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 vibration resistance. With stable and reliable performance, light weight, small size and low cost, the company has developed rapidly and is now able to mass produce high-brightness and high-performance products of various colors in the visible spectrum. The domestic LED production of red, green, orange and yellow accounts for about 12% of the world total. The industrial goal during the â€œTenth Five-Year Planâ€ period is to achieve an annual output of 30 billion, 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 super high-brightness LED dies, breaking through the key technologies of GaN materials, achieving medium-to-large production of blue, green and white LEDs. It is predicted that by 2005, the international market demand for LED will be about 200 billion, with sales reaching 80 billion US dollars.
In the LED industry link, upstream is the production of LED substrate wafers 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 and the market-oriented industrialization must go through. In a sense, it is the link between the industry and the market. Only the packaged product can become a terminal product, and it can be put into practical application to provide services to customers. The industrial chain is interlocking and seamless. The special LED packaging technology of LED packaging is mostly developed and evolved on the basis of discrete device packaging technology, but it has great speciality.
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 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. The conventional Î¦5mm type LED package bonds or sinters a square die with a side length of 0. 25mm on the lead frame. The positive electrode of the die passes through the spherical contact point and the gold wire, and is bonded to the inner lead to be 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), to act as a lens or diffuse lens Function, control the divergence angle of the light; the refractive index of the die is too much related to the refractive index of the air, so that the critical angle of total reflection inside the die is small, and only a small part of the light generated by the active layer is taken out, most of which are easy to be in the tube The inside of the core is absorbed by multiple reflections, and it is easy to cause total reflection to cause excessive light loss. The epoxy resin with corresponding refractive index is used as a transition to improve the light exiting efficiency of the die. The epoxy resin used to form the envelope must have moisture resistance, insulation, mechanical strength, and a high refractive index and transmittance for light emitted from the die. The choice of packaging materials with different refractive indices, the influence of package geometry on photon escape efficiency is different, and 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 by 0.2-0.3nm/Â°C, and the spectral width increases, which affects the color vividness.
In addition, when the forward current flows through the pn junction, the heating loss causes the junction region to produce a temperature rise. At around 1Â°C, the luminous intensity of the LED is reduced by about 1%, and the heat dissipation of the package is maintained. Purity and luminescence 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 20 mA. However, the light output of LEDs will increase with the increase of current. At present, many power LEDs can drive currents up to 70mA, 100mA or even 1A. It is necessary to improve the package structure, the new LED package design concept and low thermal resistance package structure and Technology to improve thermal properties. 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 â€‹â€‹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 100Im/W, green LEDs are 501m/W, and the luminous flux of single LEDs has reached dozens. Im.
LED chips and packages no longer follow the traditional design concept and manufacturing mode of Gong. 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 internal structure of the die and package enhances the probability of photon emission inside the LED, improves the light efficiency, solves the heat dissipation, optimizes the design of light extraction and heat sink, improves the optical performance, and accelerates the surface mount SMD process. .
Product packaging structure type Since the 1990s, LED chip and material manufacturing technology has made many breakthroughs in research and development, transparent substrate trapezoidal structure, textured surface structure, chip flip-chip structure, commercial ultra-high brightness (above 1cd) Red, orange, yellow, green, and blue LED products have been asked one after another. As shown in Table 1, in 2000, applications in low- and medium-light flux special lighting were applied. 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, variety, specifications of the product.
The types of LED product package structures are shown in Table 2, and are also 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 occupied a certain share in the LED display market and has an accelerated development trend.
Some solid-state lighting sources have been launched, which will become the medium and long-term development direction of LEDs in the future. The lead-type package LED foot package adopts the lead frame as the pin of various package appearances. 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 in the package are still in the package. keep improving. 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 which is pinned by the negative pole. The rack is distributed 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 properties, good process adaptability, high product reliability, and can be made into a transparent or colorless transparent and colored scattering or colorless 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: Î¦2 mm, Î¦3 mm, Î¦4.4 mm, Î¦5 mm, Î¦7 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 the various voltage indicators of 5-24V. 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.
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 the epoxy resin is dropped in the reflection cover, and the PCB board of the die is bonded. Bonded in place and then cured.
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