High-power LEDs have the advantages of long life, low pollution, low power consumption, energy saving and impact resistance. Compared with traditional lighting fixtures, high-power LEDs not only have good monochromaticity, high optical efficiency, strong light efficiency, but also can meet different needs of high color rendering index. Despite this, the packaging process of high-power LEDs has strict requirements.
It is embodied in: low cost; maximum system efficiency; easy replacement and maintenance; multiple LEDs can be modularized;
According to various factors to be considered in high-power LED packaging technology , several points have been proposed in the key technologies of packaging. mainly include:
(1) In terms of high-power LED heat dissipation : Consider a low thermal resistance package. The LED chip is a solid-state semiconductor device that is the core of the LED light source. Since the high-power LED chips are of different sizes, the driving method uses a constant current driving method.
The electric energy can be directly converted into light energy. Therefore, the LED chip needs to absorb most of the input electric energy during the lighting process, and a large amount of heat is generated in the process. Therefore, the heat dissipation technology for high-power LED chips is an important technology for LED packaging technology, and also a key problem that must be solved in the process of high-power LED packaging .
(2) The heart of the LED is a semiconductor wafer. One end of the chip is attached to a bracket, one end is a negative pole, and the other end is connected to the positive pole of the power source. Therefore, the high light extraction package structure is also an important key technology in the high power LED packaging process.
In the process of illuminating the LED chip, during the emission process, due to the difference in refractive index at the interface, the loss of photon reflection and the total loss of reflection may be caused. Therefore, a relatively high refractive index transparent layer may be coated on the surface of the chip. glue.
This layer of transparent plastic must have high light transmittance, high refractive index, good fluidity, easy spraying, and good thermal stability. Currently commonly used transparent adhesive layers are epoxy resin and silica gel.
LED package form
With the development of science and technology, LEDs are available in a variety of package formats, including leaded packages, surface mount chip packages, on-chip chip direct packages, and system package packages.
Low-power LED packages are typically in the form of leaded LED packages. Leaded LED packages are also common. Ordinary LEDs are basically in a lead package. The heat of the lead-type LED package is radiated from the lead frame of the negative electrode to the PCB board, and the heat dissipation problem is also better solved. However, there are certain disadvantages, that is, the thermal resistance is large, and the service life of the LED is short.
Surface Mount SMD Package (SMT) is a new type of LED package that encapsulates already packaged LED devices into a fixed location. The advantages of SMT packaging technology are high reliability, easy automation, and high frequency characteristics. The SMTLED package is one of the most popular chip package processes in the electronics industry today.
Chip-on-board (COB) LED packaging technology is a direct mounting technology that is a process of directly attaching a chip to a printed circuit board, then stitching the leads, and finally protecting the chip and the lead using organic glue. The COB process is mainly used in high power LED arrays. Has a high degree of integration.
System-packaged (SIP) LED packaging technology is a technology developed in recent years.
It mainly meets the requirements of system portable and system miniaturization. Compared to other LED packages, SIP packages are the most integrated and relatively low cost. Multiple LED chips can be assembled in one package.
In the actual application process, according to the requirements of the experiment, the author uses the form of SMT package. In the high-power LED packaging process, taking into account the high-power LED cooling factor, I use the traditional way of using aluminum substrate cooling. Through practical applications, we found that SMT packaging technology meets the actual thermal requirements. The concrete physical map of the physical aluminum substrate is shown in Figure 1.
High-power LED process
LED packaging is a multidisciplinary process technology. It involves research content such as optics, thermals, electricity, mechanics, materials, and semiconductors. Therefore, the packaging technology of high-power LED is a complex and comprehensive subject. Good LED packaging needs to take into account the factors of each discipline. The following is a brief introduction to the LED packaging process flow.
LED illuminators are wafers, and different wafers vary in price and size. The size of the wafers is different, which brings certain difficulties to the LED package. Tests have also been made in the selection of stents.
The choice of bracket and shape plastic mold determines the overall dimensions of the LED package. The bracket carries the LED chip, so the choice of the bracket depends on the actual length of the side of the LED chip.
The choice of solid crystal glue is mainly considering its cohesive force. The thickness of the solid crystal glue coated by the same size determines the thermal resistance of the packaged LED.
Silver rubber has a lower thermal resistance than insulating rubber. The size of the thermal resistance determines the light output efficiency of the LED. Similarly, silver glue has another feature, that is, good thermal conductivity.
The welding wire process can use mechanical welding wire and artificial welding wire. Since the LED package produced in this experiment is small, the artificial welding wire is used. Under the early high power microscope, the positive and negative electrodes of the chip are soldered to the two pin corners of the bracket using gold wires. Be patient and careful during the welding process. The entire LED packaging process flow is produced according to the process shown in Figure 2.
There are many factors that must be considered in the packaging process of LEDs, and every step must be carefully considered. Thermal resistance affects the light output efficiency of the LED. Thermal performance and chromatic stability affect the performance of the LED. We have to choose the right material in the LED packaging process.
Improve existing device structures, seek better ways to cool LEDs, and reduce thermal resistance of LEDs. The choice of materials is very important during the packaging process, but the thermal interface and optical interface are equally important. For LED luminaires, not only the above factors must be considered, but also the driving power supply of LEDs, the integration of modules, and the application fields are considered together. Therefore, there is a long way to go for research on high-power LED packaging technology.
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