What do you Know about Alumina Ceramic Substrate?
Alumina (Al2O3) ceramics are currently the most widely used ceramic substrate materials for packaging. They possess excellent properties such as high strength, resistance to high temperatures, thermal shock resistance, electrical insulation, and strong corrosion resistance. Additionally, they are inexpensive and readily available. Common alumina substrates are white and are often used in LED substrates and high-frequency circuit boards. However, for certain applications that require minimizing light reflection from alumina substrates, black alumina products have been developed.
Crystal Structure, Classification, and Properties of Alumina Ceramic Substrates
Alumina has several polymorphs, including α-Al2o3、β-Al2o3 andγ-Al2o3. Among these, α-Al2O3 is the most stable, with a dense crystal structure and stable physical and chemical properties. It is widely used in industrial applications due to its high density and mechanical strength.
Alumina ceramics are classified based on purity:
Corundum Ceramics:Purity >99% Al2O3
99 Ceramics, 95 Ceramics, and 90 Ceramics: Purities of 99%, 95%, and 90% respectively
High Alumina Ceramics: Purity >85% Al2O3
For example, 99.5% alumina ceramics have a bulk density of 3.95 g/cm³, flexural strength of 395 MPa, a linear expansion coefficient of 8.1×10-6, thermal conductivity of 32 W/(m·K), and insulation strength of 18 kV/mm.
Alumina ceramic substrates can be divided into different models like 90 ceramics, 96 ceramics, and 99 ceramics based on their purity. The main difference lies in the amount of doping, with higher purity corresponding to better electrical and mechanical properties. For instance, 99.6% pure alumina ceramic substrates have a dielectric constant of 9.9@1MHz, while 96% pure ones have a dielectric constant of 9.6@1MHz. This slight difference in dielectric constant can significantly impact the performance of microwave RF designs, leading to qualitative differences in the devices made from 99 ceramics compared to 96 ceramics. Generally, higher purity alumina ceramic substrates are more expensive.
Types of Alumina Ceramics
1. High-Purity Alumina Ceramics
High-purity alumina ceramics contain more than 99.9% Al2O3. These ceramics have a sintering temperature of 1650–1990°C and a transmission wavelength of 1–6 μm. They are typically used to replace platinum crucibles in molten glass production due to their light transmission and alkali metal corrosion resistance. In the electronics industry, they are used as substrates for integrated circuits and high-frequency insulation materials.
2. Ordinary Alumina Ceramics
These are classified into various grades like 99 ceramics, 95 ceramics, 90 ceramics, and 85 ceramics based on their Al2O3 content. The electrical and mechanical properties of these ceramics vary with purity. Occasionally, ceramics with Al2O3 content of 80% or 75% are also included in the ordinary alumina ceramics category.
Typical Applications of Different Al2O3 Content Ceramics
-99 Ceramics: Used for high-temperature crucibles, refractory tubes, and special wear-resistant materials like ceramic bearings, seals, and water valve plates.
- 95 Ceramics: Primarily used for corrosion-resistant and wear-resistant components.
- 85 Ceramics: Often mixed with talc to improve electrical and mechanical properties and can be sealed with metals like molybdenum, niobium, and tantalum, used in vacuum device components.
- 75 Ceramics and 95 Ceramics: Commonly used in thick-film circuit substrates, while thin-film circuit substrates often use 97 or 99 ceramics.
Color-Based Classification of Alumina Ceramic Substrates
Alumina ceramic substrates can be white, purple, or black. White alumina substrates are commonly used for LED substrates and high-frequency circuit boards. However, black alumina products are used where light reflection needs to be minimized.
Advantages of Alumina Ceramic Substrates
- Excellent Insulation: Alumina ceramic substrates have excellent insulation properties, effectively isolating circuits and preventing failures caused by leakage.
- High-Temperature Resistance: They maintain stable performance in high-temperature environments, enduring long-term operation without deformation, ablation, or oxidation.
- High Strength and Hardness: These substrates can withstand mechanical pressure and impact, making them resistant to breakage or wear.
- Chemical Stability: They exhibit strong corrosion resistance to most chemicals, allowing stable operation in chemically aggressive environments.
- Good Machinability: Alumina ceramic substrates can be drilled, milled, cut, and processed into complex geometries with high precision. The thin-film lithography process allows for micro-scale circuit design, and due to their higher dielectric constant compared to standard PCB substrates, smaller device sizes can be achieved, a significant advantage in the trend towards miniaturization.
Challenges in Manufacturing High-Purity Alumina Substrates
As the basic unit of spacecraft, the material performance of the substrate directly impacts the reliability of on-orbit operations. However, there are several challenges in manufacturing high-purity alumina substrates:
1. Low Material Purity: If the high-purity alumina powder used is not pure enough after complex processes like tape casting, drying, cutting, and sintering, it can increase dielectric loss, reducing circuit performance.
2. Weak Mechanical Strength: Insufficient mechanical strength can cause cracks and material peeling during the assembly and testing of thin-film circuit products.
3. Surface Defects: Surface defects on the ceramic substrate can degrade the adhesion of circuit film layers, leading to issues like line edge burrs, affecting signal quality, or even causing excess material problems.
These challenges limit the application of alumina substrates in spaceborne microwave components due to the non-repairability and high reliability requirements of satellites. Therefore, the characteristics of high-purity alumina substrates are crucial for microwave circuit usage.
Key Points in High-Purity Alumina Substrate Preparation
Research and practice have shown that solvent systems, tape casting thickness, and debinding and sintering process parameters are critical in the preparation process. Proper control of these factors affects the final substrate thickness, uniformity, appearance quality, and surface roughness.
1. Optimization of the Tape Casting Solvent System
The solvent's primary role is to dissolve binders, plasticizers, and other additives, disperse powders, and provide appropriate viscosity to the slurry. Different solvent systems have different environmental adaptability and can affect the solid content, surface state, and film formation effect of the slurry.
For example, ethanol, which easily volatilizes in air, can cause the slurry to form films too quickly, leading to cracking. Through repeated experimentation, a new solvent system was developed: a mixed solvent of anhydrous ethanol and butyl acetate, with a solid content around 56 wt.% and a slurry viscosity of approximately 5Pa·s, suitable for tape casting.
2. Thickness Control of Tape-Cast Films
The thickness of a single-layer tape-cast film impacts the final substrate thickness tolerance. Experiments have shown that the film thickness is mainly influenced by the slurry state and the height of the doctor blade. By fine-tuning the solid content and sintering temperature, the final dimensions of the sintered substrate can be accurately controlled.
3. Control of Substrate Appearance and Flatness
Good appearance and flatness are essential for the engineering application of high-purity alumina substrates. The key steps in controlling these properties are lamination, debinding, and sintering.
- Lamination Process Control: To avoid surface defects and flatness issues during lamination and isostatic pressing, a wide tape lamination technique is employed, trimming the central part after isostatic pressing to avoid defects and uneven areas at the edges.
- Debinding Process Control: To efficiently remove organic materials and avoid warping or cracking during debinding, the green substrate is stacked in the furnace. Experiments determined that a stacking method without a cover plate optimizes the process, preventing warping and cracking.
- Sintering Process Control: Sintering is a critical step, where the choice of cover and support plates is crucial to prevent contamination. Tests showed that using porous high-purity alumina for cover plates and dense high-purity alumina for support plates improves flatness and appearance quality.
4. Unique Characteristics of Black Alumina Substrates
Black alumina ceramic substrates are increasingly demanded due to their unique properties.
Why Use Black Alumina Substrates?
Black alumina ceramic substrates are primarily used in semiconductor integrated circuits and electronic products that are sensitive to light. These substrates offer high mechanical strength, excellent insulation, and light-shielding properties, making them suitable for applications in multilayer ceramic substrates, optoelectronics, digital integrated circuits, and microwave circuit ceramic housings. For example, using black alumina ceramics in crystal oscillators can reduce their size by 30-100 times.
Composition of Black Alumina Substrates
Black alumina is created by adding transition metal oxides to the alumina powder during synthesis, which creates energy levels that absorb visible light, giving it a black color. Typically, black alumina consists of three parts:
- The alumina base, with a content between 90%-96 wt%
- Black pigments
- Sintering aids
Commonly used black oxides include Fe2O3, CoO, NiO, V2O5, Cr2O3, and MnO. These oxides form spinel compounds at high temperatures.
Hitech Circuits has extensive experience in PCB manufacturing, and we can also provide the services of components sourcing and assembly. If you are looking for a dependable PCB supplier, Hitech is a great choice. For any inquiries, please email sales3@hitechpcb.com. For more information about our products, please visit https://www.hitechpcba.com/