Silver-Bismuth Alloy Bonding Material for Semiconductor Heat Resistance
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Solution Overview
Problem
Current bonding materials for semiconductor devices face challenges in achieving both high heat resistance and stress-relaxation ability, particularly when used in high-temperature applications, leading to degradation and reduced bonding strength over time.
Innovation Solution
A sheet-like bonding material composed of a silver-bismuth alloy, with a bismuth content between 1 to 5 mass %, undergoes a solid-phase diffusion reaction when heated, forming a diffusion layer in metal targets to enhance bonding strength and heat resistance without degrading the resin material.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a sinterable metal or metal paste with organic solvent is used as bonding material, then heat resistant temperature after bonding becomes high (nearly equal to melting point of solid metal), but the organic component decomposes at about 200 to 300° C. requiring bonding at temperatures where bonding target is not degraded
Solution Approach 1:
The patent changes the bonding mechanism from organic solvent-based sintering to solid-phase diffusion reaction. By using a eutectic alloy composition (4-12 wt% Bi in Ag), the bonding material undergoes solid-phase diffusion at lower temperatures (150-300°C) without organic decomposition, achieving both low bonding temperature and high heat resistance.
Solution Approach 2:
The patent creates a composite bonding material system consisting of Ag-Bi eutectic alloy particles combined with metal powder (Cu, Al, or alloy). This composite structure enables solid-phase diffusion bonding at low temperatures while maintaining high heat resistance and electrical conductivity.
2Strength
If metal-sintered material is used as bonding material, then elastic modulus is higher than conventional solders, but stress-relaxation ability in heat cycle decreases making it difficult to keep bonding strength over long period
Solution Approach 1:
The patent changes the material state from sintered (with voids) to fully dense through solid-phase diffusion. The Ag-Bi eutectic alloy forms a continuous metallic matrix that eliminates voids, achieving both high elastic modulus and excellent stress-relaxation ability for long-term reliability.
3Reliability
If particles having lubricating ability or fillers of resin are mixed in bonding material to relax stress, then stress-relaxation ability improves, but heat resistant temperature decreases due to degradation of resin in high temperature
Solution Approach 1:
The patent replaces long-lived resin fillers with a eutectic alloy system that is specifically designed to be active only during the bonding process. The Ag-Bi alloy provides stress-relaxation through its low melting point and fluidity during bonding, then forms a stable, high-temperature-resistant structure after bonding.
Solution Approach 2:
The patent utilizes phase transition of the Ag-Bi eutectic alloy (melting at 90-110°C) to enable stress-relaxation during bonding. The material transitions from solid to liquid state during heating, allowing stress relaxation, then solidifies to form a stable, high-temperature-resistant bonding structure.
4Ease of manufacture
If bonding is made by melting a bonding material itself such as solder, then bonding can be achieved, but temperature must be higher than heat resistant temperature of bonding portion limiting choices and causing degradation of bonding target
Solution Approach 1:
The patent changes the bonding mechanism from melting (liquid-phase) to solid-phase diffusion. The Ag-Bi eutectic alloy undergoes solid-phase diffusion at 150-300°C, which is lower than the heat resistant temperature of most bonding targets, enabling bonding without degradation while maintaining good processability.
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 silver-bismuth alloy bonding material achieves a balance between heat resistivity and stress-relaxation ability, enabling the creation of a high-temperature compatible and reliable semiconductor device with improved durability and thermal conductivity.
Implementation Method 1
when heated in a state being in contact with a metal material as a bonding target, forms in the metal material a diffusion layer of silver due to solid-phase diffusion reaction
Data Source
AI summary
The present invention has an object to achieve bonding which satisfies both in heat resistivity and in stress-relaxation ability, and the bonding material according to this invention is a sheet-like bonding material 1 made of a silver-bismuth alloy which, when heated in a state being in contact with a metal material as a bonding target (for example, surface layers 2f, 3f), forms in the metal material (as its material, for example, gold, silver or copper) a diffusion layer Ld2, Ld3 of silver due to solid-phase diffusion reaction, so as to be bonded to the metal material, said bonding material being characterized by containing not less than 1 mass % but not more than 5 mass % of bismuth.