High-power LED packaging technology

The article is mainly about the introduction of high-power LED chip packaging technology. It includes the packaging requirements of high-power LEDs, the key technologies of packaging, the form of packaging, and the process flow of high-power LED packaging technology.

The process design of the LED includes the design of the chip and the packaging of the chip. At present, the packaging technology of high-power LED and its heat dissipation technology are hotspots in today's society. Since the high-power LED packaging process is simpler to say, the actual process is very complicated. And LED packaging technology directly affects the life of the LED. Therefore, in the process of high-power LED packaging, many factors must be considered, such as light, heat, electricity, machinery and many other factors. Optical aspects should consider the high-power LED light decay problem, thermal considerations should consider the heat dissipation of high-power LEDs, electrical considerations should be taken into account the design of the drive power of high-power LEDs, mechanical considerations should be taken into account the packaging form of LEDs in the packaging process. Wait.

1 High power LED package requirements and key technologies

(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, high light efficiency, but also can meet different needs of high color rendering index. Despite this, the packaging process for high-power LEDs has strict requirements.

Specifically: 1 low cost; 2 system efficiency maximization; 3 easy to replace and maintain; more than 4 LEDs can be modularized; 5 low heat dissipation coefficient and other simple requirements.

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 possible total reflection loss may be applied, so that a relatively high refractive index of the chip surface 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.

2 LED package form

With the development of science and technology, LEDs can be packaged in a variety of forms, including leaded packages, surface mount chip packages, chip-on-chip packages, and system package packages.

Low-power LED packages are typically packaged in a leaded LED package. 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 an already packaged LED device into a fixed location. The advantages of SMT packaging technology are high reliability, easy automation, and high frequency characteristics. The SMT LED package is one of the most popular chip package processes in the electronics industry today.

The chip-on-board (COB) LED package technology is a direct mount 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 primarily used in high power LED arrays. Has a high degree of integration.

System-packaged (SIP) LED packaging technology is a technology that has been 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 heat dissipation 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.

3 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. The process is shown in Figure 2.

The illuminator of the LED is a wafer, and different wafers have different prices and different sizes. 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 LED chip.

The choice of solid crystal glue is mainly considering its cohesive force, and its particle size is large. The thickness of the same applied solid glue determines the thermal resistance of the packaged LED.

The solid glue used by the author is silver glue. 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 wire bonding process can use mechanical wire and artificial wire. Because the LED package produced in this experiment is small, the artificial 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 in accordance with the process shown in Figure 2.

4 Conclusion

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 choose the right materials for the LED packaging process. Improve existing device structures, seek better ways to dissipate LEDs, and reduce thermal resistance of LEDs. The choice of materials (heat sink, phosphor, potting) 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 should be considered, but also the driving power of the LED, the integration of the modules, and the application fields should be considered together. Therefore, there is a long way to go for research on high-power LED packaging technology.

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