Lead-Free Solder Alloy Composition for Stable Multi-Reflow Interconnects
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Solution Overview
Problem
Existing lead-free solder alloys face reliability issues due to repetitive melting and freezing cycles, leading to reduced product life and compromised interconnect quality, particularly in multilevel electronics assembly processes.
Innovation Solution
A lead-free solder alloy with a melting point range of 225-260°C, comprising specific alloying elements like copper, nickel, and germanium, which enhances viscosity and mechanical properties, reducing die tilt and improving thermo-mechanical reliability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional Pb-free solder alloys (SnAgCu, Sn-0.7Cu) are used in multilevel assembly processes, then the solder joints can be formed initially, but the interconnect quality degrades during repetitive melting and freezing cycles, leading to reduced product life and reliability
Solution Approach 1:
The patent applies parameter changes by precisely controlling the composition ranges of alloying elements (Cu: 1-9 wt.%, Ni: 0.1-1 wt.%, Mn: 0.05-0.5 wt.%, P: 0.05-0.1 wt.%) to achieve optimal melting characteristics and intermetallic compound formation, thereby maintaining interconnect quality through multiple thermal cycles
Solution Approach 2:
The patent uses composite materials by combining multiple alloying elements (Cu, Ni, Mn, P) with tin to create a complex solder alloy system that forms specific intermetallic compounds (Cu6Sn5, Cu3Sn, Ni3Sn4, Mn3Sn, Mn2Sn, MnSn2) which collectively improve reliability during repetitive melting and freezing cycles
2Temperature
If high-temperature Pb-free solder (80Au20Sn) is used to replace high-Pb solders, then the melting temperature requirement is met, but the material becomes expensive and forms high-stress interconnects due to high modulus and brittleness
Solution Approach 1:
The patent replaces expensive AuSn solder with a cost-effective tin-based alloy system using common, inexpensive elements (Cu, Ni, Mn, P) that can be easily manufactured and processed, achieving comparable or superior performance without the high cost and brittleness of gold-based solders
Solution Approach 2:
The patent adjusts the melting temperature parameter by optimizing the composition of Cu (1-9 wt.%), Ni (0.1-1 wt.%), Mn (0.05-0.5 wt.%), and P (0.05-0.1 wt.%) in the tin-based alloy to achieve the required melting point range while maintaining cost-effectiveness and manufacturability
3Ease of manufacture
If Sn-0.7Cu solder is used for first level interconnect, then the initial joint formation is achieved, but die tilt or movement occurs during secondary reflow processing
Solution Approach 1:
The patent uses composite materials by adding Ni (0.1-1 wt.%) and Mn (0.05-0.5 wt.%) to the Sn-Cu base alloy, creating intermetallic compounds (Ni3Sn4, Mn3Sn, Mn2Sn, MnSn2) that enhance die attachment strength and prevent die tilt or movement during secondary reflow processing while maintaining ease of initial joint formation
Solution Approach 2:
The patent modifies the composition parameters by increasing Cu content to 1-9 wt.% (higher than conventional 0.7%) and adding Ni and Mn elements to change the melting characteristics and intermetallic compound formation, thereby improving die positioning stability during secondary reflow while maintaining manufacturability
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 alloy minimizes die tilt and void formation, enhances shear strength, and improves thermo-cycling behavior compared to conventional alloys, ensuring stable interconnects across multiple reflow cycles.
Implementation Method 1
Solders go through a quasi-physical and mostly reversible phase transition during processing
Implementation Method 2
comprising specific alloying elements like copper, nickel, and germanium, which enhances viscosity and mechanical properties
Data Source
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AI summary
A lead-free solder alloy comprising: from 1 to 9 wt.% copper, at least one of: from greater than 0 to 1 wt.% nickel, from greater than 0 to 10 wt.% germanium, from greater than 0 to 1 wt.% manganese, from greater than 0 to 10 wt.% aluminium, from greater than 0 to 10 wt.% silicon, from greater than 0 to 9 wt.% bismuth, from greater than 0 to 5 wt.% indium, from greater than 0 to 1 wt.% titanium, from greater than 0 to 2 wt.% lanthanum, from greater than 0 to 2 wt.% neodymium, optionally one or more of: up to 1 wt.% chromium, up to 1 wt.% gallium, up to 1 wt.% cobalt, up to 1 wt.% iron, up to 1 wt.% phosphorous, up to 1 wt.% gold, up to 1 wt.% tellurium, up to 1 wt.% selenium, up to 1 wt.% calcium, up to 1 wt.% vanadium, up to 1 wt.% molybdenum, up to 1 wt.% platinum, up to 1 wt.% magnesium, up to 5 wt.% silver, up to 1 wt.% zinc, up to 2 wt.% rare earth metals, excluding lanthanum and neodymium, and the balance tin together with any unavoidable impurities.