What are main ceramic metallization methods for ceramic substrates?

With the continuous increasing power of semiconductor chips.the development trend of light weight and high integration 5th generation wireless systems, in order to make the device run more smoothly in the long-term, the importance of the interior heat dissipation of the devices particularly important. Undoubtedly, the critical issue puts forward more stringent requirements for the packaging heat dissipation materials. How to do a good job in chip thermal management will be a long-term issue that must be faced and solved for all telecommunications equipment manufacturers.

In the packaging structure of power electronic components, the packaging substrate is engaged as a role undertake and maintain the conduction of internal and external circuits, and has functions such as heat dissipation and mechanical support. It has attracted the attention of more and more manufacturers.

What is considered a good substrate material?

1. Firstly, High thermal conductivity, low dielectric constant, good heat resistance and pressure resistance is must

2. Secondly, it should offer sufficient strength and stiffness to support and protect chips and electronic components;

3.In addition, the thermal expansion coefficient of the substrate is close to the chip material (such as Si, GaAs) to avoid thermal stress damage to the chip;

4. Moreover, the cost is as low as possible to meet the needs of large-scale industrial production applications;

5. Lastly, good processing, assembly and installation performance is needed.

Technical ceramics, as typical inorganic non-metallic materials, seem to be completely incompatible with metal ones. However, people began to consider how to combine ceramics and metals because of the prominent advantages respectively of the two materials, then to show its advantages together, and then, the Ceramic metallization technology was born. Over the years, ceramic metallization has been a hot topic, and many scholars a home and at abroad have carried out in-depth research on it.

Advantages of ceramic materials

1. Low communication loss: The dielectric constant of the ceramic material itself makes the signal loss lower.

2. High thermal conductivity: The heat on the chip can be conducted to the ceramic sheet directly without an insulating layer, which can achieve relatively heat dissipation better.

3. Matching thermal expansion coefficient: The thermal expansion coefficient of ceramic is close to and chip, and it will not cause too much deformation when the temperature difference changes drastically, resulting in problems such as line de-soldering and internal stress.

4. High bonding force: The bonding strength of the metal layer of the ceramic circuit board product and the Ceramic Substrate is high, and the maximum can reach 45MPa (more than the strength of the 1mm thick ceramic sheet itself).

5. High operating temperature: Ceramics can withstand high and low temperature cycles with large fluctuations, and can even operate normally at high temperatures of 500-600℃.

6. High electrical insulation: The ceramic material itself is an insulating material and can withstand a high breakdown voltage.

Ceramics must be metallized first in order to achieve the direct bonding between ceramics and metals, that`s to say, a metal film that is firmly bonded to the ceramics but not easily melted is applied to the surface of the ceramics to make them conductive, and then sintered in a high-temperature reducing atmosphere furnace to let the glass phase interpenetrate each other so that the metal film can be closely adhered to the ceramic surface; then it is connected with metal leads or other metal conductive layers by a welding process to form an assembly. It can be said that the quality of the ceramic metallization will directly affect the final packaging effect.

The commonly used preparation methods of ceramic metallization mainly include Mo-Mn method, activated Mo-Mn method, active metal brazing method, direct bonded copper method (DBC), and magnetron sputtering method:

1. Mo-Mn method

The Mo-Mn method is mainly based on refractory metal powder Mo, then adds a small amount of low-melting Mn metallization formula and a kind of special binder to coats it on the surface of Al2O3 ceramics; Furthermore, sinters to form a metallized layer. The disadvantage of the traditional Mo-Mn method is that the sintering temperature is high, the energy consumption is too much, and the lack of activator in the formulation leads to low sealing strength.

2. Active Mo-Mn method

The activated Mo-Mn method is an improvement method based on the traditional Mo-Mn method. The main directions of improvement are: adding an activator and replacing molybdenum powder with molybdenum manganese oxide. This improved method is to reduce the metallization temperature. The disadvantage of the activated Mo-Mn method is that the process is complicated and the cost is high, but its adhesion strength is firm and can greatly improve the wettability. Therefore, it is still the earliest invented, but widely used process in the ceramic-metal brazing process till now.

3. Active metal brazing method

The active metal brazing method is also a widely used ceramic-metal hermetic process. It is 10 years later than the Mo-Mn method. It is characterized by fewer manufacturing processes, and the ceramic-metal sealing can be completed with only one heating process. . The brazing alloy contains active elements, such as Ti, Zr, Hf, and Ta, and the added active elements react with Al2O3 to form a reactive layer with metallic properties at the interface. This method can be easily adapted to large-scale production. Compared to the manganese process, this method is relatively simple and economical.

The disadvantage of the active metal brazing method is that the active brazing material is single, which limits its application to a certain extent, and is not suitable for continuous production, only suitable for large-scale, single-piece production or small batch production.

4. Direct bonding copper method (DBC)

DBC is a ceramic metallization method for bonding copper foil on the ceramic surface (mainly Al2O3 and ALN), and it is a new process developed with the rise of chip-on-board (COB) packaging technology. The basic principle is to introduce oxygen element between Cu and ceramics, and then form Cu/O eutectic liquid phase at 1065~1083, and then react with ceramic matrix and copper foil to form CuAlO2 or Cu(AlO2)2, and in the middle The bonding between the copper foil and the substrate is realized under the action of the phase.

5. Magnetron sputtering

Magnetron sputtering is a method of depositing multilayer films on substrates by magnetron technology, which has advantages over other deposition techniques, including better adhesion, less contamination, and improved crystallinity of the deposited samples to obtain high quality film. The metallization layer obtained by this method is very thin, which can ensure the accuracy of the size of the parts. Direct plating copper (DPC) is one of mainstream method for magnetron sputtering. The DPC process supports PTH (Plated Through Hole)/Vias (Through Hole). High-density assembly is possible - the line/space (L/S) resolution can reach 20μm, thereby realizing the lightweight, miniaturization and integration of equipment.

As a new type of material, ceramic metallization has many unique advantages. In the near future, ceramic metallization materials will play a more important role.