Lead-Free Solder Alloy Composition for Thermal Shock Reliability
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Solution Overview
Problem
Current lead-free solder alloys used in surface-mount technology lack sufficient ductility and thermal shock reliability while maintaining a stable melting point, which is essential for semiconductor devices, especially when the reflow process is performed at temperatures of 200° C. or less.
Innovation Solution
A lead-free solder alloy composition comprising bismuth (Bi) between 56 wt% and 57.5 wt%, indium (In) between 0.05 wt% and 1.0 wt%, and tin (Sn) with optional additions of germanium, phosphorus, or gallium, excluding silver and nickel, to achieve improved ductility and thermal shock reliability without significant melting point changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If lead-free solder alloy uses conventional compositions (e.g., Sn-58Bi), then melting point is maintained, but ductility and thermal shock reliability are insufficient
Solution Approach 1:
The patent applies parameter changes by precisely adjusting the chemical composition parameters of the solder alloy. Specifically, it limits Bi content to 56-57.5 wt%, In content to 0.05-1.0 wt%, and controls the In/Bi ratio between 1.0×10^-3 to 1.8×10^-2. This parameter optimization resolves the contradiction by achieving improved ductility and thermal shock reliability while maintaining melting point within 134-140°C.
Solution Approach 2:
The patent creates a composite material system by combining multiple elements (Sn, Bi, In, and optional Ge, P, Ga) in specific proportions. This composite approach allows the solder alloy to simultaneously achieve good ductility, thermal shock resistance, and stable melting point, resolving the technical contradiction that cannot be solved by single-element additions.
2Strength
If silver (Ag) is added to improve certain properties, then alloy performance may be enhanced, but thermal shock properties are degraded
Solution Approach 1:
The patent applies the extraction principle by explicitly excluding silver (Ag) and nickel (Ni) from the solder alloy composition. By removing these elements that would degrade thermal shock properties, the patent achieves improved thermal shock reliability while maintaining adequate ball shear strength through the optimized Sn-Bi-In system.
Solution Approach 2:
The patent converts the potential harm of excluding Ag (which might reduce strength enhancement) into a benefit by achieving superior thermal shock resistance. The exclusion of Ag, which would otherwise degrade thermal shock properties, is transformed into a positive outcome through the optimized combination of Bi and In that provides both strength and thermal shock reliability.
Data Source
AI summary
A lead-free solder alloy includes bismuth (Bi), content of which is equal to or greater than 56 wt % and equal to or less than 57.5 wt %, indium (In), content of which is equal to or greater than 0.05 wt % and equal to or less than 1.0 wt %, and the balance of tin (Sn) and another unavoidable impurity. The lead-free solder alloy of the disclosure may enable bonding with improved ductility and thermal shock reliability while not having a large melting point change compared to an Sn-58Bi alloy.