At present, with the development of LED industry at home and abroad in the direction of high efficiency, high density, high power, etc., the overall LED has made rapid progress, and the power is also increasing. The development of superior performance heat dissipation materials has become an urgent task to solve the LED heat dissipation problem . Generally speaking, LED luminous efficiency and service life will decrease with the increase of junction temperature. When the junction temperature reaches above 125 ℃, the LED will even fail. In order to keep the LED junction temperature at a lower temperature, a heat-dissipating substrate material with high thermal conductivity and low thermal resistance and a reasonable packaging process must be used to reduce the overall packaging thermal resistance of the LED.

At this stage, the commonly used substrate materials are Si, metals and metal alloy materials, ceramics and composite materials, etc., their thermal expansion coefficient and thermal conductivity are shown in the following table. Among them, the cost of Si materials is high; the inherent conductivity and thermal expansion coefficient of metals and metal alloy materials do not match the chip materials; ceramic materials are difficult to process and other shortcomings, it is difficult to meet the various performance requirements of high-power substrates at the same time.

Since the development of power LED packaging technology, the available heat dissipation substrates mainly include epoxy resin copper-clad substrates, metal-based copper-clad substrates, metal-based composite substrates, ceramic copper-clad substrates, etc.

Epoxy copper-clad substrate is the most widely used substrate in traditional electronic packaging. It plays the role of support, conduction and insulation. Its main features are: low cost, high moisture absorption resistance, low density, easy processing, easy realization of fine graphic circuits, suitable for large-scale production, etc. However, because the base material of FR-4 is epoxy resin, the thermal conductivity of organic materials is low, and the high temperature resistance is poor, so FR-4 cannot meet the requirements of high-density, high-power LED packaging, and is generally only used in low-power LED packaging.

The metal-based copper-clad substrate is a new type of substrate after FR-4. It is made by directly bonding the copper foil circuit and the polymer insulation layer through the thermally conductive bonding material to the metal with high thermal conductivity and the base. Its thermal conductivity is about 1.12 W / m · K, compared with FR- 4 has a greater improvement. Due to its excellent heat dissipation, it has become the most widely used product in the market of high-power LED heat dissipation substrates. But it also has its inherent shortcomings: the thermal conductivity of the polymer insulating layer is low, only 0.3 W / m · K, resulting in the heat cannot be directly transferred from the chip to the metal base; the thermal expansion coefficient of the metal Cu and Al Larger, may cause more serious thermal mismatch problems.

The most representative material of the metal-based composite substrate is aluminum silicon carbide. Aluminum silicon carbide is a metal matrix composite material that combines the low expansion coefficient of SiC ceramics and the high thermal conductivity of metal Al. It combines the advantages of two materials with low density, low thermal expansion coefficient, high thermal conductivity, and high rigidity. A series of excellent features. The thermal expansion coefficient of AlSiC can be adjusted by changing the content of SiC to match the thermal expansion coefficient of adjacent materials, thereby minimizing the thermal stress of both.

Common ceramic substrate materials are Al2O3, aluminum nitride, SiC, BN, BeO, Si3N4, etc. Compared with other substrate materials, ceramic substrates have the following characteristics in mechanical properties, electrical properties, and thermal properties:

(1) Mechanical properties. Mechanical strength, can be used as a supporting member; good workability, high dimensional accuracy; smooth surface, no micro-cracks, bending, etc.

(2) Thermal properties. The thermal conductivity is large, the thermal expansion coefficient is matched with Si and GaAs and other chip materials, and the heat resistance is good.

(3) Electrical properties. The dielectric constant is low, the dielectric loss is small, the insulation resistance and insulation breakdown power are high, the performance is stable under high temperature and high humidity conditions, and the reliability is high.

(4) Other properties. Good chemical stability, no hygroscopicity; oil-resistant, chemical-resistant; non-toxic, pollution-free, small emission of alpha rays; stable crystal structure, not easy to change within the use temperature range; abundant raw material resources.

For a long time, Al2O3 and BeO ceramics are two main substrate materials for high-power packaging. However, these two kinds of substrate materials have inherent shortcomings. Al2O3 has low thermal conductivity, and the coefficient of thermal expansion does not match the chip material. Although BeO has excellent comprehensive performance, the production cost is high and highly toxic. Therefore, from the aspects of performance, cost and environmental protection, these two substrate materials cannot be used as the most ideal materials for the development of high-power LED devices in the future. Aluminum nitride ceramics have excellent properties such as high thermal conductivity, high strength, high resistivity, small density, low dielectric constant, non-toxicity, and thermal expansion coefficient matching Si, and will gradually replace traditional high-power LED substrate materials and become A ceramic substrate material with the most development prospect in the future.