Zn Alloy Bonding Layer for Lead-Free Soldering
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Solution Overview
Problem
Conventional Pb-free soldering structures using Sn-based solders exhibit weak bonding strength, brittleness, and the formation of inter-metallic compounds (IMCs) that lead to voids and cracks, reducing the reliability of electronic joints.
Innovation Solution
A soldering structure and method incorporating a Zn alloy bonding layer, which reacts with lead-free solders to inhibit the growth of Ag3Sn platelets and Cu3Sn phase, thereby enhancing bonding strength and preventing void formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Cu bonding layer is employed with Pb-free solder, then the soldering structure can be fabricated without lead, but the bonding strength is weak and the bonding interface becomes brittle
Solution Approach 1:
The patent changes the material composition parameter of the bonding layer from pure Cu to a Cu-Zn alloy system with specific Zn content (5-30 at%). This compositional parameter change fundamentally alters the intermetallic compound formation behavior, preventing the creation of brittle Cu3Sn phase while maintaining soldering reliability without lead
Solution Approach 2:
The patent employs a composite bonding layer structure consisting of Cu-Zn alloy with controlled composition. The composite nature of the alloy (combining Cu and Zn in specific ratios) creates a unique interface chemistry that prevents harmful IMC formation while enhancing bonding strength, resolving the contradiction between reliability and strength
2Object-affected harmful factors
If Pb-free solder is used, then the environmental safety is improved, but Ag3Sn platelets and Cu3Sn phase are generated at the interface causing brittleness
Solution Approach 1:
The patent introduces Zn as an intermediary element in the bonding layer that mediates the interaction between Cu substrate and Sn-based solder. The Zn forms a protective intermediate zone that controls the diffusion process, preventing direct formation of harmful Ag3Sn platelets and Cu3Sn phase at the solder-interface boundary while maintaining environmental safety of Pb-free solder
Solution Approach 2:
By changing the chemical composition parameter of the bonding layer (adding Zn to Cu), the patent alters the thermodynamic and kinetic parameters of the diffusion process. This parameter change suppresses the formation of unstable Ag3Sn platelets and controls IMC formation, achieving stable interface composition with Pb-free solder
3Strength
If a thicker IMC layer is formed, then the bonding interface is more robust, but voids and pores are generated reducing soldering strength
Solution Approach 1:
The patent changes the compositional parameter of the bonding layer (Cu-Zn ratio) to control the IMC formation kinetics. This parameter change results in the formation of a thinner, more uniform IMC layer without voids, as the Zn modifies the diffusion rates and phase transformation behavior during soldering, achieving both strength and reliability
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 Zn alloy bonding layer improves the reliability of soldering joints by reducing the growth of Ag3Sn platelets and Cu3Sn phase, minimizing voids and cracks, and maintaining soldering strength, resulting in a more solid and durable electronic joint.
Implementation Method 1
Since the thick IMC of the conventional soldering structure generates voids or pores in the internal interface, the soldering strength deteriorates. In specific, as the solid state diffusion takes place in the generation of the Cu3Sn phase, voids or pores may be generated in the interface.
Implementation Method 2
After a reflow process is carried out in the conventional soldering process, the Sn-based solder undergoes an undercooling during the cooling process. The undercooling of the Sn solder causes a formation and separation of Ag3Sn particles
Implementation Method 3
The undercooling of the Sn solder causes a formation and separation of Ag3Sn particles, which flow into the undercooled molten Sn. Accordingly, the Pb-free solder is irregularly constituted. The Ag3Sn particles rapidly grow in the molten Sn and form Ag3Sn platelets.
Data Source
AI summary
A soldering structure using Zn includes a bonding layer which contains Zn; and a lead-free solder which bonds and reacts to the bonding layer. The bonding layer can be a Zn alloy layer or a multilayer including a Zn layer. Accordingly, the characteristics of the soldering structure can be improved by involving the high reactive Zn to the interfacial reaction of the soldering.