Ceramic Circuit Substrate Brazing for Heat Cycle Resistance
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Solution Overview
Problem
Ceramic circuit substrates in high-voltage and high-current power modules face heat cycle resistance issues due to thermal stress from differences in thermal expansion coefficients between ceramic and metal components, leading to cracks and insulation failures, and existing bonding methods struggle to control brazing material diffusion and residual stress effectively.
Innovation Solution
A ceramic circuit substrate with a brazing material comprising Ag, Cu, and an active metal, bonded in a continuous heating furnace at 720-800°C for 5-30 minutes, with controlled Ag diffusion distance of 5-20 µm, and a bond void fraction of no greater than 1.0%, to enhance heat cycle resistance and reduce thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If bonding is performed at high temperature (no less than 790°C) to ensure bondability, then the bonding strength is improved, but Ag diffusion into the copper plate increases and residual stress increases, deteriorating heat cycle resistance
Solution Approach 1:
The patent changes the bonding temperature parameter from conventional high temperature (790°C or higher) to a lower range (700-850°C, preferably 720-800°C). This parameter change reduces Ag diffusion into the copper plate and lowers residual stress while maintaining adequate bonding strength through optimized brazing material composition containing Sn and active metals
Solution Approach 2:
The patent uses a composite brazing material composition comprising Ag (65-95 wt%), Cu (5-30 wt%), Sn (0.1-15 wt%), and active metals (0.5-10 wt% each of Ti, Zr, Hf, Nb). This composite material provides both bondability at lower temperatures and control over Ag diffusion, resolving the contradiction between bonding strength and heat cycle resistance
2Reliability
If bonding temperature is reduced to suppress Ag diffusion and residual stress, then heat cycle resistance is improved, but bonding strength deteriorates
Solution Approach 1:
The composite brazing material with specific composition ratios enables bonding at lower temperatures (700-850°C) while maintaining adequate bonding strength. The Sn and active metal components facilitate bonding at these temperatures, preventing strength deterioration despite the temperature reduction needed to improve heat cycle resistance
Solution Approach 2:
The patent optimizes multiple parameters simultaneously: bonding temperature (700-850°C), brazing material composition (Ag-Cu-Sn-active metals), and heating time (5-30 minutes at bonding temperature). This multi-parameter optimization ensures bonding strength is maintained even at lower temperatures that improve heat cycle resistance
3Reliability
If brazing material layer thickness is increased to ensure continuous coverage, then bonding reliability is improved, but Ag diffusion distance increases and cracks develop in the ceramic substrate
Solution Approach 1:
The patent changes the approach from controlling thickness to controlling diffusion distance. By lowering the bonding temperature to 700-850°C and limiting heating time to 5-30 minutes, the Ag diffusion distance is controlled to 5-20 µm. This allows the use of thinner brazing material layers (reducing the risk of cracks) while maintaining bonding reliability through the optimized composition and controlled diffusion
4Strength
If heating time is extended to ensure complete bonding, then bonding strength is improved, but Ag diffusion distance increases and residual stress increases, deteriorating heat cycle resistance
Solution Approach 1:
The patent optimizes the heating time parameter to 5-30 minutes at the bonding temperature (700-850°C). This time window is sufficient to achieve complete bonding and adequate strength while limiting Ag diffusion distance to 5-20 µm and controlling residual stress, thereby improving heat cycle resistance compared to conventional longer heating times
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 solution provides ceramic circuit substrates with excellent heat cycle resistance characteristics, suppressing cracks and copper plate separation, enabling the production of high-performance power modules with improved reliability and productivity.
Implementation Method 1
the diffusion distance of Ag, which is a brazing material component, is 5-20 µm
Implementation Method 2
bonding a copper plate to one surface of a ceramic substrate and bonding a copper heat dissipation board to the other surface via a brazing material
Implementation Method 3
the stress loads increase more and more during heat cycles due to differences in the coefficients of thermal expansion of the ceramic substrates and the metal plates
Data Source
Figure 1~2
AI summary
[Problem] To provide a ceramic circuit substrate and power module with excellent heat cycle resistance characteristics. [Solution] Provided is a ceramic circuit substrate formed by bonding a ceramic substrate and a copper plate via a brazing material comprising Ag, Cu, and an active metal, wherein the bond void fraction is no greater than 1.0% and the diffusion distance of the Ag, which is a brazing material component, is 5-20 µm. Provided is a method for manufacturing a ceramic circuit substrate characterized in that the heating time in a temperature range 400-700°C in a process for raising the temperature to a bonding temperature is 5-30 minutes and bonding is performed by maintaining the bonding temperature at 720-800°C for 5-30 minutes.