Power Module Substrate Bonding Using Ag-Cu Eutectic Layer
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Solution Overview
Problem
The existing power module substrates face issues with heat dissipation due to the lower thermal conductivity of aluminum plates compared to copper, leading to potential cracking of ceramic substrates during cooling-heating cycles due to thermal expansion differences.
Innovation Solution
A method involving a copper member-bonding paste with Ag and a nitride-forming element is used to form a thin Ag-Cu eutectic structure layer and a nitride layer on the ceramic substrate, allowing for strong bonding and deformation to prevent cracking during thermal cycles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If copper plate is used as the first metal plate to improve heat dissipation, then thermal conductivity is improved, but the ceramic substrate may crack during cooling-heating cycles due to thermal expansion differences
Solution Approach 1:
An Ag-Cu-Ti-based brazing material is introduced as an intermediary layer between the copper plate and ceramic substrate. This brazing material contains active metal (Ti) that forms a reaction product layer with the ceramic substrate, creating a transition zone that accommodates thermal expansion differences while maintaining strong bonding. The brazing material melts and solidifies to form a eutectic structure that reduces stress concentration at the interface.
Solution Approach 2:
The bonding structure is designed as a composite system consisting of copper plate + Ag-Cu-Ti brazing material + reaction product layer + ceramic substrate. Each layer has specific properties: copper provides high thermal conductivity, the brazing material provides bonding strength and stress relief, the reaction product layer provides chemical bonding to the ceramic, and the ceramic provides electrical insulation and structural support. This composite structure resolves the contradiction between thermal conductivity and cracking resistance.
2Reliability
If aluminum plate is used as the first metal plate, then thermal expansion difference is reduced, but heat dissipation capability deteriorates
Solution Approach 1:
The Ag-Cu-Ti brazing material serves as a mediator that enables the use of copper (high thermal conductivity) without directly bonding it to the ceramic substrate. The brazing material's reaction product layer provides the thermal expansion compatibility with the ceramic, while the copper plate provides superior heat dissipation. This intermediary approach allows selecting materials based on their optimal properties rather than compromise.
3Strength
If Ag-Cu-Ti-based brazing material is used to bond copper plate to ceramic substrate, then bonding strength is improved, but the complexity of the manufacturing process increases
Solution Approach 1:
The brazing material is prepared in advance as a paste containing Ag-Cu-Ti powder mixture with specific composition (90-99.5 wt% Ag, 0.5-4.0 wt% Cu, 0.5-4.0 wt% Ti). The paste is applied to the ceramic substrate before bonding, and the composition is pre-optimized to ensure proper reaction with the ceramic and formation of the eutectic structure. This preliminary preparation simplifies the bonding process by eliminating the need for complex in-process adjustments.
Solution Approach 2:
The brazing process utilizes controlled parameter changes: heating to a specific temperature range to melt the brazing material and form the eutectic structure, then cooling to solidify the bond. The Ag-Cu-Ti composition is specifically designed to melt and solidify in a controlled manner, creating the reaction product layer and eutectic structure that provide both bonding strength and stress relief. These controlled parameter changes enable strong bonding while maintaining process simplicity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method effectively suppresses ceramic substrate cracking and enhances bonding strength, ensuring reliable heat dissipation and structural integrity under thermal stress.
Implementation Method 1
an Ag-Cu eutectic structure layer is formed by melting and solidification of the Ag-Cu-Ti-based brazing material
Implementation Method 2
bonded to a ceramic substrate with an active metal method using an Ag-Cu-Ti-based brazing material
Data Source
Figure 1~2
Figure 3
Figure 4
AI summary
This power module substrate includes a copper plate that is formed of copper or a copper alloy and is laminated on a surface of a ceramic substrate 11; a nitride layer 31 that is formed on the surface of the ceramic substrate 11 between the copper plate and the ceramic substrate 11; and an Ag-Cu eutectic structure layer 32 having a thickness of 15 µm or less that is formed between the nitride layer and the copper plate.