Sn-Bi Solder Joining for High-Temperature Reliability
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Solution Overview
Problem
The existing soldering methods using Sn—Pb solder and solder pastes with Cu, Al, or Ag alloys face issues with joint strength under elevated temperatures, Sn remaining after soldering, and the formation of laminar intermetallic compounds that lead to stress and potential cracking, making them unreliable for high-temperature applications.
Innovation Solution
A joining method using a low melting point metal like Sn or its alloys, combined with Cu—Al or Cu—Cr alloys, where heat treatment forms an intermetallic compound between the metals, enhancing joint strength and preventing Sn remelting, with the Cu—Al or Cu—Cr alloys promoting rapid diffusion and forming a reliable, high-temperature-resistant joint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a solder paste including low melting point metal (Sn) balls and high melting point metal (Cu) balls is used for bonding with temperature hierarchy, then the melting point of the joining material is reduced to enable low-temperature soldering, but Sn remains after soldering due to slow diffusion rates, causing deteriorated joint strength under elevated temperatures and potential remelting in subsequent soldering steps
Solution Approach 1:
The invention changes the chemical composition parameters of the joining material by replacing conventional Sn-Cu solder paste with a low melting point alloy containing Bi (e.g., Sn-Bi, Pb-Bi, or Sn-Pb-Bi alloys with melting points of 130°C to 200°C). This parameter change enables the joining material to melt and form intermetallic compounds at lower temperatures while maintaining sufficient diffusion rates to prevent Sn remelting in subsequent reflow steps, thereby resolving the contradiction between low melting point requirement and joint strength reliability.
Solution Approach 2:
The invention uses composite alloy materials combining multiple metals (Sn, Bi, Pb, Cu, Ag, In) with specifically controlled compositions to achieve both low melting point and high reliability. The multi-component alloy system creates a eutectic structure that melts at low temperature while forming stable intermetallic compounds (such as Cu6Sn5, Cu3Sn, Ag3Sn) that prevent remelting and maintain joint strength under elevated temperatures.
2Reliability
If heating is extended to convert all low melting point metal to intermetallic compound to prevent Sn remaining, then joint strength under elevated temperatures is improved, but productivity is significantly reduced due to impractically long heating time
Solution Approach 1:
The invention changes the melting point parameter of the joining material to 130°C to 200°C by using Bi-containing alloys, which enables complete conversion to intermetallic compounds within practical heating times (10 to 60 minutes at 200°C to 300°C). The lower melting point accelerates diffusion kinetics, allowing full reaction without requiring excessively long heating times, thus resolving the contradiction between reliability and productivity.
Solution Approach 2:
The invention utilizes phase transition at a lower temperature (130°C to 200°C) to enable complete metallurgical reaction and intermetallic compound formation within practical timeframes. The eutectic melting and subsequent solidification process promotes rapid diffusion and complete conversion to intermetallic compounds without requiring extended heating, thereby maintaining both high reliability and productivity.
3Ease of manufacture
If conventional Sn-Pb solder or Sn-based solder paste is used for mounting electronic parts, then ease of manufacture is maintained, but Sn remains after soldering due to slow diffusion, causing deteriorated heat resistance and potential flow out in subsequent reflow soldering steps
Solution Approach 1:
The invention changes the composition parameters of the solder paste by incorporating Bi-containing low melting point alloys (Sn-Bi, Pb-Bi, or Sn-Pb-Bi with 30% to 70% Bi by weight). This parameter change reduces the melting point to 130°C to 200°C and accelerates diffusion rates, enabling complete conversion to intermetallic compounds without requiring process changes, thus maintaining ease of manufacture while dramatically improving heat resistance and preventing Sn flow out.
Solution Approach 2:
The invention replaces conventional long-lasting Sn-Pb solder with a low melting point Bi-containing alloy that is designed to completely react and form intermetallic compounds during a controlled heating process. The joining material is consumed to form the joint, with no residual low melting point metal remaining, thereby eliminating the reliability issues of Sn flow out while maintaining manufacturing 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 achieves high joint reliability and heat resistance, preventing Sn remelting and joint failure even under reflow conditions, with improved shear strength and thermal shock resistance, and reduced Sn content in the joint, ensuring strong and durable electronic device connections.
Implementation Method 1
heat treatment of heat-treating the first metal member and the second metal member with the joining material located therebetween at a temperature at which the low melting point metal composing the joining material melts
Implementation Method 2
an intermetallic compound produced by the reaction of the low melting point metal composing the joining material with the Cu—Al alloy or the Cu—Cr alloy composing the first metal and/or the second metal
Implementation Method 3
the Cu—Al or Cu—Cr alloys promoting rapid diffusion
Data Source
AI summary
In joining a first metal member composed of a first metal to a second metal member composed of a second metal with a joining material interposed therebetween, the joining material including a low melting point metal having a lower melting point than the first metal and/or the second metal, the low melting point metal composing the joining material is Sn or an alloy containing Sn, at least one of the first metal and the second metal is a metal or an alloy which forms an intermetallic compound with the low melting point metal composing the joining material, and heat treatment is performed at a temperature at which the low melting point metal melts in a state of locating the joining material between the first metal member and the second metal member.