Sn-Ag-Cu-Bi Solder Paste Composition for Low-Temperature Power Joints
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Solution Overview
Problem
Existing solder alloys for power semiconductor devices do not adequately address the issues of high liquidus temperature, low mechanical strength, and insufficient thermal fatigue resistance, particularly in high-temperature environments, while also failing to consider mountability concerns such as leaching, misalignment, and void formation during manufacturing and use.
Innovation Solution
A solder alloy composition with specific ranges of Ag, Cu, and Bi contents, optimized to achieve a low liquidus temperature, high tensile strength, and improved thermal fatigue resistance, along with a spherical solder powder and flux components for enhanced fluidity and reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If Sn-Ag-Cu-Bi solder alloys with low liquidus temperature are used for step soldering, then the solder alloy can be used for second-time soldering without melting the first solder joint, but the mechanical strength and thermal fatigue resistance are insufficient
Solution Approach 1:
The patent applies parameter changes by precisely controlling the composition ratios of Ag (2.0-4.0 mass%), Cu (0.6-0.75 mass%), and Bi (4.0-7.0 mass%) to achieve optimal balance between low liquidus temperature and high mechanical strength. This compositional parameter optimization allows the solder alloy to maintain tensile strength of 84 MPa or more while having liquidus temperature below 217°C
Solution Approach 2:
The patent uses composite material principles by creating a multi-element Sn-Ag-Cu-Bi alloy system where each element contributes specific properties: Sn provides base strength, Ag lowers liquidus temperature and improves wetting, Cu enhances strength, and Bi further reduces melting point. The synergistic combination of these elements resolves the contradiction between low temperature and high strength
2Temperature
If solder alloys with high Bi content are used to lower liquidus temperature, then the solder can withstand step soldering processes, but the solidus temperature becomes too low causing mountability issues such as leaching and misalignment
Solution Approach 1:
The patent applies parameter changes by establishing an optimal Bi content range (4.0-7.0 mass%) that is high enough to achieve liquidus temperature below 217°C for step soldering compatibility, but controlled to maintain solidus temperature above 150°C to prevent mountability issues. This precise parameter control resolves the contradiction between low liquidus and appropriate solidus temperatures
3Strength
If conventional solder alloys are used to ensure high strength, then mechanical properties are maintained, but thermal fatigue resistance in high-temperature environments is insufficient
Solution Approach 1:
The patent uses composite material principles by incorporating Bi into the Sn-Ag-Cu system to create Sn-Ag-Cu-Bi alloy that maintains high tensile strength (84 MPa or more) while significantly improving thermal fatigue resistance. The Bi element forms specific intermetallic compounds that enhance high-temperature stability and prevent degradation under thermal cycling conditions
Solution Approach 2:
The patent applies parameter changes by optimizing the Cu content (0.6-0.75 mass%) which plays a crucial role in enhancing both strength and thermal fatigue resistance. The specific Cu concentration promotes formation of strengthening precipitates that maintain structural integrity under thermal stress while preserving mechanical strength
Data Source
Figure 1
AI summary
Provided are a solder alloy, a solder powder, a solder paste, and a solder joint, which have excellent mountability, high strength, and excellent thermal fatigue resistance due to low liquidus temperature and not too low solidus temperature. The solder alloy has an alloy composition consisting of, by mass: Ag: 2.0 to 4.0%; Cu: 0.51 to 0.79%; and Bi: more than 4.0% and 8.0% or less, with the balance being Sn. The solder alloy has a liquidus temperature of less than 217°C.