The types of LED product packaging structures are shown in Table 2, which are also classified according to the characteristics such as luminous color, chip material, luminous brightness and size. A single die usually constitutes a point light source, and a plurality of dies can usually constitute a surface light source and a line light source for information, status indication and display. Light-emitting displays are also formed by combining multiple dies with appropriate optical structures through appropriate connections (including series and parallel connections) to form light-emitting segments and light-emitting points of the light-emitting display. Surface mount LED can gradually replace pin LED, and the application design is more flexible. It has occupied a certain share in the LED display market, and has the trend of accelerating development. The listing of some products of solid lighting source will become the medium and long-term development direction of LED in the future. The lead frame is used as the lead of various package shapes, which is the first package structure successfully developed and put into the market. There are many varieties, high technical maturity, and the internal structure and reflective layer of the package are still improving. Standard LED is considered by most customers as the most convenient and economical solution in the display industry. A typical traditional LED is placed in a package that can withstand 0. 1W input power, and 90% of its heat is emitted from the negative lead frame to the PCB board and then to the air. How to reduce the temperature rise of pn junction during operation must be considered in packaging and application. Most of the packaging materials are epoxy resin cured at high temperature, which has excellent optical properties, good process adaptability and high product flexibility. Can be made into colored transparent or colorless transparent and colored scattering or colorless scattering lens packages. Different lens shapes constitute various shapes and sizes. For example, the circle is divided into 2mm, 3mm, 4.4mm, 5mm, 7mm, etc. According to the diameter, different components of epoxy resin can produce different luminous effects. There are many different packaging structures of color point light source: epoxy resin packaging with ceramic base has good working temperature performance, the pin can be bent into the required shape and the volume is small; Plastic reflector package with metal base is an energy-saving indicator lamp, which is suitable for indicating power supply. CMOS oscillator circuit chip and LED die are packaged in a flashing way, which can generate flashing light with strong visual impact by itself. The bichromatic type consists of two dies with different luminous colors, which are encapsulated in the same epoxy lens. Besides two colors, a third mixed color can be obtained, which is widely used in large-screen display systems and can be packaged into two-color display devices. The voltage type combined with constant current source chip and LED die package can directly replace various voltage indicator lamps of 5-24V. The surface light source is formed by combining a plurality of LED dies on a designated position of a miniature PCB, using a plastic reflective frame cover and encapsulating epoxy resin. The different design of PCB determines the arrangement and connection mode of external leads, including dual in-line and single in-line structures. Point light sources and surface light sources have developed hundreds of packaging shapes and sizes for the market and customers.
LED light-emitting display can be composed of digital tube or inclined tube, symbol tube and rectangular tube, and can be designed into various shapes and structures according to actual needs. Take the digital tube as an example, there are three packaging structures: reflective cover, monolithic integration and single seven-segment, and the connection modes are * * * anode and * * * cathode. One is usually called a digital tube, and more than two are usually called displays. The reflector type has the characteristics of large font, material saving and flexible assembly. Usually, it is made of white plastic with seven sections of shell with reflecting cavities, and a single LED die is bonded to a PCB aligned with the seven reflecting cavities of the reflector. The center of the bottom of each reflection cavity is the light-emitting area formed by the die. Epoxy resin is dripped into the reflector by pressure welding bonding wires, and the reflector is aligned and bonded with the PCB with bonding die, and then cured. The reflector is divided into two types: empty seal and solid seal. The former uses epoxy resin containing scattering agent and dye, which is mostly used in single and two-position devices. The latter is covered with color filter and light equalizing film, and the tube core and bottom plate are coated with transparent insulating glue to improve luminous efficiency, which is generally used for digital display with more than four digits. Monolithic integration is to make a large number of seven-segment digital display graphics molds on the luminescent material wafer, then dice and divide them into monolithic graphics molds, and bond, pressure weld and package the shell and lens (commonly known as fisheye lens). A single seven-segment LED chip is cut into light-emitting strips containing one or more lamps, and the same seven strips are bonded on a digital kovar frame, and then are packaged by pressure welding and epoxy resin. Single-chip and single-chip products are characterized by miniaturization, which can adopt dual in-line packaging, and most of them are special products. LED light column display is a high-density package that places 10 1 die (at most 20 1 die) on a circuit board with a length of 16 mm, and passes through a transparent cover of 13- 15 by using the principle of optical refraction.
The electroluminescent mechanism of semiconductor pn junction determines that LED can not produce white light with continuous spectrum, and a single LED can not produce more than two kinds of high-brightness monochromatic light. Only when the blue or ultraviolet LED die is coated with fluorescent powder during packaging can broadband spectrum be indirectly generated and white light can be synthesized. Or several (two or three or more) dies emitting different colors of light are packaged in a module housing, and a white LED is formed by mixing the colors of light. These two methods have been put into practical use. In 2000, Japan produced 6,543.8 billion white LEDs, which developed into a product that emits white light stably. Many white LEDs are designed and assembled into electric light sources with low requirements for luminous flux, paying attention to local decoration and pursuing new trends. In 2002, surface mount package LED(SMDLED) was gradually accepted by the market and gained a certain market share. Switching from pin package to SMD is in line with the development trend of the whole electronics industry, and many manufacturers have introduced such products.
Most of the early SMD LED were improved by transparent plastic SOT-23, and were woven and packaged in coiled containers. On the basis of SOT-23, the former emits monochromatic light, while the latter emits dichromatic or trichromatic light. In recent years, SMD LED has become a hot spot, which has solved the problems of brightness, viewing angle, flatness, reliability and consistency. Lighter PCB and reflective layer materials are used, less epoxy resin needs to be filled in the reflective layer of the display, and heavier carbon steel pins are removed. By reducing the volume and weight, the product weight can be easily reduced by half, and the final application is more perfect, especially suitable for indoor and semi-outdoor full color.
Table 3 shows several sizes of common SMD LED and the optimal viewing distance (plus necessary clearance) calculated according to the sizes. Pad is an important channel for heat dissipation. The data of SMD LED provided by manufacturers are all based on 4.0×4.0mm pads, and the pads can be designed to be equal to the pins through reflow soldering. Ultra-high brightness LED products can be packaged in PLCC -2, and the high brightness die is assembled in a unique way. The thermal resistance of the product is 400K/W, it can be welded in CECC mode, and the luminous intensity is 1250mcd under the driving current of 50mA. The one-,two-,three-and four-digit seven-segment digital patch LED display device has a character height of 12.7mm and a wide range of display sizes. PLCC package avoids the manual insertion and pin alignment procedures required for seven-segment digital tube display, meets the production requirements of automatic pick-and-place equipment, and has flexible application design space and bright and clear display. Multicolor PLCC package has an external reflector, which can be easily combined with LED or light guide. The reflective optical design is used instead of the transmissive optical design to provide uniform illumination for a large area. A power patch LED package working at 3.5V and 1A driving conditions is developed. LED chips and packages are developing towards high power, and the light flux at high current is 10-20 times larger than that of φ 5 mm LED. Therefore, effective heat dissipation and non-deteriorating packaging materials must be adopted to solve the problem of light attenuation. Therefore, shell and packaging are also its key technologies, and LED packaging that can withstand several W powers has emerged. Since the beginning of 2003, 5W series of white, green, blue-green and blue power LEDs have been supplied. The light output of white LED reaches 187 1lm, and the light efficiency is 44.5438+0lm/w. A large area LED with power of 10W has been developed. The size of bi is 2.5×2.5mm, and it can work at 5A current, and the light output is 200 1lm. As a solid lighting source, it has great development space.
Luxeon series power LED is to flip-chip A 1GalnN power on a silicon carrier with solder bumps, and then put the flip-chip silicon carrier into a heat sink and package, and bond wires for packaging. This package is the best for the design of light extraction efficiency, heat dissipation performance and increasing working current density. Its main characteristics are: low thermal resistance, generally only 14℃/W, only110 of conventional LED; The flexible gel with high reliability and stable packing will not cause the gold wire to be disconnected from the lead frame due to internal stress caused by sudden temperature change within the range of -40- 120℃, the epoxy lens will not turn yellow, and the lead frame will not be contaminated by oxidation; The optimized design of reflector cup and lens makes the radiation pattern controllable and the optical efficiency is the highest. In addition, its output optical power and external quantum efficiency are excellent, which develops LED solid-state light source to a new level.
The packaging structure of Norlux series power LED is a multi-chip combination based on hexagonal aluminum plate (making it non-conductive). The diameter of the base is 3 1.75mm, and the light-emitting area is located in the center of the base, with a diameter of about (0.375×25.4)mm, which can accommodate 40 LED dies, and the aluminum plate is used as a heat sink at the same time. The bonding wire of the chip is connected with the positive electrode and the negative electrode through two contacts made on the substrate, and the number of chips mounted on the substrate is determined according to the required output light power, so that the combined packaging of ultra-high brightness AlGaInN and AlGaInP chips is realized, and the emitted light is monochromatic, colored or synthetic white, respectively, and finally the high refractive index material is used for packaging according to the optical design shape. This kind of package adopts conventional die high-density combination package, which has high light output efficiency, low thermal resistance, better protection for die and bonding wire, and higher light output power under high current. It is also a promising LED solid light source.
In application, the packaged products can be assembled on a metal core PCB board with an aluminum interlayer to form a power density led. PCB board is used as the wiring of device electrode connection, and aluminum core sandwich can be used as heat sink to obtain higher luminous flux and photoelectric conversion efficiency. In addition, the packaged SMD LED is small in size, and can be flexibly combined to form modular, light guide plate, spotlight, reflective and other colorful light sources.
The thermal characteristics of power LED directly affect the working temperature, luminous efficiency, luminous wavelength and service life of LED. Therefore, the packaging design and manufacturing technology of power LED chip is particularly important. COB packaging can directly package multiple chips on the metal-based printed circuit board MCPCB, and directly dissipate heat through the substrate, which not only reduces the manufacturing process and cost of the bracket, but also has the heat dissipation advantage of reducing thermal resistance.
From the perspective of cost and application, COB will become the mainstream direction of future lighting design. The LED module encapsulated by COB is equipped with multiple LED chips on the bottom board. Using multiple chips can not only improve the brightness, but also help to realize the reasonable configuration of LED chips, reduce the input current of a single LED chip and ensure high efficiency. Moreover, this kind of surface light source can greatly expand the heat dissipation area of the package and make the heat more easily conducted to the shell.
In order to enter the field of general lighting, production cost is the first limiting factor. To reduce the cost of semiconductor lighting fixtures, we should first consider how to reduce the packaging cost of LED. The traditional method of LED lamps is: LED light source discrete device →MCPCB light source module →LED lamps, which is mainly based on the lack of applicable core light source components, which is not only labor-consuming and time-consuming, but also costly. In fact, if we take the route of "COB light source module →LED lamp", we can not only save labor and time, but also save the cost of device packaging.
In terms of cost, compared with the traditional COB light source module, it can save device packaging cost, light engine module manufacturing cost and secondary light distribution cost in lighting applications. In the lighting system with the same function, the overall cost can be reduced by about 30%, which is of great significance to the application and popularization of semiconductor lighting. In terms of performance, COB light source module can effectively avoid the disadvantages of spot light and glare in the combination of discrete light source devices by reasonably designing and molding microlenses, and can effectively improve the color rendering of light source (up to more than 90%) without reducing the efficiency and life of light source by adding appropriate red chip combination.
In application, COB light source module can make the installation and production of lighting factory easier and more convenient. In production, the existing technology and equipment can fully support the large-scale high-yield manufacturing of COB light source modules. With the expansion of LED lighting market, the demand for lamps is increasing rapidly. We can gradually form a series of mainstream products of COB light source module for mass production according to the needs of different lighting applications. First, the process:
1) cleaning: use ultrasonic to clean PCB or LED bracket and dry it.
2) Mounting: after the silver paste is prepared on the bottom electrode of the LED die, it is expanded, the expanded die is placed on the crystal thorn table, and the die is mounted on the corresponding pads of PCB or LED one by one with a crystal thorn pen under a microscope, and then the silver paste is sintered and solidified.
3) Bonding: use aluminum wire or gold wire welder to connect the electrode to the LED die as the lead wire for current injection. When LED is directly mounted on PCB, aluminum wire welder is generally used.
4) packaging: epoxy resin is used to protect the LED die and bonding wires through dispensing. Dispensing glue on PCB has strict requirements on the shape of cured glue, which is directly related to the brightness of finished backlight. This process will also undertake the task of lighting phosphor.
5) Welding: If the backlight is SMD-LED or other packaged LED, the LED needs to be welded to the PCB before the assembly process.
6) Film cutting: various diffusion films and reflective films required by backlight are cut by a punch.
7) Assembly: According to the requirements of drawings, manually install various materials of backlight in the correct position.
8) Test: Check whether the photoelectric parameters and light uniformity of the backlight are good.
9) Packaging: the finished products are packaged and put into storage as required.
Second, the packaging process.
1, the task of LED packaging
The external leads are connected to the electrodes of the LED chip, which protects the LED chip and improves the light extraction efficiency. The key processes are assembly, pressure welding and packaging.
2, LED packaging form
LED packaging forms can be said to be various, mainly according to different applications, adopting corresponding external dimensions, heat dissipation measures and luminous effects. According to the packaging form, LEDs are divided into lamp LEDs, top LEDs, side LEDs, surface mounted LEDs, high-power LEDs and so on.
3.LED packaging process flow
Three. Description of packaging process
1, chip detection
Microscopic examination: whether there are mechanical damage and pits on the surface of the material.
Whether the chip size and electrode size meet the process requirements; Is the electrode pattern complete?
Step 2 expand the film
Because the LED chips are still closely arranged after dicing, the spacing is very small, which is not conducive to the operation of the later process. We use a film stretching machine to expand the film attached to the chip, that is, we can stretch the distance between LED chips to about 0.6 mm, or we can use manual expansion, but it is easy to cause bad problems such as chip falling and waste.
3, dispensing
Apply silver glue or insulating glue to the corresponding position of LED bracket. The process difficulty lies in the control of dispensing amount, and the height and location of colloid have detailed process requirements. Because there are strict requirements for the storage and use of silver glue and insulating glue, the proofing, stirring and use time of silver glue are all matters that must be paid attention to in the process.
Step 4 prepare glue
Contrary to dispensing, glue making is to apply silver glue to the electrode on the back of LED with a glue making machine, and then install the LED coated with silver glue on the LED bracket. The efficiency of dispensing is much higher than dispensing, but not all products are suitable for dispensing.
5. Handmade thorns
The expand LED chips are place on that fixture of the puncture table, the LED bracket is placed under the fixture, and the LED chips are puncture to corresponding positions one by one with a needle under a microscope. Compared with automatic mounting, manual punching has an advantage that it is convenient to replace different chips at any time, and it is suitable for products that need to install multiple chips.
6. Automatic shelves
Automatic mounting is actually a combination of two steps: gluing and chip mounting. First, apply silver glue on the LED bracket, then use the suction nozzle to suck the LED chip up and move it, and then put it in the corresponding bracket position. In the process of automatic mounting, it is mainly necessary to be familiar with the equipment operation programming, and at the same time, to adjust the gluing and installation accuracy of the equipment. In the selection of suction nozzle, try to use bakelite suction nozzle to prevent damage to the surface of LED chip, especially blue-green chip. Because the steel nozzle will scratch the current diffusion layer on the chip surface.
7. Sintering
The purpose of sintering is to solidify the silver paste, and the sintering needs to monitor the temperature to prevent batch defects. The sintering temperature of silver paste is generally controlled at 65438 050℃ and the sintering time is 2 hours. It can be adjusted to 170℃ and 1 hour according to the actual situation. Insulation glue is generally 150℃ 1 hour.
The silver glue sintering furnace must be opened every 2 hours, and the sintered products should be replaced according to the process requirements. It is not allowed to open it at will. The sintering furnace shall not be used for other purposes to prevent pollution.
8, pressure welding
The purpose of pressure welding is to lead the electrode to the LED chip and complete the connection of the internal and external leads of the product. There are two bonding processes for LED: gold wire ball bonding and aluminum wire bonding. The picture on the right shows the bonding process of aluminum wire. Press the first point on the electrode of the LED chip, then pull the aluminum wire above the corresponding bracket, press the second point, and then pull the aluminum wire off. The gold wire ball welding process is to burn a ball first and then press the first point, and other processes are similar. Bonding is a key link in LED packaging technology, and the main processes that need to be monitored are the arch wire shape, solder joint shape and tension of bonded gold wire. The in-depth study of pressure welding technology involves many problems, such as gold wire material, ultrasonic power, pressure of pressure welding, selection of cutting knife, movement track of cutting knife and so on.
9, dispensing and packaging
There are three kinds of LED packaging: glue, potting and plastic packaging. Basically, the difficulties in process control are bubbles, lack of materials and black spots. The design is mainly about the selection of materials, and the combination of epoxy and bracket is selected. Generally speaking, top LEDs and side LEDs are suitable for distribution packaging. Manual dispensing packaging requires a high level of operation, and the main difficulty is to control the amount of dispensing, because epoxy will thicken during use. The dispensing of white LED still has the problem of color difference caused by phosphor precipitation.
10, potting and packaging
The LED lamp is encapsulated by potting. The process of potting is to inject liquid epoxy resin into the LED molding cavity, then insert the pressure welded LED bracket, put it in the oven to cure the epoxy resin, and then take the LED out of the molding cavity for molding.
1 1. Forming and packaging
Put the pressure-welded LED bracket into the mold, close the upper and lower molds with a hydraulic press and vacuum, put the solid epoxy resin into the rubber channel of the mold with a hydraulic ejector pin, heat it at the entrance of the rubber channel, and the epoxy resin enters each LED molding groove along the rubber channel for curing.
12, curing and post-curing
Curing refers to the curing of encapsulated epoxy resin. Generally, the curing conditions of epoxy are 135℃ and 1 hour. Molding and packaging are generally carried out at 150℃ for 4 minutes.
13, post curing
Post-curing is to completely cure epoxy resin and thermally age LED. Post-curing is very important to improve the bonding strength between epoxy resin and stent. The general condition is 120℃ for 4 hours.
14, steel cutting and slicing
Because the LEDs are connected together in production, the LEDs in the lamp package cut off the connecting ribs of the LED bracket by cutting ribs. SMD-LED is on PCB, and it needs dicing machine to complete the separation.
15, test
Test the photoelectric parameters of LED, check the overall dimensions, and classify LED products according to customer requirements.
16 packaging
And packaging the finished product. Ultra-bright LEDs need anti-static packaging.