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 “Tenth Five-Year Plan†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.
LED package specificity
LED packaging technology 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. In a conventional Φ5mm LED package, a square die with a side length of 0.25 mm is bonded or sintered on a lead frame. The positive electrode of the die passes through a spherical contact point and a gold wire, and is bonded to an inner lead connected to a pin, and the negative electrode passes through the reflection. The 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/°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 1°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 ​​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.
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