Aluminum nitride (AlN) ceramics exhibit excellent overall properties and have become a widely researched next-generation advanced ceramic material in recent years. It possesses high thermal conductivity, low dielectric constant, low dielectric loss, excellent electrical insulation, a thermal expansion coefficient compatible with silicon, and non-toxicity, making it an ideal material for high-density, high-power, and high-speed integrated circuit substrates and packaging.
Although hot pressing and isostatic pressing are suitable for producing high-performance AlN ceramics, these methods are costly and have low production efficiency, which cannot meet the increasing demand for AlN ceramic substrates in the electronics industry. To solve this problem, many manufacturers have adopted the tap casting process to make AlN ceramic substrates in recent years. Tap casting has thus become the main forming process for AlN ceramic substrates used in the electronics industry.
1. Ball Milling and Slurry Preparation
In the preparation of AlN slurry, organic solvents such as dispersants, binders, and plasticizers are typically added to achieve the desired rheological properties for easy casting. Additionally, Y₂O₃ is often added as a sintering aid to promote sintering under normal atmospheric pressure. The viscosity of the slurry has a significant impact on the performance of the substrate. Factors influencing the viscosity include milling time, the amount of organic solvents, dispersants, binders, and plasticizers. Therefore, the choice of slurry formulation and process control has a substantial effect on the performance of the ceramic substrate.
2. Tap Casting
Tap casting forming is a high-efficiency process that facilitates continuous and automated production, reducing costs and enabling mass production. The thickness of the produced substrate can range from less than 10 μm to over 1 mm. Tap casting is a critical step in the practical application of AlN ceramic substrates and holds great potential for future applications. Compared to other forming methods, Tap casting has several advantages:
- Simple equipment and process, enabling continuous production.
- Capable of producing single-phase or multi-phase ceramic thin films.
- Minimal defects, uniform performance, high production efficiency, and continuous operation.
- Suitable for both large and small batch production, making it ideal for industrial manufacturing.
- Particularly suitable for the preparation of large, thin ceramic components, which is a feature difficult to achieve with pressing or extrusion techniques.
3. Degassing
The green body of the substrate produced by tap casting contains a large amount of organic materials, resulting in a high porosity and low strength. If sintered directly, it may lead to excessive shrinkage, warping of the substrate, and adhesion between green bodies during sintering, which affects the yield and thermal conductivity. To prevent these defects, the green body is pre-fired in a nitrogen atmosphere furnace at 1100°C before sintering. This helps improve the strength of the green body, reduce porosity, and obtain AlN substrates with high flatness and good performance.
4. Sintering
After degassing, the AlN substrates undergo high-temperature sintering. The sintering process for high thermal conductivity AlN substrates focuses on sintering methods, the addition of sintering aids, and the control of the sintering atmosphere.
Since AlN is a covalent compound with a small self-diffusion coefficient, densification during sintering is very difficult. Rare-earth metal oxides and alkaline earth metal oxides are typically used as sintering aids to promote sintering, though temperatures above 1800°C are usually required. There are three primary ways to achieve dense and high-performance AlN ceramics:
- Use of ultra-fine powders;
- Hot pressing or isostatic pressing;
- Introduction of sintering aids.
The five main sintering techniques for AlN substrates include hot-press sintering, pressureless sintering, microwave sintering, spark plasma sintering (SPS), and self-propagating high-temperature synthesis (SHS). Among these, hot-press sintering is currently the primary method for producing high thermal conductivity, dense AlN ceramics.
Xiamen Juci's AlN powder has the characteristics of high purity,low oxygen content,high sintering activity,and sharp size distribution. and it is widely used for tap casting AlN substrate.
