How to choose the appropriate substrate for ceramic circuit boards?

A ceramic circuit board is a circuit board based on ceramic materials, usually using high-purity alumina ceramic as the substrate. Compared with traditional organic glass fiber circuit boards, it has higher mechanical strength, better high-temperature resistance, and better thermal conductivity. In addition, ceramic circuit boards also have better dimensional stability and chemical stability.

However, ceramic circuit boards also have some disadvantages, such as higher cost, difficulty in processing, and high interface stress between the conductive layer and the substrate. Therefore, when selecting a ceramic circuit board, it is necessary to consider multiple factors based on application requirements and budget and select the most appropriate substrate material and process.

Types and properties of ceramic substrates

There are three main types of ceramic substrates in the market: Alumina (Al2O3), Aluminum nitride (AlN), and Silicon nitride (Si3N4). They each have different advantages and disadvantages, and it is not advisable to choose a substrate based on the price alone. Generally speaking, they can be compared from the following aspects:

Ⅰ: Thermal conductivity:

thermal conductivity is the ability of a material to conduct heat, and the unit is W/(m.k). The higher the thermal conductivity, the better the heat dissipation effect of the material. According to the data, the thermal conductivity of the 3 ceramic substrates is as follows:

As can be seen from the table, AlN has the highest thermal conductivity, followed by Si3N4, and alumina has the lowest. This means that AlN is more suitable for applications that require high heat dissipation, such as high-power LED chips, while alumina is more suitable for low-power devices. Si3N4 is a compromise between the two, with moderate thermal conductivity and other properties.

Ⅱ: Dielectric constant:

Dielectric constant is a measure of the degree of polarization of a material in an electric field, and the unit is dimensionless. The lower the dielectric constant, the less the material affects the electric field, and it is suitable for high-frequency, microwave, radio frequency, and other fields. According to the data, the dielectric constants of the three ceramic substrates are as follows:

As can be seen from the table, Si3N4 has the lowest dielectric constant, Al2O3 has the highest, and AlN is in between.

Ⅲ: Coefficient of expansion:

The coefficient of thermal expansion is the degree of volume change of a material when the temperature changes and the unit is 1/℃. The closer the coefficient of thermal expansion is to Si material (3.7×10-6/℃), the less stress and cracks are generated when the material is combined with Si chips, which improves the reliability of the chips. According to the data, the coefficients of thermal expansion of the three ceramic substrates are as follows:

As can be seen from the table, Si3N4 has the closest coefficient of thermal expansion to Si material, Al2O3 has the farthest, and AlN is in between.

In summary, different types of ceramic substrates have their own advantages and disadvantages in different performance aspects. When applying them, they need to be considered comprehensively. They can start from three aspects: thermal conductivity, dielectric constant, and coefficient of thermal expansion, and choose the most suitable one.

Ceramic Circuit Board Processes

In addition to the substrate material, another important factor affecting the performance and cost of ceramic circuit boards is its manufacturing process. Common processes for ceramic circuit boards include the following:

• Direct bonded copper ceramic (DBC):

This is a process that directly bonds copper foil to a ceramic substrate, using the redox reaction between copper and oxide to form a copper oxide layer at high temperatures, thereby achieving a strong Bond. This process has the advantages of strong current carrying capacity and good heat dissipation effect, but it also has the disadvantages of large interface stress between the conductive layer and the substrate and high cost.

• Direct copper-plated ceramic (DPC):

This is a process that uses magnetron sputtering technology to composite a metal layer on the surface of a ceramic substrate and uses electroplating and photolithography technology to create circuit patterns. This process has the advantages of small interface stress and low cost, but it also has shortcomings such as weak current carrying capacity and poor heat dissipation effect.

• Low-temperature co-fired ceramics (LTCC):

This is a process in which multi-layer ceramic substrates and metallic conductive layers are stacked together and co-fired at low temperatures. This process has the advantages of multi-layer wiring, three-dimensional packaging, high-density integration, and low dielectric loss, but it also has the disadvantages of complex manufacturing process, high cost, and large package size.

• High-temperature co-fired ceramics (HTCC):

This is a process in which multi-layer ceramic substrates and metallic conductive layers are stacked together and co-fired at high temperatures. This process has the advantages of high mechanical strength, good high-temperature performance, and good chemical stability, but it also has shortcomings such as large dielectric loss, difficulty in multi-layer wiring, and high cost.

In Conclusion

Different ceramic circuit board processes have their own characteristics and application ranges. When choosing, in addition to the material, you also need to consider the design requirements and budget of the product, weigh the pros and cons, and choose the most suitable one.

Ceramic circuit boards are a new type of electronic packaging material that has many advantages over traditional circuit boards. However, they also have some limitations and challenges, and the most appropriate substrate materials and processes need to be selected according to different application scenarios and requirements. This article briefly introduces three common ceramic substrate materials and four common ceramic circuit board processes, as well as their performance and characteristics. We hope you find it helpful. If you want to know more about ceramic circuit boards, you can check out our previous blog and email sales@bstceramicpcb.com!