Bi-Sn Solder Alloy Composition for Low-Temperature Durable Joints
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Solution Overview
Problem
Solder alloys containing Bi tend to be hard and brittle, leading to reduced reliability of joint structures due to thermal and mechanical stresses, such as thermal fatigue and external impacts, which are not adequately addressed by existing lead-free solder alloys.
Innovation Solution
A solder alloy composition comprising 45 to 63% Bi, 0.1 to 0.7% Sb, 0.05 to 1% In, and the balance Sn, with optional additions of P, Ga, Ge, Mn, Ti, Al, Cr, V, or Mo, to enhance ductility and strength while maintaining a low liquidus temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If Bi is added to lower the liquidus temperature, then the heating temperature can be set low, but the ductility of the solder alloy is reduced and the joint portion becomes hard and brittle
Solution Approach 1:
The patent optimizes the compositional parameters of the solder alloy by precisely controlling the content ranges of Bi (45-63 mass%), Sb (0.1-0.7 mass%), In (0.05-1 mass%), and Sn (balance). This parameter optimization resolves the contradiction by finding the optimal composition point where the liquidus temperature is sufficiently low while maintaining adequate ductility through the synergistic effects of multiple alloying elements.
Solution Approach 2:
The patent creates a composite solder alloy system combining Bi, Sb, In, and Sn elements. The composite material approach allows the alloy to benefit from the low melting point of Bi, the ductility enhancement from Sb and In, and the base properties of Sn, thereby resolving the contradiction between low temperature processing and maintained ductility that cannot be achieved with single-element additions.
2Temperature
If Bi content is increased to achieve low liquidus temperature, then heating temperature can be reduced, but the joint portion becomes more brittle and resistant to thermal and mechanical stresses decreases
Solution Approach 1:
The patent establishes specific parameter ranges for each alloying element to simultaneously achieve low heating temperature and high stress resistance. By controlling Bi content at 45-63 mass% (not excessively high), Sb at 0.1-0.7 mass%, and In at 0.05-1 mass%, the alloy maintains both low melting characteristics and sufficient mechanical strength to resist thermal and mechanical stresses.
Solution Approach 2:
The patent introduces Sb and In as intermediary elements that mediate between Bi's low melting point benefit and its brittleness drawback. Sb (0.1-0.7 mass%) and In (0.05-1 mass%) act as intermediaries that enhance ductility and stress resistance while allowing Bi to provide the low liquidus temperature, thereby resolving the contradiction between low heating temperature and stress resistance.
3Temperature
If existing lead-free solder alloys are used, then the liquidus temperature can be lowered, but the heat cycle resistance and drop impact resistance are insufficient
Solution Approach 1:
The patent develops a composite solder alloy containing Bi (45-63 mass%), Sb (0.1-0.7 mass%), In (0.05-1 mass%), and Sn (balance). This composite material provides superior heat cycle resistance and drop impact resistance compared to existing lead-free alloys by combining the low melting point advantage of Bi with the ductility and strength enhancement from Sb and In, while the specific composition ratios ensure adequate resistance to thermal and mechanical stresses.
Solution Approach 2:
The patent changes the compositional parameters from conventional lead-free solder alloys by significantly increasing Bi content to 45-63 mass% (higher than typical compositions) and optimizing Sb (0.1-0.7 mass%) and In (0.05-1 mass%) content. These parameter changes result in a liquidus temperature of 170°C or lower while simultaneously achieving excellent heat cycle resistance and drop impact resistance through the synergistic effects of the optimized composition.
Data Source
AI summary
A solder alloy includes 45 mass % or more and 63 mass % or less of Bi, 0.1 mass % or more and less than 0.7 mass % of Sb, 0.05 mass % or more and 1 mass % or less of In, and a balance including Sn.