At present, with the domestic and foreign LED industry to high efficiency, high density, high power and other directions of development, from 2017 to 2018, it can be seen that the overall domestic LED has made rapid progress, power is also growing, the development of superior performance of cooling materials has become an urgent task to solve the problem of LED cooling.
In general, the LED luminous efficiency and service life declines with the increase of the junction temperature, when the junction temperature of 125 ℃ above, the LED can appear even failure.
In order to keep LED junction temperature at a low temperature, it is necessary to adopt heat dissipation substrate material with high thermal conductivity and low thermal resistance and reasonable packaging technology to reduce the overall thermal resistance of LED packaging.
At present, common substrate materials include Si, metal and metal alloy materials, ceramics and composites, etc., their thermal expansion coefficient and thermal conductivity are shown in the table below.
Si material cost is high;
The intrinsic conductivity and thermal expansion coefficient of metal and metal alloy materials do not match the chip materials;
Ceramic material is difficult to process and other shortcomings, it is difficult to meet the high power substrate performance requirements.
Since the development of power LED packaging technology, the heat dissipation substrates available mainly include epoxy resin copper clad substrates, metal copper clad substrates, metal composite substrates and ceramic copper clad substrates.
Epoxy resin coated copper substrate is the most widely used substrate in traditional electronic packaging.
It supports, conducts electricity and insulates.
Its main features are: low cost, high moisture absorption resistance, low density, easy processing, easy to achieve micro graphics circuit, suitable for large-scale production.
However, as 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 and high-power LED packaging, and is generally only used in low-power LED packaging.
Metal base copper clad substrate is a new type of substrate after fr-4.
It is made by directly bonding copper foil circuit and macromolecule insulation layer with metal and base with high thermal conductivity through thermal conductive bonding material, and its thermal conductivity is about 1.
12 W/m·K, which is a big improvement over fr-4.
Due to its excellent heat dissipation, it has become the most widely used product in the market of high-power LED heat dissipation substrate.
But it also has its inherent disadvantages: the thermal conductivity of polymer insulation layer is low, only 0.
3 W/m·K, so the heat cannot be transferred directly from the chip to the metal base;
The thermal expansion coefficient of Cu and Al is large, which may cause serious thermal mismatch.
The most representative material of metal matrix composite substrate is aluminum carbide.
Aluminum carbide is a metal matrix composite material that combines the low expansion coefficient of SiC ceramics with the high thermal conductivity of metal Al. It combines the advantages of the two materials and has a series of excellent characteristics such as low density, low thermal expansion coefficient, high thermal conductivity and high stiffness.
The thermal expansion coefficient of AlSiC can be adjusted by changing the content of SiC to match the thermal expansion coefficient of adjacent materials so as to minimize the thermal stress of the two materials.
The common ceramic substrate materials are Al2O3, aluminum nitride, SiC, BN, BeO, Si3N4, etc. Compared with other substrate materials, ceramic substrate has the following characteristics in mechanical properties, electrical properties, thermal properties:
(1) mechanical properties.
Mechanical strength, which can be used as supporting member;
Good machinability and high dimensional accuracy;
The surface is smooth, without micro-cracks, bending, etc.
(2) thermal properties.
Thermal conductivity, thermal expansion coefficient and Si and GaAs and other chip materials match, good heat resistance.
(3) electrical properties.
Low dielectric constant, low dielectric loss, high insulation resistance and insulation damage, stable performance and high reliability under high temperature and humidity conditions.
(4) other properties.
Good chemical stability, no hygroscopicity;
Oil and chemical resistance;
Non-toxic, pollution-free, small alpha ray emission;
The crystal structure is stable and it is not easy to change in the temperature range.
Abundant raw material resources.
Al2O3 and BeO ceramics have long been two main substrate materials for high-power packaging.
But these two kinds of substrate materials are inherent shortcomings, Al2O3 thermal conductivity is low, thermal expansion coefficient and chip material does not match;
Although BeO has excellent comprehensive performance, it has high production cost and high toxicity.
Therefore, in terms of performance, cost and environmental protection, these two kinds of substrate materials cannot be considered as the most ideal materials for the development of high-power LED devices in the future.
Aluminum nitride ceramics with high thermal conductivity, high strength, high resistivity, small density, low dielectric constant, non-toxic, and Si matching thermal expansion coefficient and other excellent properties, will gradually replace the traditional high-power LED substrate material, become the most promising ceramic substrate material in the future.