Production process of Si3N4-AMB copper clad substrate

---

As the power density and operating temperature of the third-generation SiC-based power module devices continue to increase, the device also puts forward higher requirements for the heat dissipation capability and reliability of the packaging substrate. Traditional DBC ceramic substrates have been difficult to meet the packaging requirements of high temperature, high power, high heat dissipation, and high reliability. The copper/ceramic interface bond strength formed by the active metal vacuum brazing (AMB) process is higher, and Si3N4 ceramics have better mechanical properties and good thermal conductivity than AI203 and AIN, so Si3N4-AMB copper-clad substrates It has stronger service reliability at high temperature and is the first choice for SiC device packaging substrates.

Preparation process of Si3N4-AMB substrate

The Si3N4-AMB copper-clad substrate uses the characteristics of active metal elements (Ti, Zr, Ta, Nb, V, Hf, etc.) that can wet the ceramic surface, and the copper layer is vacuum brazed on the Si3N4 ceramic plate through the active metal solder. According to public information, in the preparation of Si3N4-AMB copper-clad substrates, the preparation of active solder and active metal vacuum brazing are the current focus and difficulty.

Si3N4-AMB copper clad substrate production vacuum brazing process flow chart

① Selection of active solder

Ti, Zr, Hf, V, Nb, etc. are several common active metal elements, which can infiltrate the surface of ceramics and are widely used in the active sealing of ceramics and metals. Among them, the Ag-Cu-Ti alloy with Ti as the active element is the most widely studied and widely used active solder by scholars. It can wet most ceramic surfaces at a temperature of 800 ~ 950 ° C. Vacuum brazing The joint has high strength and stable performance, so it can better realize the sealing of ceramics and metals, and ceramics and ceramics. The activity of Ti element is better, and it is also the most commonly used active metal element in the process of ceramic metallization, which is more conducive to the wetting of the solder on the ceramic surface.

②Usage form of Ag-Cu-Ti active solder

The use form of Ag-Cu-Ti active solder varies with the form of Ti element and the combination of solder:
●Pre-coat Ti powder (or TiH2 powder) paste, and then add preformed solder sheet (usually Ag72Cu28 alloy solder sheet). This method is often difficult to control the uniform distribution of Ti or TiH2 on the ceramic surface, and the Ti content provided is often too high, and when the Ti content in the solder is too high, more brittle intermetallic compounds will be produced in the solder layer and affect connection strength.

●Preliminarily plate a layer of Ti film on the ceramic surface by PVD or CVD, and then add Ag-Cu solder. The problem in this case lies in the oxidation of Ti and the low process efficiency and high cost caused by the large-area metallization of the ceramic surface.

●Use Ag-Cu-Ti solder pads. Its main preparation methods are smelting and rolling method, powder metallurgy method, layered composite method and mechanical alloy method. Although the Ag-Cu-Ti solder sheet has the advantages of convenient use and low pollution, it is prone to active element Ti during the preparation process. Oxidation and segregation of the alloy lead to brittle alloy, extremely low yield, and poor performance of welded joints.
●Ag-Cu-Ti solder paste is used. Mix Ag powder, Cu powder and Ti powder according to the required ratio, or use silver-copper alloy powder or silver-coated copper powder instead of Ag powder, Cu powder, TiH2 powder instead of Ti powder, or directly use Ag-Cu-Ti ternary Alloy powder is prepared, and organic components such as solvent, thixotropic agent and leveling agent are added to form Ag-Cu-Ti solder paste. For silver-copper-titanium solder paste, its preparation process is mature, simple and efficient, but a large amount of organic matter volatilizes when heated in a high vacuum, resulting in a less dense vacuum brazing interface and more voids, which is harmful to the subsequent vacuum brazing process. The requirements are more stringent.

③Active metal vacuum brazing

Active metal vacuum brazing is the most important link in the preparation process of Si3N4-AMB substrate. In the preparation of Si3N4-AMB copper-clad substrate, the active metal solder is coated between the ceramic and the copper sheet, and then the copper sheet is vacuum brazed on both sides of the Si3N4 ceramic through a high-temperature vacuum vacuum brazing process to form a copper -Structure of solder-Si3N4-solder-copper.

Copper/Si3N4 interface voids

The void ratio of the copper/Si3N4 ceramic interface is one of the important factors to characterize the quality of the substrate, and it is also one of the bottlenecks that Si3N4-AMB products are always difficult to break through. According to the report on the official website of Rogers, there is a hidden danger of partial discharge around the interface cavity, which greatly affects the electrical performance of the substrate and reduces the reliability of the substrate. Therefore, the control of the voids at the copper/ceramic interface is very important, and a good interface void rate (tending to 0%) can ensure the service reliability of the substrate under high temperature and high current.

① Causes of void formation

Taking Ag-Cu-Ti active solder (solder paste and solder sheet) as an example, the main reasons for the voids at the Si3N4-AMB interface are as follows:
Surface quality of raw materials: scratches, pits, oxidation, organic pollution and other problems on the surface of ceramics, oxygen-free copper and soldering sheets before welding will have a negative impact on the wetting and spreading of the solder, bringing potential problems to the vacuum brazing interface Hollow risk.

Deactivation of active elements: The active element Ti of Ag-Cu-Ti is very sensitive to oxygen. In the process of high-temperature vacuum brazing, the vacuum degree is often required to be better than 10-3Pa. If the vacuum degree cannot meet the welding requirements, Ti will be oxidized and deactivated. Solder cannot wet the surface of Si3N4 ceramics, resulting in large areas of virtual soldering and missing soldering.

●Vacuum brazing process parameters: Ag-Cu-Ti active solder often wets the Si3N4 surface above 800°C. If the vacuum brazing temperature is too low or the holding time is too short, the reaction between Ti and the ceramic surface will not be sufficient Sufficiently, the solder cannot completely wet the ceramic surface.

②Measures to reduce void rate

In the process of preparing silicon nitride copper-clad substrates by the AMB process, degreasing and deoxidizing the Si3N4 ceramics and copper sheets, and providing a higher vacuum vacuum brazing environment are currently known methods to reduce interface voids. Welding pressure is the most important factor affecting the void rate. Appropriate pressure can not only make the base metal and solder form a close contact, which is conducive to the contact reaction melting, but also can enhance the fluidity of the molten solder and squeeze out the vacuum brazing interface. gas, thereby reducing the void rate.

AMB silicon nitride substrate

In addition, the vacuum + nitrogen welding atmosphere is more conducive to reducing the welding void rate than the vacuum atmosphere, which also has a certain inspiration for the AMB process, but it should be noted that nitrogen may chemically react with Ti at high temperatures, and other inert gases (helium gas, argon, etc.) may be more suitable for the AMB process.

SIMUWU vacuum brazing furnace can meet the process requirements of AMB vacuum vacuum brazing.

Learn More:

Vacuum Hardening Stainless Steel

Aluminum Alloy Brazing in Vacuum Furnace

Metallographic Inspection Knowledge of Bolt Quenching And Tempering