At present, with the development of the LED industry at home and abroad in the direction of high efficiency, high density, high power, etc., the development of heat dissipation materials with superior performance has become a top priority to solve the LED heat dissipation problem。In general, LED luminous efficiency and service life will decrease with the increase of junction temperature, when the junction temperature reaches more than 125 ° C, LED will even fail。In order to keep the LED junction temperature at a low temperature, it is necessary to use high thermal conductivity, low thermal resistance of the heat dissipation substrate material and reasonable packaging process to reduce the overall packaging thermal resistance of the LED。

Figure 1 LED cooling ceramic substrate

Common substrate materials at this stage areSi, metals (Al, Cu, W, Mo) and metal alloy materials (Cu/W, Cu/Mo), ceramics (Al₂O₃、AlN, SiC, BN) and composite materials, their thermal expansion coefficient and thermal conductivity are shown in Table 1。The cost of Si material is high;The inherent electrical conductivity and thermal expansion coefficient of metal 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 substrate。

Table 1 Thermal expansion coefficient and thermal conductivity of common high-power LED packaging substrates

Epoxy copper-coated substrate (FR-4)It is the most widely used substrate in traditional electronic packaging。It has three functions: support, conduction and insulation。Its main characteristics are: low cost, high hygroscopic resistance, low density, easy to process, easy to realize micro graphics circuit, suitable for large-scale production。However, because the substrate of FR-4 is epoxy resin, the thermal conductivity of organic materials is low, and the high temperature resistance is poor, so FR-4 can not adapt to the requirements of high-density and high-power LED packaging, and is generally only used in low-power LED packaging。

Metal clad Copper substrate (MCPCB)It is a new type of substrate after FR-4。It is made by directly bonding copper foil circuit and polymer insulation layer with metal (Cu, Al) and base with high thermal conductivity through thermal conductive bonding material, as shown in Figure 2。Its thermal conductivity is about 1.12 W/m·K, compared with FR-4 is a large increase。Due to its excellent heat dissipation, MCPCB has become the most widely used product in the current high-power LED cooling substrate market。However, MCPCB also has its inherent disadvantages: the thermal conductivity of the polymer insulation layer is low, only 0.3 W/m·K, resulting in heat can not 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 problem。

Metal matrix composite substrate(MMC)The most representative material is aluminum silicon carbide (AlSiC)。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, which combines the advantages of the two materials and has a low density (2.79g/cm³), low coefficient of thermal expansion, high thermal conductivity, high stiffness and a series of excellent characteristics。The coefficient of thermal expansion of AlSiC can be adjusted by changing the content of SiC to match the coefficient of thermal expansion of adjacent materials, thus minimizing the thermal stress of both。Figure 3 is a schematic diagram of multi-chip packaging with AlSiC as the substrate。

FIG. 3 Schematic diagram of a multi-chip package based on AlSiC

For a long time,Al₂O₃和BeO ceramics are two main substrate materials for high-power packaging。But both substrate materials have inherent drawbacks, Al₂O₃Low thermal conductivity, thermal expansion coefficient with chip material(e.g. Si) does not match;Although BeO has excellent comprehensive properties, it has high production cost and is highly toxic。Therefore, from the aspects of performance, cost and environmental protection, these two substrate materials can not be used as the most ideal material for the development of high-power LED devices in the future。AlN ceramics have high thermal conductivity, high strength, high resistivity, low 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 materials, become the most promising ceramic substrate materials in the future。

Aluminium nitride(AlN) is a synthetic mineral that does not occur naturally in nature。The crystal structure type of AlN is hexagonal wurtzite type, withThe density is small (3.26g/cm³), high strength, good heat resistance (about 3060℃ decomposition), high thermal conductivity, corrosion resistanceEqual advantage。

FIG. 4 Schematic diagram of AlN wurtzite crystal structure

AlN is a strongly covalently bonded compound whose heat transfer mechanism is lattice vibration (i.e. phonon heat transfer).。Due to the small ordinal number of Al and N, AlN has a high thermal conductivity by nature, and its theoretical value can be as high as 319W/m·K。However, in the actual product, because the crystal structure of AlN may be completely evenly distributed, and there are many impurities and defects, as shown in Figure 5, its thermal conductivity is generally only 170-230W/m·K。In the heat conduction process, various defects in the crystal (such as lattice distortion, dislocation, impurities, pores, micro-cracks), grain boundaries, interfaces, second phases, and phonons themselves will interfere with and scatter phonons, thus greatly reducing the thermal conductivity of the substrate。

FIG. 5 Various factors affecting the thermal conductivity of AlN substrate

Since the AlN substrate is not conductive, its surface must be metallized and graphed before it can be used as a high-power LED cooling substrate。However, AlN and metal are two kinds of materials with completely different physical and chemical properties, and the most prominent difference between the two is the different bonding methods of the formation of compounds。AlN is a strong covalent bond compound, and metals generally behave as metal bond compounds, so compared with other chemically bonded compounds, AlN and metal infiltration is poor at high temperatures, and it is difficult to achieve metallization。Therefore, how to achieve the metallization and graphics of the AlN substrate surface has become a crucial issue in the development of high-power LED cooling substrates。At present, the most widely used methods for metallization of AlN substrates are: (1) mechanical connection method, (2) thick film method, (3) active metal brazing method, (4) co-firing method, (5) film method, (6) direct copper coating method。

FIG. 6 Schematic diagram of AlN substrate metallized by thick film method

FIG. 7 Schematic diagram of bonding interface between AlN substrate and active metal filler metal

Figure 8 Multilayer metal AlN ceramic substrate

FIG. 9 Direct copper coating process of AlN substrate