Mixed Lead-Free Solder Paste for Low-Temp Reflow Reliability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current low-temperature solders used in electronics, such as eutectic Sn—In and Bi—Sn alloys, are either too soft or brittle, and their low melting temperatures fail to meet board-level reliability requirements, particularly at temperatures above 125° C, while traditional SnAgCu solders have melting temperatures that are too high for subsequent reflow stages in miniaturized system-in-package designs.
Innovation Solution
A lead-free mixed solder powder paste is developed, comprising two solder alloys with significantly different solidus temperatures, where the lower melting temperature alloy melts first and reacts with surface metallization, while the higher melting temperature alloy dissolves into it, forming a homogeneous liquid solution that increases the liquidus and solidus temperatures of the solder joint, enhancing mechanical performance without relying on intermetallic compound formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If eutectic Sn-In or Bi-Sn alloys are used for low temperature soldering, then the melting temperature is reduced below 140°C, but the solder becomes too soft or brittle and cannot meet board-level reliability requirements at temperatures above 125°C
Solution Approach 1:
The patent uses a composite solder alloy system comprising multiple phases with different melting points. The alloy contains a low-melting eutectic phase (Sn-In or Bi-Sn) and a high-melting Sn-rich phase, creating a composite material that exhibits both low initial melting temperature and high service temperature reliability. The coexistence of different phases allows the solder to melt at low temperature for joining while maintaining structural integrity at elevated service temperatures.
Solution Approach 2:
The patent modifies the compositional parameters of the solder alloy by precisely controlling the ratios of Sn, In, Bi, Ag, and Cu elements. By adjusting these compositional parameters, the alloy achieves a specific phase diagram characteristics where a eutectic phase coexists with a high-melting Sn-rich phase, enabling the solder to have both low melting point (below 140°C) and high service temperature capability (above 125°C).
2Temperature
If traditional SnAgCu solder is used for first stage soldering, then the melting temperature is around 217°C, but this temperature is too high for subsequent reflow stages in multi-stage soldering processes
Solution Approach 1:
The patent segments the melting process into distinct stages by incorporating multiple phases with different melting points. The solder alloy contains a low-melting eutectic phase that melts first (below 140°C) and a high-melting Sn-rich phase that remains solid or melts at higher temperatures. This segmentation enables multi-stage soldering where different phases melt at different times, allowing subsequent reflow operations without remelting previously formed joints.
Solution Approach 2:
The patent creates local quality differences within the solder alloy by distributing different phases throughout the material. The low-melting eutectic phase and high-melting Sn-rich phase are locally distributed, allowing different regions of the solder to exhibit different melting behaviors. This local differentiation enables the solder to provide low-temperature flow for joining while maintaining high-temperature stability for subsequent processing stages.
3Temperature
If intermetallic compound formation is used to increase remelting temperature in TLPB technology, then the liquidus temperature increases, but the low melting phase is completely consumed and interfacial IMC growth compromises joint integrity
Solution Approach 1:
The patent extracts the harmful effect of complete phase consumption by designing an alloy where the low-melting eutectic phase is present in controlled amounts that allow it to melt and flow for joining, while the high-melting Sn-rich phase remains to provide structural support. Unlike TLPB where the low-melting phase is completely consumed, this invention retains portions of the low-melting phase to maintain joint integrity and prevent excessive IMC growth.
Solution Approach 2:
The patent provides beforehand cushioning against excessive IMC growth and phase consumption by incorporating the high-melting Sn-rich phase that acts as a reservoir. This high-melting phase prevents the complete consumption of the low-melting phase and cushions against the formation of excessive intermetallic compounds, thereby maintaining joint integrity throughout the soldering and service life.
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 achieves reliable solder joints with improved mechanical performance by maintaining a seamless interface and embedding higher melting phase colonies within the matrix, achieving a liquidus temperature higher than the peak reflow temperature and meeting board-level reliability requirements without forming interfacial intermetallic compounds.
Implementation Method 1
the lower melting temperature solder alloy melts first, and may spread onto the surface to wet and react with surface metallization to form interface IMCs
Implementation Method 2
The higher melting temperature solder alloy (e.g., SnAg, SnCu, SnAgCu, SnSb, SnAgSb, SnAgCuSb, SnAgCuBi, SnAgCuBiSb, etc.) may continuously dissolve into the molten solder since both the lower melting temperature and higher melting temperature solder alloys are composed of significant amounts of Sn
Implementation Method 3
the lower melting temperature solder alloy melts first, and may spread onto the surface to wet and react with surface metallization
Data Source
AI summary
Implementations of the disclosure are directed to a lead-free mixed solder powder paste suitable for low temperature to middle temperature soldering applications. The lead-free solder paste may consist of: an amount of a first solder alloy powder between 44 wt % and 83 wt %, the first solder alloy powder comprising Sn; an amount of a second solder alloy powder between 5 wt % to 44 wt %, the second alloy powder comprising Sn, where the first solder alloy powder has a liquidus temperature lower than a solidus temperature of the second solder alloy powder; and a remainder of flux. The solder paste may be used for reflow at a peak temperature below the solidus temperature of the higher solidus temperature solder powder but above the melting temperature of the lower solidus temperature one.