Ceramic substrates are widely used in power electronics, electronic packaging, hybrid microelectronics, and multi chip modules due to their excellent thermal conductivity and airtightness. Aluminum oxide is currently the most widely used ceramic substrate material. It not only has advantages such as high mechanical strength, insulation, high temperature resistance, good stability, high cost-effectiveness, and good resistance to thermal shock, but also can form a sealed brazing with metal. In addition, its manufacturing process is mature and low-cost. It has been widely used in thick film circuits, thin film circuits, hybrid circuits, multi chip components, and high-power IGBT modules, and is currently the most widely used ceramic substrate material.

Types of alumina ceramics

Alumina ceramics can be divided into two types based on their purity: pure high type and ordinary type.

① High purity alumina

High purity alumina ceramics are ceramic materials with an Al2O3 content of over 99.9%. Due to their high sintering temperature of 1650-1990 ℃ and transmission wavelength of 1-6 μ m. Generally, molten glass is made to replace platinum crucibles, and its transparency and resistance to alkali metal corrosion are used as sodium lamps; It can be used as an integrated circuit substrate and high-frequency insulation material in the electronics industry.

② Ordinary alumina ceramics

Ordinary alumina ceramics can be classified into 99 ceramics, 95 ceramics, 90 ceramics, 85 ceramics, and other varieties based on the different Al2O3 content (referring to Al2O3 content of 99%, 95%, 90%, 85%, etc.). The main difference is that the doping amount on the substrate is different. The lower the doping amount, the higher the purity of the substrate. The electrical and mechanical properties of alumina ceramic substrates with different purities have certain differences. Ceramics with higher purity have higher dielectric constant, lower dielectric loss, and better substrate smoothness. Sometimes those with Al2O3 content of 80% or 75% are also classified as ordinary alumina ceramic series.

Among them, 99 alumina ceramic materials are used to make high-temperature crucibles, refractory furnace tubes, and special wear-resistant materials, such as ceramic bearings, ceramic seals, and water valve plates; 95 alumina ceramics are mainly used as corrosion-resistant and wear-resistant components; Due to the frequent addition of some talc, the electrical performance and mechanical strength of 85 ceramics are improved. They can be sealed with metals such as molybdenum, niobium, and tantalum, and some are used as vacuum devices. 75 porcelain and 95 porcelain are widely used ceramic materials for thick film circuit substrates, and ceramic substrates for thin film circuits are mostly 97 porcelain or 99 porcelain.

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

According to color, there are three types of alumina ceramic substrates: white, purple, and black. The common aluminum oxide substrate is white, used for LED substrates, high-frequency circuit substrates, etc. However, some applications require avoiding the reflection of light from the aluminum oxide substrate, resulting in the formation of black aluminum oxide products.

The advantages of alumina ceramic substrates

● Good insulation performance: Aluminum oxide ceramic substrates have good insulation performance, which can effectively isolate circuits and avoid faults caused by leakage and other issues.

Excellent high-temperature resistance: The alumina ceramic substrate can maintain stable performance in high-temperature environments, withstand long-term operation in high-temperature environments, and is not easy to deform, burn, or oxidize.

● High strength and hardness: Aluminum oxide ceramic substrates have high strength and hardness, can withstand certain mechanical pressure and impact forces, and are not easy to break or wear.

Excellent chemical stability: The alumina ceramic substrate has good corrosion resistance to most chemicals and can operate stably in chemically aggressive environments.

● Good processing performance: The alumina ceramic substrate has good processing performance, which 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 based on thin film lithography technology, with accuracy reaching the micrometer level. Many passive devices can be designed based on alumina ceramic substrates. Due to their higher dielectric constant compared to general PCB substrates, the designed devices have a small size, which has significant advantages in the development trend of miniaturization of various component modules.

Application of alumina ceramic substrate

① Ceramic substrates for chip resistors

The advantages of resistive alumina ceramic substrates include small size, light weight, low coefficient of thermal expansion, good reliability, high thermal conductivity and density, greatly improving the reliability of circuits and wiring density. It is a carrier material for chip resistance components.

② Ceramic substrates for hybrid integrated circuits

A hybrid integrated circuit is a packaging method that should contain at least two or more components, one of which is active. Install them on metal ribbon insulation sheets already made of thick or thin films, and complex circuits manufactured using this technology are called hybrid integrated circuits. The substrate serves as a mechanical support for the circuit, providing a deposition site for the conduction band material, dielectric material, and resistance material 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 aluminum oxide substrates with smooth surfaces are still widely used. Due to different levels of aluminum oxide content, the quality and grade of the substrate also vary. Common types include 99.6% aluminum oxide and 96% aluminum oxide. The former is generally suitable for thin film circuits, while for thick film circuits, 96% aluminum oxide substrate can meet their process requirements. Aluminum oxide ceramics for multi-layer co firing generally use aluminum oxide ceramic raw ceramic sheets between 90 and 95 porcelain as the basic material.

③ Power device substrate

For power electronic device packaging, in addition to basic wiring (electrical interconnection) functions, the substrate also requires high thermal conductivity, insulation, heat resistance, voltage resistance, and thermal matching performance. Metallized ceramic substrates represented by DBC and DPC have superior performance in thermal conductivity, insulation, voltage resistance, and heat resistance, and have become the preferred materials for power device packaging, gradually gaining market recognition. 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 affordable.

Due to the relatively low thermal conductivity of alumina ceramics (20-30W/m · K) and poor matching of thermal expansion coefficients with semiconductor materials such as Si and SiC, the application of alumina substrates in high-power devices is limited.

④ Aluminum oxide ceramic substrate for LED

The high-power LED heat dissipation substrate is mainly composed of ceramic substrates. The commonly used high-power ceramic substrates in the market are LTCC (low-temperature co fired ceramic) and DPC (directly copper plated ceramic), with ceramic materials such as alumina and aluminum nitride. Aluminum oxide ceramic substrates for LED have the characteristics of high heat dissipation and high air tightness, which can improve the luminous efficiency and lifespan of LED. And its good airtightness gives it a high degree of weather resistance, making it suitable for various environments.