30-December-2025
However, the rapid development of high-power power electronics (such as IGBT modules and third-generation semiconductors) has driven the demand for superior heat dissipation. This necessitates the use of ceramic materials with much higher thermal conductivity, such as Aluminum Nitride (AlN) or Silicon Nitride (Si3N4). These advanced ceramics are typically of high purity and contain little to no glassy phase. For these materials, the traditional Mo-Mn metallization method is often ineffective.
This is where Active Metal Brazing (AMB) becomes the solution of choice.
The core mechanism of the Mo-Mn process relies on the "glassy phase" within the ceramic migrating and penetrating into the porous molybdenum layer at high temperatures. However, the Mo-Mn method faces significant challenges with two specific categories of ceramics:
1.High-Purity Ceramics: Such as 99% or higher Alumina. Due to the lack of a sufficient glassy phase, the Mo-Mn layer cannot establish a strong mechanical interlock, resulting in very low sealing strength.
2.Non-Oxide Ceramics: Such as Aluminum Nitride (AlN) and Silicon Nitride (Si3N4). These ceramics are chemically stable and extremely difficult to wet with traditional oxide glass phases.
3. To address this, scientists developed Active Metal Brazing (AMB). Unlike the Mo-Mn method, AMB does not rely on physical penetration of a glass phase. Instead, it utilizes a chemical reaction to "grow" a wettable reaction layer directly on the ceramic surface. This means metallization and joining can be completed in a single-step process, greatly simplifying the workflow.
The core of AMB lies in the "Active Element."
The filler metal typically used is a Silver-Copper alloy (Ag-Cu eutectic), but with small amounts of active metals added. The most common addition is Titanium (Ti), though Zirconium (Zr) or Hafnium (Hf) are sometimes used.
The microscopic reaction mechanism is as follows:
1. Melting and Segregation: During vacuum heating, the brazing filler melts. Because active elements like Ti have an extremely high affinity for oxygen and nitrogen, they preferentially segregate towards the interface between the ceramic and the liquid filler.
2. Chemical Reaction:
· For oxide ceramics (like Alumina), Ti extracts oxygen from the ceramic surface to form titanium oxides (TiO-type oxides).
· For nitride ceramics (like AlN), Ti reacts with nitrogen to form Titanium Nitride (TiN).
3. Formation of Reaction Layer: These reaction products (TiO or TiN) form an extremely thin, continuous Reaction Layer on the ceramic surface. Crucially, this layer possesses metallic characteristics.
4. Wetting and Bonding: The bulk liquid Ag-Cu filler can wet this newly formed reaction layer perfectly, achieving a solid bond between the ceramic and the metal part.
In simple terms: The active element acts as a mediator, "biting" into the ceramic surface chemically, while the bulk solder "bites" into the active element.
Compared to the complex "Metallization Sintering -> Nickel Plating -> Brazing" three-step process of the Mo-Mn method, Active Metal Brazing is typically a one-step process, offering higher efficiency.
As with all electric vacuum processes, cleanliness is paramount. The ceramic and metal parts must undergo rigorous ultrasonic cleaning, degreasing, and drying to remove oils and impurities, ensuring the uniformity of the chemical reaction.
AMB filler metals usually exist in the form of Paste or Foil.
· Ag-Cu-Ti paste is screen-printed onto the ceramic surface, or an Ag-Cu-Ti foil is sandwiched between the ceramic and the metal component.
· Appropriate fixture pressure is applied to ensure intimate contact.
This is the most critical step. Since Titanium (Ti) oxidizes extremely easily at high temperatures, AMB must be performed in a High Vacuum or a high-purity inert gas (like Argon) atmosphere.
· Temperature: Typically between 800°C - 900°C (depending on the composition; the Ag72-Cu28 eutectic point is 780°C, and adding Ti alters the melting point slightly).
· Atmosphere Control: Vacuum levels are usually required to be better than 10-3 Pa. Note that AMB typically does not use a Hydrogen atmosphere, as Titanium readily absorbs hydrogen to form brittle Titanium Hydride, leading to joint embrittlement.
After furnace cooling, the ceramic-to-metal bond is complete. AMB joints typically possess excellent airtightness and strength without requiring any additional plating layers.
1. Simplified Process: Combines metallization and brazing into a single step, significantly shortening the production cycle.
2. Wide Applicability: It is the only reliable method for joining advanced non-oxide ceramics like Aluminum Nitride (AlN), Silicon Nitride (Si3N4), and Silicon Carbide (SiC), as well as high-purity alumina.
3. Low Thermal Resistance: Because there is no thick, thermally insulating glass phase intermediate layer (as in Mo-Mn), and because highly conductive Ag-Cu fillers are used, AMB Substrates exhibit exceptional thermal conductivity, making them ideal for high-power heat dissipation scenarios.
· Higher Cost: Active brazing filler metals containing Titanium (especially foils) are expensive.
· Equipment Requirements: Extremely strict requirements for vacuum levels and atmospheric cleanliness in the brazing furnace.
· Flow Control: The flowability of active fillers can sometimes be difficult to control, potentially leading to overflow on the ceramic surface.
· IGBT Module Substrates: In electric vehicles (EVs) and high-speed rail traction inverters, AMB-Si3N4 substrates (Silicon Nitride copper-clad laminates) are used for their extreme mechanical strength and thermal performance.
· Aerospace Sensors: For ceramic-to-metal packaging in high-temperature, harsh environments.
· Cutting Tools: Brazing Diamond or Cubic Boron Nitride (CBN) tips onto steel bodies.
Summary: If Mo-Mn metallization is the "foundation" of ceramic sealing, then Active Metal Brazing (AMB) is the key to "high performance." By breaking the limitations of the glass phase mechanism, AMB achieves direct metallurgical bonding for a wide range of advanced ceramics through chemical reaction.We will also introduce other ceramic metallization methods in subsequent articles. If you are interested in these products, please click here to contact us!
