High power LED packaging technology and development trend

First, the high-power LED package due to the structure and complex process, and directly affects the performance and lifetime of LED, has been a hot research topic in recent years, especially in high power white LED package is a hot research topic in the. The function of LED package includes: 1 mechanical protection, in order to improve the reliability; 2 to strengthen the heat, to reduce the chip junction temperature, improve the performance of LED; 3 optical control, improve light efficiency, optimize the beam distribution; 4 power supply management, including AC / DC conversion, and power control.

The selection of LED packaging methods, materials, structures and processes are mainly determined by the factors such as chip structure, photoelectric / mechanical properties, specific application and cost. After 40 years of development, LED packaging has gone through the LED (Lamp LED), LED (SMD), power LED (Power) and other stages of development. With the increasing power of the chip, especially the development of solid-state lighting technology, new and higher requirements for the optical, thermal, electrical and mechanical structures of LED packaging are put forward. In order to effectively reduce the thermal resistance and improve the efficiency of light output, it is necessary to design a new technology.

Two, the key technology of high power LED packaging

High power LED packaging mainly involves light, heat, electricity, structure and process, as shown in figure 1. These factors are independent of each other and influence each other. Among them, the light is the purpose of LED packaging, heat is the key, electricity, structure and technology is a means, and performance is the embodiment of the level of packaging. From the process compatibility and reduce production costs, LED packaging design should be carried out simultaneously with the chip design. Otherwise, after the completion of the chip manufacturing, may be due to the need to adjust the structure of the chip packaging, thereby extending the product development cycle and process costs, and sometimes even impossible.

Specifically, the key technologies of high power LED packaging include:

(a) low thermal resistance packaging process

For the existing LED light efficiency level, due to the conversion of about 80% of the input energy into heat, and the LED chip area is small, therefore, chip cooling is the key problem to be solved in LED packaging. It mainly includes the layout of the chip, the selection of packaging material (substrate material, thermal interface material) and the process, heat sink design, etc..

The thermal resistance of LED package mainly includes the internal thermal resistance and thermal resistance of the material (heat sink and heat sink). The role of the heat sink is to absorb the heat generated by the chip, and heat conduction to the heat sink, and heat exchange with the outside world. Commonly used heat sink substrate materials include silicon, metal (such as aluminum, copper), ceramics (such as, AlN, SiC) and composite materials, etc.. Such as Nichia, the third generation of LED using CuW as substrate, the 1mm flip chip on the CuW substrate, the thermal resistance is reduced, the luminous power and improve the efficiency; Lamina Ceramics company developed the LTCC substrate, as shown in Figure 2 (a), and developed the corresponding LED sealing technology. Firstly, the high power LED chip and the corresponding ceramic substrate suitable for eutectic welding are prepared, and then the LED chip is directly welded with the substrate. Because the substrate integrated eutectic solder layer, ESD protection circuit, drive circuit and control circuit of compensation, not only has the advantages of simple structure, high thermal conductivity and the thermal interface material, less, greatly improve the heat dissipation performance, puts forward solutions for the high power LED array package. The high thermal conductivity of copper coated ceramic plate developed by German Curmilk company, a ceramic substrate (or AlN) and conductive layer (Cu) under high pressure and high temperature sintering, without the use of adhesive, so the good thermal conductivity, high strength, high insulating property, such as shown in Figure 2 (B). The thermal conductivity of aluminum nitride (AlN) is 160W/mk, and the coefficient of thermal expansion is equivalent to that of silicon, which reduces the thermal stress.

The results show that the interface has a great influence on the thermal resistance, and it is difficult to obtain a good cooling effect if the interface is not properly handled. For example, at room temperature, there may be an interfacial gap in a good interface at high temperature, and the warpage of the substrate may also affect the bonding and local cooling. The key to improve the LED packaging is to reduce the interface and interface thermal resistance, enhance heat dissipation. Therefore, the selection of thermal interface material (TIM) between the chip and the substrate is very important. LED commonly used for TIM conductive adhesive and conductive adhesive, due to the low thermal conductivity, generally 0.5-2.5W/mK, resulting in high interfacial thermal resistance. The thermal resistance of the interface can be greatly reduced by the use of low temperature eutectic solder, solder paste or conductive adhesive with nano particles.

(two) the high rate of light taking package structure and process

In the process of using LED, radiation photons produced in the composite to launch the loss, mainly includes three aspects: the internal structure and chip absorbing material; a photon in the outgoing interface due to the reflection loss caused by the refractive index difference; and due to the incident angle is greater than the critical angle of total reflection loss. Therefore, a lot of light can not shoot out from the chip to the outside. By coating on the chip surface with a layer of relatively high refractive index transparent adhesive layer (sealant), due to the adhesive layer in between the chip and the air, thus effectively reducing the loss of photons in the interface, improve the light extraction efficiency. In addition, the role of potting glue also includes mechanical protection of the chip, stress release, and as a light guide structure. Therefore, it requires high light transmittance, high refractive index, good thermal stability, good fluidity, easy spraying. In order to improve LED