Copper Joining Paste Composition for Void-Free Pressureless Sintering
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Solution Overview
Problem
Existing copper pastes for joining fail to prevent void formation during sintering, leading to insufficient joining strength, especially when joining large areas or in pressureless conditions.
Innovation Solution
A copper paste formulation using copper powders with different particle sizes and a solvent with a specific crystallite size increase rate and boiling point, ensuring high packing density and controlled sintering to minimize voids.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional copper paste with standard particle sizes is used, then the paste can be easily applied, but voids are formed during sintering leading to insufficient joining rate
Solution Approach 1:
The copper paste uses a segmented particle size distribution with three distinct ranges: fine particles (0.1-1 μm) to fill voids, intermediate particles (1-3 μm) for packing density, and coarse particles (3-10 μm) for structural framework. This segmentation prevents void formation during sintering while maintaining high joining rate.
Solution Approach 2:
The invention changes the particle size parameters by introducing a specific distribution across three ranges rather than using uniform particles. The fine particle range (0.1-1 μm) specifically addresses void filling, while the overall distribution optimizes both packability and sintering behavior to eliminate harmful voids.
2Strength
If high copper powder content is used to improve joining strength, then the paste becomes difficult to apply and may form defects
Solution Approach 1:
The paste formulation uses local quality differentiation through particle size distribution: fine particles (0.1-1 μm) provide flowability and ease of application, intermediate particles (1-3 μm) contribute to packing density, and coarse particles (3-10 μm) provide structural strength. This local differentiation allows high copper content (85-95 wt%) while maintaining applicability.
Solution Approach 2:
The invention creates a composite particle system combining three different copper particle size ranges in specific proportions (fine: 30-70 wt%, intermediate: 20-50 wt%, coarse: 10-30 wt%). This composite structure achieves both high joining strength through dense packing and good applicability through the flow properties of fine particles.
3Ease of operation
If sintering is performed in pressureless state for simplicity, then the process is easier, but voids are formed reducing joining quality
Solution Approach 1:
The invention performs preliminary action by pre-distributing fine copper particles (0.1-1 μm) throughout the paste formulation before sintering. These fine particles are positioned to fill interstices between larger particles during application, creating a pre-configured dense structure that prevents void formation during pressureless sintering.
Solution Approach 2:
The multi-size particle distribution enables self-service during sintering: fine particles naturally migrate to fill voids created by coarser particles, and the crystallite growth characteristics of the fine particles promote densification without external pressure. The system self-corrects potential void formation through the inherent properties of the particle distribution.
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 paste achieves a high joining rate and strength without voids, even in large joining areas and pressureless conditions, enhancing thermal and electrical properties.
Implementation Method 1
one of the copper powders has an increase rate of a crystallite size, (D2−D1)/D1×100, of 5% or more, where D1 (nm) represents a crystallite size at 150° C. and D2 (nm) represents a crystallite size at 250° C.
Implementation Method 2
In the process of sintering the coating film, the solvent contained in the coating film is volatilized and removed as the temperature increases.
Implementation Method 3
the solvent contained in the coating film is volatilized and removed as the temperature increases
Data Source
AI summary
A copper paste for joining is provided that is unlikely to generate voids in a sintering process of a coating film formed thereof to achieve a sufficiently high joining rate to a target member to be joined. The copper paste for joining contains: two or more copper powders having different particle sizes; and a solvent; wherein one of the copper powders has an increase rate of a crystallite size, (D2−D1)/D1×100, of 5% or more, where D1 (nm) represents a crystallite size at 150° C. and D2 (nm) represents a crystallite size at 250° C.; the solvent has a boiling point of 150° C. or higher and lower than 300° C.; and the proportion of the total amount of the copper powders is 88 mass % or more in 100 mass % of the copper paste.