Lead-Free Solder Foil for High-Temperature Diffusion Joining
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Solution Overview
Problem
Current lead-free solder materials for high-temperature applications in power electronics lack the necessary temperature resistance and reliability, with existing solutions being either too costly or requiring complex processes, such as additional pressing forces or heat treatments, to achieve effective connections above 150°C.
Innovation Solution
A lead-free soldering foil composed of a soft solder matrix with dispersed high-melting metal particles, where each particle is completely covered by the soft solder, allowing for the formation of a high-melting intermetallic phase with a remelting temperature above 400°C without additional processing steps like pressing forces or heat treatments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional lead-free solders (Sn-based) are used, then environmental compatibility is improved, but temperature resistance deteriorates (stable only up to 150°C)
Solution Approach 1:
The invention uses a composite solder material consisting of a lead-free soft solder matrix (Sn-Ag-Cu alloy) with dispersed hard solder particles (high-melting metal intermetallic compounds) embedded throughout. This composite structure allows the soft solder to provide environmental compatibility and low-temperature melting (217-250°C) while the hard solder particles provide high-temperature stability (remelting temperature >400°C), resolving the contradiction between environmental friendliness and temperature resistance.
2Temperature
If Au80Sn20 solder is used, then temperature resistance is improved (melting point 280°C), but cost deteriorates (high material cost)
Solution Approach 1:
The invention applies local quality by concentrating the expensive high-melting metal intermetallic compounds only where needed - as dispersed particles within the solder joint - rather than using them as the bulk solder material. The matrix uses inexpensive lead-free soft solder, while the hard solder particles locally provide the necessary high-temperature stability, significantly reducing overall material cost compared to Au80Sn20 while maintaining temperature resistance.
3Reliability
If diffusion soldering with pressing force is applied, then connection reliability is improved, but process complexity deteriorates
Solution Approach 1:
The invention enables self-service by designing the solder material itself to provide the necessary diffusion and bonding functions without requiring external pressing forces. The dispersed hard solder particles create capillary channels and concentration gradients that drive spontaneous diffusion of metal atoms during heating, allowing the solder to self-organize and form reliable connections through material-intrinsic mechanisms rather than external mechanical intervention.
4Temperature
If sintering technique is used, then temperature resistance is improved, but process complexity deteriorates (pressing force absolutely necessary)
Solution Approach 1:
The invention replaces the mechanical pressing system with a materials-based solution. Instead of relying on external pressing forces to enable diffusion and bonding, the solder material's own composition (soft solder matrix with hard solder particles) creates chemical and concentration gradients that drive spontaneous diffusion during heating. This substitutes mechanical action with material-intrinsic diffusive and capillary forces, eliminating the need for complex pressing equipment while achieving high-temperature resistant connections.
5Temperature
If reaction solders with thin layered foils are used, then temperature resistance is improved, but manufacturing precision deteriorates (layer thickness control)
Solution Approach 1:
The invention applies segmentation by dividing the solder material into a continuous soft solder matrix phase and dispersed hard solder particle phase. Rather than requiring precise control of multiple thin layers, the hard solder is distributed as discrete particles throughout the matrix, with particle size and spacing controlling the diffusion process. This segmented structure is much easier to manufacture with conventional mixing and casting techniques while achieving the same high-temperature performance.
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
This solution enables a rapid, pore-free formation of a high-melting intermetallic connection zone with a remelting temperature over 400°C, suitable for power electronics, while maintaining mechanical and electrical conductivity, and adapting thermal expansion coefficients for stress absorption.
Implementation Method 1
diffusion soldering process, in which a substance that deviates from the original composition of the soft solder is created during the soldering process, firmly connecting the components to be joined
Implementation Method 2
the soft solder matrix (5), in which particles (6) of a high-melting metal component (7), a hard solder component, are distributed in a dispersed manner... complete conversion of the soft solder (8) of the soft solder matrix (5) into intermetallic phases (9)
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
The invention relates to a lead-free solder foil for diffusion soldering and to the method for producing the same, with which metallic components and/or metallized/metal-coated components, i.e. metallic surface layers of adjacent components, can be connected to one another. The object of the invention is to provide a cost-effective, environmentally friendly and non-toxic, lead-free solder foil for diffusion soldering with which surface layers of the components to be soldered can be connected to another in such a way that a continuous layer of a high-melting connecting zone in the form of an intermetallic phase with a re-melting temperature of higher than 400°C is produced in a process temperature range typical for soft soldering, i.e. at about 240°C, and in soldering times of less than 5 minutes, without a subsequent heat treatment and without a pressing force being exerted during the soldering. The lead-free solder foil (1) according to the invention for diffusion soldering comprises a composite solder material (4), which is produced by roll-bonding and is then roughened in such a way that, in a lead-free soft solder environment of a soft solder matrix (5), compact particles (6) of a high-melting metal component (7) are completely surrounded by the lead-free soft solder (8), wherein the dispersed particles (6) of the high-melting metal component (7) have in the direction of the foil thickness a thickness of 3 µm to 20 µm, the distances between the particles (6) in the soft solder matrix (5) are 1 µm to 10 µm, each of the particles of the high-melting metal component (7) is enclosed on all sides by the 1 µm to 10 µm thick layer of the lead-free soft solder (8), and the solder foil (1) has adjacent to the metallic surface layers (3) of the components (2) to be joined an outer encasing layer (10), the layer thickness of which is 2 µm to 10 µm and which consists of soft solder (8).