Mixed-Powder Lead-Free Solder Paste for Multi-Reflow Strength
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Solution Overview
Problem
Existing lead-free solder alloys struggle to maintain bond shear strength and durability during multiple board-level reflow operations, especially at high temperatures, due to issues with intermetallic compound formation and thermal expansion mismatch.
Innovation Solution
A lead-free solder paste composed of mixed solder powders, including an SnSbCuAg alloy with a high melting temperature and an Sn-rich alloy with a lower melting temperature, optimized in ratios to form a composite structure that maintains high bond shear strength and ductility, avoiding interfacial intermetallic compound formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high melting temperature Pb-free solder alloys are used to replace conventional high lead solders, then environmental compliance and lead-free requirements are met, but bond shear strength and durability during multiple reflow operations deteriorate
Solution Approach 1:
The patent uses a composite solder powder system combining Sn-3.0Ag-0.5Cu-0.5Sb alloy powder (60-80 wt%) with Sn-5.0Sb alloy powder (20-40 wt%). This composite structure leverages the strengths of both alloys: the SAC alloy provides good wetting and initial bonding, while the high-Sb alloy forms protective intermetallic compounds that enhance high-temperature durability. The synergistic combination resolves the contradiction between meeting lead-free requirements and maintaining bond strength through multiple reflow cycles.
Solution Approach 2:
The patent optimizes specific compositional parameters including the Sn:Ag:Cu:Sb ratio in the SAC alloy (3.0:0.5:0.5:0.5) and the Sb content (5.0%) in the second alloy. These parameter adjustments are critical for achieving the right balance between melting characteristics, intermetallic formation rate, and bond strength retention at elevated temperatures during multiple reflow operations.
2Temperature
If intermetallic compounds are formed through TLPB technology to achieve higher remelting temperature, then melting temperature increases, but continuous IMC growth causes deterioration of joint reliability
Solution Approach 1:
The patent applies partial TLPB action by limiting intermetallic compound formation to controlled layers at the interface rather than allowing continuous growth through the entire joint. The Sn-5.0Sb alloy powder provides Sb that forms a protective intermetallic layer at the substrate interface, raising the effective remelting temperature, while the bulk of the joint retains ductile solder matrix that prevents brittle failure. This partial IMC formation approach achieves temperature enhancement without the reliability deterioration caused by excessive IMC growth.
3Temperature
If high melting temperature solder is used for die-attachment, then high temperature resistance is achieved, but thermal expansion mismatch causes stress concentration and functional failures
Solution Approach 1:
The patent modifies the thermal expansion characteristics by adjusting the alloy composition, particularly the Sb content (5.0% in the second alloy) and the presence of Ag and Cu elements. These compositional changes alter the coefficient of thermal expansion to better match the substrate and die materials, reducing thermal stress concentration during temperature cycling while maintaining high-temperature resistance through controlled intermetallic formation.
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 achieves superior bond shear strength and durability above 270°C, accommodating thermal expansion mismatch and surviving multiple reflow processes, outperforming traditional lead-based solders.
Implementation Method 1
the liquid phase line temperature of the SnSb-based alloy powder is higher than the liquid phase line temperature of the Sn-based powder
Implementation Method 2
Transient liquid phase bonding (TLPB) technology is aimed at achieving a higher remelting temperature of a solder joint through the formation of intermetallic compounds (IMCs) between low melting temperature alloys and high melting temperatures alloys
Implementation Method 3
accommodating thermal expansion mismatch
Data Source
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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 10wt% to 90wt% 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; 10wt% to 90wt% of a second solder alloy powder, the second solder alloy powder consisting of an Sn solder alloy including at least 80wt% of Sn; and a remainder of flux.