Low-Silver Tin Solder Alloy for Thermal Cycling and Drop Shock
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Solution Overview
Problem
Existing lead-free solder alloys face challenges in achieving both good thermal cyclic reliability and drop/shock performance, with high silver content improving thermal reliability but compromising drop/shock performance, and low silver content showing the opposite, while also experiencing undercooling issues that affect mechanical performance.
Innovation Solution
A low-silver lead-free solder alloy composition comprising 2.4-2.6 wt.% silver, 0.5-0.8 wt.% copper, 1.5-3.2 wt.% bismuth, 0.03-0.05 wt.% cobalt, 0.03-0.07 wt.% antimony, and 0.03-0.07 wt.% nickel, with balance tin, which reduces undercooling temperature, process temperature, and improves thermal cycle and drop/shock reliability by refining the grain structure and enhancing mechanical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high silver content is used in lead-free solder alloy, then thermal cyclic reliability is improved, but drop/shock performance deteriorates
Solution Approach 1:
The patent optimizes the silver content parameter to a specific range (2.4-2.6 wt.%) rather than using high silver content, and introduces additional alloying elements (bismuth at 1.5-3.2 wt.%, cobalt at 0.03-0.05 wt.%, antimony at 0.03-0.07 wt.%) to achieve both improved thermal cyclic reliability and drop/shock performance through compositional parameter optimization
Solution Approach 2:
The patent creates a multi-element composite solder alloy system (Sn-Ag-Cu-Bi-Co-Sb-Ni) that combines multiple elements with complementary functions: tin as base metal, silver for thermal reliability, copper for strength, bismuth for melting point control, cobalt and antimony for grain refinement, and nickel for oxidation resistance, achieving synergistic effects that resolve the contradiction between thermal reliability and mechanical strength
2Strength
If low silver content is used in lead-free solder alloy, then drop/shock performance is improved, but thermal cyclic reliability deteriorates
Solution Approach 1:
The patent precisely controls the silver content parameter within 2.4-2.6 wt.% and balances it with other elements (copper: 0.5-0.8 wt.%, bismuth: 1.5-3.2 wt.%, cobalt: 0.03-0.05 wt.%, antimony: 0.03-0.07 wt.%, nickel: 0.03-0.07 wt.%) to achieve optimal combination of drop/shock performance and thermal cyclic reliability
Solution Approach 2:
The multi-element composite alloy system provides synergistic effects where cobalt and antimony refine grain structure to improve mechanical strength for drop/shock resistance, while bismuth adjusts melting characteristics, and nickel provides oxidation resistance, collectively maintaining thermal cyclic reliability without requiring high silver content
3Reliability
If standard SAC305 alloy composition is used, then thermal cyclic performance is maintained, but undercooling temperature is high and aging effects increase
Solution Approach 1:
The patent modifies the alloy composition parameters compared to SAC305 by reducing silver content (2.4-2.6 wt.% vs 3.0 wt.%) and adding bismuth (1.5-3.2 wt.%), cobalt (0.03-0.05 wt.%), antimony (0.03-0.07 wt.%), and nickel (0.03-0.07 wt.%), which collectively reduce undercooling temperature and minimize aging effects while preserving thermal cyclic performance
Solution Approach 2:
The enhanced composite alloy system (Sn-Ag-Cu-Bi-Co-Sb-Ni) extends the life of the solder joint by reducing aging effects through nickel's oxidation resistance and cobalt's grain refinement, while bismuth's presence modifies the solidification behavior to reduce undercooling, maintaining thermal cyclic performance equivalent to SAC305
Data Source
Figure 1A~1B
Figure 2A~2B
Figure 3
AI summary
A lead-free, tin based solder alloy comprises silver, copper, cobalt and optionally bismuth and antimony. The alloy may further comprise nickel. Silver is present in an amount from 2.0% to 2.8% by weight of the solder. Copper is present in an amount from 0.2% to 1.2% by weight of the solder. Bismuth may be present in an amount from 0.0% to 5.0% by weight of the solder. In some embodiments bismuth may be present in an amount from about 1.5% to 3.2% by weight of the solder. Cobalt is present in an amount from 0.001 % to 0.2% by weight of the solder. Antimony may be present in an amount between about 0.0% to about 0.09 % by weight of the solder. The balance of the solder is tin.