products.

Most of the early SMD LEDs used an improved SOT-23 with a transparent plastic body, the overall size was 3.04 × 1.11mm, and the reel container was packaged with tape. On the basis of SOT-23, the SLM-125 series and SLM-245 series of high-brightness SMD with lenses are developed. The former is single-color emitting, and the latter is two-color or three-color emitting. In recent years, SMD LED has become a development hotspot, which has well solved the problems of brightness, viewing angle, flatness, reliability, consistency, etc. It adopts lighter PCB board and reflective layer material, and displays the ring that needs to be filled in the reflective layer. Less oxygen resin, and remove the heavier carbon steel material pins, by reducing the size and weight, the product weight can easily be reduced by half, and finally make the application more perfect, especially suitable for indoor, semi-outdoor full color display application.

Table 3 shows several sizes of common SMD LEDs and the optimal viewing distance calculated based on the size (plus the necessary clearance). The pad is an important channel for its heat dissipation. The data of the SMD LED provided by the manufacturer is based on the pad of 4.0×4.0mm, and the pad can be designed to be equal to the pin by reflow soldering. Ultra-high-brightness LED products can be packaged in PLCC (plastic-encapsulated lead chip carrier)-2, with an external dimension of 3.0 × 2.8 mm. The high-brightness die is assembled by a unique method, and the thermal resistance of the product is 400K/W. CECC welding, its luminous intensity can reach 1250mcd under 50mA driving current. The character height of the seven-segment one-, two-, three- and four-digit digital SMD LED display devices is 5.08-12.7mm, and the display size selection range is wide. The PLCC package avoids the manual insertion and pin alignment process required by the pin seven-segment digital display, and meets the production requirements of automatic pick-and-place equipment. The application design space is flexible, and the display is bright and clear. The multi-color PLCC package has an external reflector that can be easily combined with a light-emitting tube or light guide to replace the current transmissive optical design with a reflective type to provide uniform illumination for a large area. Developed at 3.5V, 1A drive A power SMD LED package that operates under these conditions.

6 power package

LED chips and packages are developing in the direction of high power. Under high current, the luminous flux is 10-20 times larger than that of Φ5mm LED. Effective heat dissipation and non-deteriorating packaging materials must be used to solve the problem of light decay. Therefore, the shell and packaging are also the key. technology, LED packages that can withstand several watts of power have emerged. 5W series white, green, blue-green and blue power LEDs have been available since the beginning of 2003. The white LED light output reached 1871m, and the light efficiency was 44.31m/W. Area tube; The size of the dagger is 2.5×2.5mm, it can work under the current of 5A, and the light output can reach 2001m. It has a lot of room for development as a solid-state lighting source.

Luxeon series power LEDs are made by flip-chip welding A1GalnN power type flip-chip die on a silicon carrier with solder bumps, and then placing the flip-chip soldered silicon carrier into a heat sink and a tube case, and bonding wires for packaging. This kind of package is the best for light extraction efficiency, heat dissipation performance, and design to increase the working current density. Its main features: low thermal resistance, generally only 14°C/W, only 1/10 of conventional LEDs; high reliability, filled with stable flexible gel inside the package, in the range of -40-120°C, it will not be affected by temperature. The internal stress caused by the sudden change breaks the gold wire from the lead frame and prevents the epoxy lens from turning yellow, and the lead frame will not be stained by oxidation; the optimal design of the reflector cup and lens makes the radiation pattern controllable and optical Most efficient. In addition, its output optical power, external quantum efficiency and other excellent performance, the LED solid-state light source has been developed to a new level.

The package structure of Norlux series power LED is a multi-chip combination with a hexagonal aluminum plate as the base (making it non-conductive). 40 LED dies, the aluminum plate also acts as a heat sink. 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 dies arranged on the base is determined according to the required output optical power. The die, whose emission light is monochromatic, colored or synthetic white, is finally encapsulated in the optically designed shape with a high refractive index material. This kind of package adopts the high-density combination package of conventional die, which has high light extraction efficiency, low thermal resistance, better protection of die and bonding wire, and high optical output power under high current, which is also a promising development prospect. LED solid-state light source.

In the application, the packaged product can be assembled on a metal core PCB board with an aluminum interlayer to form a power density LED. The PCB board is used for the wiring of the device electrode connection, and the aluminum core interlayer can be used as a heat sink to obtain High 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 lighting sources such as module type, light guide plate type, concentrating type, and reflection type.

The thermal characteristics of power LEDs directly affect the operating temperature, luminous efficiency, luminous wavelength, and service life of the 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 LED has increased by 100 times, and the cost has decreased by 10 times. It is widely used in large-area graphic display full-color screen, status indication, sign lighting, signal display, LCD backlight, automotive combination taillights and interior lighting. In other respects, its development prospects have attracted global lighting manufacturers to join the LED light source and market development. White LEDs with great development and application prospects are economical and environmentally friendly as solid-state lighting devices. They are gradually replacing traditional incandescent lamps. The world’s annual growth rate is more than 20%. Provinces have launched a semiconductor lighting plan. At present, the luminous efficiency of ordinary white LED is 251m/W, and experts predict that it may exceed 3001m/W in 2005. The excellent heat dissipation and optical characteristics of power LEDs are more suitable for the field of general lighting, and are considered by academia and industry as the only way for LEDs to enter the lighting market. In order to replace fluorescent lamps, white LEDs must have a light effect of 150-2001m/W, and the price per Im should be significantly lower than 0.015/Im (current price is about 0.25$/Im, red LED is 0.065/Im) , there are still many technical problems to be studied to achieve this goal, but overcoming and solving these problems is not very far away. According to the principle of solid-state luminescence physics, 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 gas discharge lamps.

8 Conclusion

There are more than 20 upstream and midstream R&D and production units and more than 150 downstream packaging companies in the domestic LED industry, and high-end packaging products have not yet been introduced to the market. At present, the industrialization research of GaN-based blue-green LED midstream process technology has been completed, and the performance indicators of the products will reach the level of similar products in the same period abroad in a short period of time, and strive to achieve a monthly production capacity of 10kk in a relatively short time. 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 terminal products, start with special products, take automobile and urban landscape lighting as market breakthroughs, and put high-power and high-brightness LEDs in a prominent position. The results will serve the Beijing Olympic Games and the Shanghai World Expo. There is no doubt that the substrate, epitaxy, chip, packaging, and application in the industrial chain need to develop together, and multi-party interactive cultivation. Packaging is the linking part of the industrial chain, which requires attention and attention.

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