Mixed-Alloy Lead-Free Solder Paste for Lower-Temperature Reflow
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Traditional lead-free Sn-rich solder alloys are not reliable enough for high-temperature electronic applications, as they degrade quickly under harsh thermal conditions, leading to a narrower process window and poor voiding performance.
Innovation Solution
A solder paste comprising two or more metal solder powders with a lower and higher melting temperature, where the higher melting temperature solder alloy includes Sn—Sb, Sn—Ag—Cu—Sb, or Sn—Ag—Cu—Sb—In, and the lower melting temperature solder alloy includes Sn—Ag—Cu or Sn—Ag—Cu—Bi, combined with flux, to reduce peak reflow temperature, widen the process window, and improve reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional binary or ternary lead-free Sn-rich solder alloys are used, then the solder paste has a simpler composition and lower cost, but the solder joint reliability deteriorates under high-temperature thermal cycling conditions
Solution Approach 1:
The patent uses composite solder alloy powders consisting of multiple phases (Sn-rich matrix with SnSb, Cu6Sn5, and other intermetallic compounds) to achieve high reliability under thermal cycling. The composite structure combines the benefits of different intermetallic phases, where SnSb provides thermal fatigue resistance while Cu6Sn5 contributes to strength, resolving the contradiction between reliability and compositional simplicity.
Solution Approach 2:
The patent optimizes the local distribution and volume fractions of specific intermetallic compounds within the solder joint. By controlling the volume fraction of SnSb (5-15%) and Cu6Sn5 (20-35%) phases in specific regions, the solder joint achieves enhanced thermal fatigue resistance and reliability without requiring complex overall composition.
2Reliability
If Sb is added to improve thermal fatigue resistance, then the solder joint reliability improves, but the peak reflow temperature increases and process window narrows
Solution Approach 1:
The patent optimizes the Sb content parameter within a specific range (5-15 wt%) to achieve the desired balance between thermal fatigue resistance and reflow temperature. By precisely controlling the Sb concentration and its distribution as SnSb intermetallic phase, the solder paste maintains reliable processing at manageable temperatures while achieving improved thermal fatigue performance.
Solution Approach 2:
The patent creates a composite solder system where Sb is combined with other elements (Cu, Ag, In, Bi) to form a multi-phase alloy system. This composite approach allows the SnSb phase to provide thermal fatigue resistance while other phases (such as Cu6Sn5, SnIn, or SnBi) help control the overall melting range and reflow temperature, thus resolving the temperature contradiction.
3Reliability
If Sb content is increased to optimize SnSb intermetallic volume fraction, then thermal fatigue resistance improves, but the solder paste viscosity increases and fillability deteriorates
Solution Approach 1:
The patent optimizes the particle size distribution and volume fraction of SnSb intermetallic phase within specific ranges to maintain adequate flux penetration and wetting during printing while achieving the desired thermal fatigue resistance. By controlling the SnSb particle characteristics rather than simply increasing overall Sb content, the patent balances reliability improvement with processing ease.
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 mixed solder paste reduces voiding, maintains or improves reliability, and allows for reflow at lower temperatures, enhancing the thermal stability and durability of solder joints in high-temperature environments.
Implementation Method 1
one of the solder powders can have a lower melting temperature than the other, comparable to or slightly lower than the melting temperature of traditional SnAgCu solder alloys
Implementation Method 2
Sb plays a key role in improving the thermal fatigue resistance of solder joints in harsh thermal cycling or thermal shock conditions
Data Source
AI summary
Some implementations of the disclosure describe a solder paste consisting essentially of: 10 wt % to 90 wt % of a first solder alloy powder, the first solder alloy powder consisting of a Sn—Sb alloy, a Sn—Ag—Cu—Sb alloy, a Sn—Ag—Cu—Sb—In alloy, a Sn—Ag—Cu—Sb—Bi alloy, or Sn—Ag—Cu—Sb—Bi—In alloy; 10 wt % to 90 wt % of a second solder alloy powder, the second solder alloy powder consisting of an Sn—Ag—Cu alloy or Sn—Ag—Cu—Bi alloy, and the second solder alloy powder having a lower solidus temperature than the first solder alloy powder; and flux.