Mixed-Alloy Lead-Free Solder Paste for Low-Temperature Reflow Reliability
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Solution Overview
Problem
Existing lead-free solders used in electronics assembly face challenges such as low melting temperatures that fail to meet board-level reliability requirements, brittleness, and the need for multiple reflow steps with different temperature solders, which can cause remelting issues.
Innovation Solution
A lead-free mixed solder paste composed of two solder powder alloys with significantly different solidus temperatures, allowing for reflow at a peak temperature below the higher solidus temperature but above the lower melting temperature, where the higher melting temperature alloy dissolves into the lower melting temperature alloy during reflow, forming a homogeneous liquid solution with embedded reinforcing colonies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If low melting temperature solder alloys (e.g., eutectic Sn-In, eutectic Bi-Sn) are used to enable lower reflow temperatures, then the reflow temperature can be reduced, but the solder joints become softer or more brittle and cannot survive board-level reliability requirements at 125°C
Solution Approach 1:
The patent uses composite solder powder consisting of two distinct alloy systems: a low melting temperature alloy (e.g., Sn-In, Sn-Bi, or Sn-Zn) and a high melting temperature alloy (e.g., SnAgCu). This composite structure allows the solder joint to melt at low temperature during reflow while the high melting alloy particles remain solid and provide structural reinforcement, enabling the joint to survive subsequent thermal stress at 125°C without remelting or losing mechanical strength.
2Temperature
If eutectic Sn-In alloy is used to achieve low melting temperature, then the reflow temperature can be reduced to around 118°C, but the solder becomes much softer than traditional SnAgCu alloys
Solution Approach 1:
The patent combines soft, low-melting eutectic Sn-In alloy particles with harder, high-melting SnAgCu alloy particles in a composite powder structure. During reflow, the Sn-In particles melt and provide easy wetting and flow, while the solid SnAgCu particles remain as reinforcing colonies that maintain joint strength and prevent excessive softening.
3Temperature
If eutectic Bi-Sn alloy is used to achieve low melting temperature, then the reflow temperature can be reduced to around 139°C, but the solder becomes brittle due to high Bi content
Solution Approach 1:
The patent combines brittle, low-melting eutectic Bi-Sn alloy particles with ductile, high-melting SnAgCu alloy particles. During reflow, the Bi-Sn particles melt and provide low-temperature flow, while the solid SnAgCu particles act as ductile reinforcement colonies that prevent brittleness and improve overall joint ductility.
4Adaptability or versatility
If multiple reflow steps with different temperature solders are used for SIP designs, then component interconnection can be achieved, but the risk of solder remelting increases during subsequent reflows
Solution Approach 1:
The patent segments the solder powder into two functional components with distinct melting behaviors: low-melting particles that enable initial wetting and bonding at lower temperatures, and high-melting particles that remain solid during subsequent reflows to prevent remelting. This segmentation allows a single solder paste to function across multiple soldering stages without remelting issues.
Solution Approach 2:
The composite solder powder combines low-melting and high-melting alloy particles, creating a material that exhibits both low-temperature wetting characteristics and high-temperature stability. This enables multi-stage soldering processes where the first reflow creates strong bonds using the melted low-melting particles, while subsequent reflows occur below the solidus temperature of the high-melting particles, preventing remelting.
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 solder joint achieves improved mechanical performance with a higher liquidus and solidus temperature, eliminating the need for interfacial intermetallic compounds and reducing the risk of weak interfaces, while maintaining strength and ductility.
Implementation Method 1
the higher solidus temperature solder powder may be dissolved into the molten lower solidus temperature solder powder to form a homogeneous liquid solution
Implementation Method 2
reflow at a peak temperature below the solidus temperature of the higher solidus temperature solder powder but above the melting temperature of the lower one
Data Source
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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 44wt% and 83wt%, the first solder alloy powder comprising Sn; an amount of a second solder alloy powder between 5wt% to 44wt%, 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.