Brief analysis of the current status and development of ceramic substrates for LED packaging

1 Comparison of plastic and ceramic materials

Plastics, especially epoxy resins, still dominate the entire electronics market due to their relatively good economics, but many special fields such as high temperature, coefficient of linear expansion, mismatch, air tightness, stability, and mechanical properties are obvious. Not suitable, even if a large amount of organic bromide is added to the epoxy resin, it does not help.

Compared with plastic materials, ceramic materials play an important role in the electronics industry. They have high electrical resistance, high frequency characteristics, high thermal conductivity, good chemical stability, high thermal stability and high melting point. These properties are highly desirable in the design and manufacture of electronic circuits, so ceramics are widely used for substrate materials of different thick films, films or circuits, and can also be used as insulators for conducting thermal paths in thermally demanding circuits. To manufacture a variety of electronic components.

2 Comparison of various ceramic materials

2.1 Al2O3

So far, alumina substrates are the most commonly used substrate materials in the electronics industry because of their high mechanical and thermal properties, high strength and chemical stability relative to most other oxide ceramics, and abundant raw material sources. A variety of technical manufacturing and different shapes.

2.2 BeO

It has a higher thermal conductivity than metal aluminum and is used in applications requiring high thermal conductivity, but it rapidly decreases after the temperature exceeds 300 °C.

The most important thing is that its toxicity limits its development.

2.3 AlN

AlN has two very important properties worth noting: one is a high thermal conductivity and the other is a coefficient of expansion that matches Si. The disadvantage is that even a very thin oxide layer on the surface has an effect on the thermal conductivity. Only a strict control of the material and process can produce a uniform ARN substrate. At present, the large-scale production technology of AlN is still immature in China. Compared with Al2O3, the price of AlN is relatively high, which is also a bottleneck restricting its development. Based on the above reasons, it can be known that alumina ceramics are widely used in the fields of microelectronics, power electronics, hybrid microelectronics, and power modules due to their superior comprehensive performance.

Ceramic substrate materials are widely used in power electronics, electronic packaging, hybrid microelectronics and multi-chip modules due to their excellent thermal conductivity and air tightness. This paper briefly introduces the current status and future development of ceramic substrates.

3 Ceramic substrate manufacturing

It is very difficult to manufacture high-purity ceramic substrates. Most ceramics have high melting point and hardness, which limits the possibility of ceramic machining. Therefore, ceramic substrates are often doped with lower melting point glass for fluxing or bonding. The final product is easy to machine. Al2O3, BeO, AlN substrate preparation process is very similar, the base material is ground to a powder diameter of about several microns, mixed with different glass flux and adhesive (including powdered MgO, CaO), in addition to the mixture Some organic binders and different plasticizers are ball milled to prevent agglomeration to make the ingredients uniform, to shape the green sheets, and finally to high temperature sintering. At present, ceramic molding has the following main methods:

Roller Rolling The slurry is sprayed onto a flat surface, partially dried to form a sheet of viscous like putty, which is then fed into a pair of large parallel rolls to obtain a uniform thickness of green slab.

• The cast slurry is applied to a moving belt to form a sheet by a sharp blade. This is a low pressure process compared to other processes.

• The powder compacted powder is sintered in a hard mold cavity and subjected to a large pressure (about 138 MPa). Although the pressure unevenness may cause excessive warpage, the sintered part produced by this process is very dense and has a small tolerance.

● Isostatic powder compacting This process uses a mold that is surrounded by water or glycerin and uses a pressure of up to 69 MPa to produce a more uniform warpage of the parts.

Extrusion Slurry Extrusion through Die This process uses a lower viscosity and makes it difficult to obtain a smaller tolerance, but this process is very economical and can result in thinner parts than other methods.

4 Comparison of substrate types and characteristics

At present, there are four types of ceramic heat-dissipating substrates, such as HTCC, LTCC, DBC and DPC. Among them, HTCC belongs to the earlier development technology, but the selection of electrode materials is limited due to the high sintering temperature, and the production cost is relatively expensive. These factors have contributed to the development of LTCC. Although LTCC reduces the co-firing temperature to about 850 ° C, the disadvantage is that dimensional accuracy and product strength are difficult to control. DBC and DPC are domestically developed and matured in recent years. DBC uses high temperature heating to combine Al2O3 with Cu. The technical bottleneck is that it is difficult to solve the micro-pores between Al2O3 and Cu. The problem is that the mass production energy and yield of the product are greatly challenged, while the DPC technology uses the direct copper plating technology to deposit Cu on the Al2O3 substrate, the process bonding material and the film process technology, and its products. It is the most commonly used ceramic heat sink substrate in recent years. However, its material control and process technology integration capabilities are relatively high, which makes the technical threshold for entering the DPC industry and stable production relatively high.

4.1 LTCC (Low-Temperature Co-fired Ceramic)

LTCC is also known as low-temperature co-fired multi-layer ceramic substrate. This technology requires first adding inorganic alumina powder and about 30% to 50% of glass material to organic binder to make it into a slurry. The scraper is used to scrape the slurry into a sheet shape, and then the sheet-like slurry is formed into a thin piece of green embryo through a drying process, and then the through hole is drilled according to the design of each layer, as the signal transmission of each layer, the internal circuit of the LTCC The screen printing technology is used to fill and print the lines on the raw embryos. The inner and outer electrodes can be respectively made of silver, copper, gold and other metals. Finally, the layers are laminated and placed at 850~900 °C. Sintering in a sintering furnace can be completed. Detailed manufacturing process LTCC production flow chart 1.

Figure 1 LTCC production flow chart

4.2 HTCC (High-Temperature Co-fired Ceramic)

HTCC is also known as high temperature co-fired multi-layer ceramics. The manufacturing process is very similar to that of LTCC. The main difference is that HTCC ceramic powder is not added to glass. Therefore, HTCC must be dried and hardened at a high temperature of 1300~1600 °C. The embryo is then drilled into the via hole, and the screen printing technology fills the hole and the printed circuit. Because of the high co-firing temperature, the selection of the metal conductor material is limited, and the main material is a high melting point but conductive. Metals such as tungsten, molybdenum, manganese, etc., which are less inferior, are finally laminated and sintered.

4.3 DBC (Direct Bonded Copper)

The direct copper coating technology uses copper oxygen-containing eutectic liquid to directly bond copper to ceramics. The basic principle is to introduce an appropriate amount of oxygen between copper and ceramic before or during the bonding process, at 1065 ° C ~ 1083 In the range of °C, copper and oxygen form a Cu-O eutectic liquid, and the DBC technology uses the eutectic liquid to chemically react with the ceramic substrate to form a CuAlO2 or CuAl2O4 phase, and on the other hand, infiltrate the copper foil to realize the combination of the ceramic substrate and the copper plate. The manufacturing flow chart of the direct copper-clad board of the ceramic substrate is shown in FIG.

Figure 2 Schematic diagram of the process of directly applying copper ceramic substrate

The direct copper-clad ceramic substrate has wide application because it has the advantages of excellent electrical conductivity and thermal conductivity of copper, high mechanical strength of ceramics, and low dielectric loss. In the past few decades, copper-clad substrates have made a significant contribution to power electronics packaging, mainly due to the following performance characteristics of direct copper substrates:

● good thermal performance;

● Capacitance performance;

● High insulation performance;

● Si-matched thermal expansion coefficient;

● Excellent electrical performance and strong current carrying capacity.

The initial research on direct copper ceramic substrates was developed to solve the problem of high current and heat dissipation, and later applied to the metallization of AlN ceramics. In addition to the above features, it has the following features that make it widely used in high-power devices:

● Strong mechanical stress, stable shape; high strength, high thermal conductivity, high insulation; strong bonding, anti-corrosion;

● Excellent thermal cycle performance, the number of cycles is up to 50,000 times, and the reliability is high;

● It can etch all kinds of graphic structures like PCB board (or IMS substrate); it is non-polluting and pollution-free;

● The use temperature is wide -55 ° C ~ 850 ° C; the thermal expansion coefficient is close to silicon, simplifying the production process of the power module.

Due to the characteristics of the direct copper-clad ceramic substrate, it has an irreplaceable feature of the PCB substrate. The thermal expansion coefficient of DBC is close to that of silicon chip, which can save the transition layer Mo film, save labor, material and reduce cost. Since the direct copper-clad ceramic substrate does not add any brazing composition, the welding layer is reduced, the thermal resistance is reduced, and the hole is reduced. Improve the yield, and the 0.3mm thick copper foil line width is only 10% of the ordinary printed circuit board under the same current carrying capacity; its excellent thermal conductivity makes the chip package very compact, thus greatly improving the power density and improving the system. And the reliability of the device.

In order to improve the thermal conductivity of the substrate, the thickness of the substrate is generally reduced. The ultra-thin (0.25 mm) DBC plate can replace BeO, and the thickness of the directly coated copper can reach 0.65 mm, so that the copper ceramic substrate can directly carry a large load. The current and the temperature increase are not obvious. The 100A current continuously passes through a 1mm wide and 0.3mm thick copper body, and the temperature rises to about 17°C. The 100A current continuously passes through a 2mm wide and 0.3mm thick copper body, and the temperature rise is only about 5°C. Compared with the brazing and Mo-Mn methods, DBC has a very low thermal resistance characteristic, taking the thermal resistance of a 10×10 mm DBC board as an example:

The thermal resistance of the 0.63 mm thick ceramic substrate DBC is 0.31 K/W, the thermal resistance of the 0.38 mm thick ceramic substrate DBC is 0.19 K/W, and the thermal resistance of the 0.25 mm thick ceramic substrate DBC is 0.14 K/W.

Alumina ceramics have the highest electrical resistance and high insulation withstand voltage, which ensures personal safety and equipment protection. In addition, DBC substrates can realize new packaging and assembly methods, making products highly integrated and compact.

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