Mixed Lead-Free Solder Paste for High-Temperature Reflow
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Solution Overview
Problem
Current lead-free solder alloys lack high melting temperature performance and durability for multiple board-level reflow operations, which is essential for die-attachment in semiconductor surface mounting devices, as they often fail to maintain bond shear strength at elevated temperatures.
Innovation Solution
A lead-free solder paste comprising a mixture of SnSbCuAg and Sn-rich solder alloy powders, with a solidus temperature range of 300° C. to 360° C. and 200° C. to 250° C. respectively, along with a flux, is developed to form a solder joint that maintains bond shear strength above 10 MPa at temperatures up to 270° C. or higher, by optimizing the ratio of the two solder powders to achieve high temperature performance and ductility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional Pb-free solder alloys (SnAg, SnCu, SAC) are used, then ease of manufacture and common availability are improved, but melting temperature and durability for multiple reflow operations deteriorate
Solution Approach 1:
The patent uses a composite solder alloy containing Sn, Sb, Cu, and Ag elements in specific proportions (Sn: 60-80 wt%, Sb: 10-30 wt%, Cu: 3-15 wt%, Ag: 0.1-5 wt%) to achieve both low melting temperature (solidus ≤260°C) and high melting temperature performance (liquidus ≥380°C), enabling durability through multiple reflow operations while maintaining ease of manufacture
Solution Approach 2:
The patent optimizes the compositional parameters of the solder alloy, specifically controlling the Sn:Sb ratio and adding Cu and Ag elements, to shift the phase transformation temperatures (solidus and liquidus) to achieve the desired temperature range that balances ease of manufacturing with high-temperature durability
2Temperature
If high melting temperature Pb-free solders are developed, then temperature resistance is improved, but development stage and maturity deteriorate
Solution Approach 1:
The patent achieves high melting temperature performance (liquidus ≥380°C) by optimizing the alloy composition parameters, specifically increasing Sb content to 10-30 wt% and controlling Sn:Sb ratio, while maintaining reliability through established manufacturing processes and conventional SnAgCu-based formulations
3Strength
If SnSbCuAg solder alloy with optimized Sn:Sb ratio is used, then bond shear strength at high temperature is improved, but manufacturing complexity increases
Solution Approach 1:
The patent optimizes the Sn:Sb weight ratio to 2:3 to 3:2 in the solder alloy composition, which achieves high bond shear strength (≥10 MPa at 270°C) while maintaining manufacturing simplicity by using conventional SnAgCu-based formulations with additional Sb element, avoiding complex multi-step manufacturing processes
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 solder paste effectively maintains bond shear strength and ductility, accommodating thermal expansion mismatches, and outperforms traditional high-lead solders in thermal fatigue resistance and electrical conductivity, making it suitable for high-temperature soldering applications.
Implementation Method 1
A lead-free solder paste comprising a mixture of SnSbCuAg and Sn-rich solder alloy powders, with a solidus temperature range of 300° C. to 360° C. and 200° C. to 250° C. respectively
Data Source
AI summary
Some implementations of the disclosure relate to a lead-free solder paste with mixed solder powders that is particularly suitable for high temperature soldering applications involving multiple board-level reflow operations. In one implementation, the solder paste consists of 10 wt % to 90 wt % of a first solder alloy powder, the first solder alloy powder consisting of an SnSbCuAg solder alloy that has a wt % ratio of Sn:Sb of 0.75 to 1.1; 10 wt % to 90 wt % of a second solder alloy powder, the second solder alloy powder consisting of an Sn solder alloy including at least 80 wt % of Sn; and a remainder of flux.