Alumina ceramic substrate

Ceramic substrates are widely used in fields such as power electronics, electronic packaging, hybrid microelectronics and multi-chip modules due to their excellent thermal conductivity and air tightness. Alumina is currently the most widely used ceramic substrate material. It not only has high mechanical strength, insulation, high temperature resistance, good stability, high cost performance, good resistance to thermal shock, and can form a sealed brazing with metal. With advantages such as mature manufacturing and processing technology and low cost, it has been widely used in thick film circuits, thin film circuits, hybrid circuits, multi-chip components, high-power IGBT modules and other fields. It is also the most widely used ceramic substrate material at present.

Types of alumina ceramics

Alumina ceramics can be divided into two types according to purity: pure high type and ordinary type.

① High purity alumina
High-purity alumina ceramic is a ceramic material with an Al2O3 content of more than 99.9%. Since its sintering temperature is as high as 1650~1990°C and the transmission wavelength is 1~6μm, it is generally made into molten glass to replace platinum crucibles, using its light transmittance and It can resist alkali metal corrosion and is used as sodium lamp tube; in the electronics industry, it can be used as integrated circuit substrate and high-frequency insulating material.
② Ordinary alumina ceramics
Ordinary alumina ceramics are divided into 99 porcelain, 95 porcelain, 90 porcelain, 85 porcelain and other varieties according to the Al2O3 content (referring to the Al2O3 content of 99%, 95%, 90%, 85%, etc.). The main difference lies in the substrate doping The amount is different. The less the doping amount, the higher the purity of the substrate. There are certain differences in the electrical and mechanical properties of alumina ceramic substrates of different purity. The higher the purity of the ceramic substrate, the higher the dielectric constant and the dielectric loss. The lower it is, the better the finish of its substrate. Sometimes those with an Al2O3 content of 80% or 75% are also classified as ordinary alumina ceramic series.

Among them, 99 alumina porcelain is used to make high-temperature crucibles, refractory furnace tubes and special wear-resistant materials, such as ceramic bearings, ceramic seals and water valves; 95 alumina porcelain is mainly used for corrosion-resistant and wear-resistant parts; 85 porcelain Because some talc is often incorporated into it, the electrical properties and mechanical strength are improved. It can be sealed with molybdenum, niobium, tantalum and other metals, and some are used as electronic vacuum device components. 75 porcelain and 95 porcelain are ceramic materials widely used as thick film circuit substrates. Ceramic substrates for thin film circuits mostly use 97 porcelain or 99 porcelain.

The main properties of alumina substrates increase with the increase in alumina content, but the higher the alumina content, the more difficult it is to prepare ceramics. 95 porcelain is generally fired above 1500°C. The firing temperature of 99 porcelain reaches above 1700℃.

According to color, alumina ceramic substrates are available in three types: white, purple, and black. Common alumina substrates are white and are used as LED substrates, high-frequency circuit substrates, etc. However, some applications require the alumina substrate to avoid reflecting light, resulting in black alumina products.

Advantages of alumina ceramic substrates

● Good insulation performance: The alumina ceramic substrate has good insulation performance, which can effectively isolate the circuit and avoid failures caused by leakage and other problems.
● Excellent high temperature resistance: The alumina ceramic substrate can maintain stable performance in high temperature environments, can withstand long-term operation in high temperature environments, and is not easy to deform, ablate or oxidize.
● High strength and hardness: The alumina ceramic substrate has high strength and hardness, can withstand a certain amount of mechanical pressure and impact, and is not easily broken or worn.
● Excellent chemical stability: Alumina ceramic substrate has good corrosion resistance to most chemicals and can operate stably in chemically aggressive environments.
● Good processing performance: Alumina ceramic substrate has good processing performance and can be used for drilling, milling, cutting and other processing processes, and can achieve complex geometric shapes and high-precision dimensional requirements. Circuit processing can be carried out based on thin film photolithography, and its accuracy can reach micron level. Many passive devices can be designed based on alumina ceramic substrates. Since its dielectric constant is higher than that of general PCB substrates, the designed devices are small in size. , which has a very significant advantage in the development trend of miniaturization of various component modules.

Applications of alumina ceramic substrates

① Ceramic substrate for chip resistors
Advantages of resistance alumina ceramic substrate: small size, light weight, small thermal expansion coefficient, good reliability, high thermal conductivity and density, which greatly improves the reliability of the circuit and the wiring density of the circuit. It is the carrier material of chip resistance elements.
② Ceramic substrate for hybrid integrated circuits
Hybrid integrated circuit (hydrid) is a packaging method that should contain at least two or more components, one of which is active. They are mounted on metal conductive tape insulation sheets that have been made of thick films or thin films. The complex circuits produced using this technology are hybrid integrated circuits. The substrate provides mechanical support for the circuit and provides a deposition place for conductive band materials, dielectric materials and resistive materials that form passive components. It also provides mechanical support for all passive and active chip components.
In hybrid integrated circuits, commonly used substrates include aluminum oxide, beryllium oxide, silicon oxide, aluminum nitride, etc. However, considering cost and performance, high-purity alumina substrates with smooth surfaces are widely used. Due to the different alumina content, the quality and grade of the substrate also vary. The common ones are 99.6% alumina and 96% alumina. The former is generally suitable for thin film circuits, while for thick film circuits, 96% alumina substrate is the best. can meet its process requirements. Multi-layer co-fired alumina ceramics generally use alumina ceramic green sheets between 90 porcelain and 95 porcelain as the basic material.

③ Due to the relatively low thermal conductivity of alumina ceramics (20-30W/m·K), power device substrates do not match well with the thermal expansion coefficients of semiconductor materials such as Si and SiC, which limits the use of alumina substrates in high-power devices. application.
④ Alumina ceramic substrate for LED
High-power LED heat dissipation substrates are mainly ceramic substrates. The most commonly used high-power ceramic substrates on the market are LTCC (low-temperature co-fired ceramics) and DPC (direct copper-plated ceramics). Ceramic materials include alumina, nitride Aluminum etc. Alumina ceramic substrates for LEDs have the characteristics of high heat dissipation and high air tightness, which can improve the luminous efficiency and life of LEDs. The good air tightness makes it highly weather resistant and can be used in various environments.
For power electronic device packaging, in addition to the basic wiring (electrical interconnection) function, the substrate is also required to have high thermal conductivity, insulation, heat resistance, voltage resistance and thermal matching performance. Metalized ceramic substrates represented by DBC and DPC have superior performance in thermal conductivity, insulation, voltage resistance and heat resistance. They have become the preferred material for power device packaging and are gradually recognized by the market. The most common substrate material for device packaging is alumina substrate (Al2O3), which is generally an alumina substrate with an alumina content of 96%. The alumina substrate technology is very mature and low-priced.

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