Lead-Free Solder Alloy Composition for Ni Leaching and Void Suppression
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Solution Overview
Problem
Existing lead-free Sn—Ag—Cu solder alloys used in electronic devices face challenges in suppressing Ni leaching and void generation at bonded interfaces, particularly in high-integration semiconductor packages like QFP and BGA, necessitating improved alloy design for enhanced tensile strength and impact resistance.
Innovation Solution
A lead-free and antimony-free solder alloy composition comprising specific balances of Ag, Cu, Ni, Co, Ge, and optionally Bi, with controlled ratios to suppress Ni leaching and void generation, achieved by optimizing the content of these elements to enhance tensile strength and prevent Ni diffusion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Ni layer is used as outermost layer on back metal to improve wettability and adhesive strength, then wettability with solder is improved, but Ni leaching occurs when Ni layer contacts molten solder
Solution Approach 1:
A barrier layer is introduced between the Ni layer and molten solder to prevent direct contact. The barrier layer acts as an intermediary that blocks Ni diffusion into the solder while allowing the Ni layer to maintain its wettability function. This resolves the contradiction by separating the conflicting requirements of Ni layer exposure for wettability and Ni layer protection against leaching.
Solution Approach 2:
The barrier layer is formed in advance on the back metal before applying the Ni layer, creating a protective structure that prevents Ni leaching before it can occur. This preliminary protective action ensures that when solder is applied, Ni atoms cannot diffuse into the molten solder, thus preventing Ni leaching while maintaining adhesive strength.
2Object-generated harmful factors
If barrier layer such as Ti is formed on back metal to suppress Ni diffusion, then Ni leaching is suppressed, but Ti layer repels molten solder without wetting when exposed
Solution Approach 1:
The back metal structure is designed with different layers having different functions: the Ti barrier layer provides Ni diffusion suppression at the interface, while the outer Ni layer provides wettability with solder. This local differentiation of material properties and functions allows each layer to optimize its specific role without compromising the other.
Solution Approach 2:
The Ni layer serves as an intermediary between the Ti barrier layer and the solder. It allows the Ti layer to perform its barrier function while the Ni layer maintains contact with solder for wettability. This intermediary structure resolves the contradiction by preventing direct exposure of Ti to solder.
3Adaptability or versatility
If conventional Sn—Ag—Cu solder alloy is used, then widespread compatibility is achieved, but heat cycle resistance and impact resistance are insufficient
Solution Approach 1:
The solder alloy is designed as a composite material containing multiple elements (Sn, Ag, Cu, Ni, Co, Ge, Bi) in specific proportions. This composite composition combines the benefits of conventional Sn-Ag-Cu alloys with additional elements that enhance heat cycle resistance, impact resistance, and wettability, while maintaining compatibility with existing electronic components and processes.
Solution Approach 2:
The chemical composition parameters of the solder alloy are optimized by adjusting the proportions of various elements. Specific ranges of Ag (1.0-4.0%), Cu (0.1-1.0%), Ni (0.005-0.3%), Co (0.003-0.1%), Ge (0.001-0.015%), and Bi (0.1-9.0%) are established to achieve improved reliability while maintaining adaptability. This parameter optimization resolves the contradiction between conventional compatibility and enhanced performance.
4Object-generated harmful factors
If Ni content is increased to suppress Ni leaching, then Ni diffusion is reduced, but alloy composition complexity increases
Solution Approach 1:
Instead of simply increasing Ni content, the alloy composition is optimized by adjusting the proportions of multiple elements (Ag, Cu, Ni, Co, Ge, Bi) within specific ranges. This multi-parameter optimization achieves Ni leaching suppression through synergistic effects rather than relying on high Ni content alone, thus managing composition complexity while improving performance.
Solution Approach 2:
The solder alloy uses a composite multi-element composition where each element contributes specific functions. The controlled presence of Ni (0.005-0.3%) combined with other elements creates a synergistic system that suppresses Ni leaching through the overall alloy structure rather than through high Ni concentration, managing complexity through functional distribution.
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 optimized solder alloy effectively suppresses Ni leaching and void generation, ensuring high tensile strength and improved bonding integrity in semiconductor packages, suitable for both BGA and QFP applications.
Implementation Method 1
a barrier layer such as Ti is usually formed on the back metal to suppress Ni from diffusing into the silicon chip
Implementation Method 2
the molten solder wets the microelectrode, so that solder bumps are formed on the microelectrodes
Data Source
AI summary
Provided are a solder alloy and a solder joint which have high tensile strength, can suppress Ni leaching and can suppress generation of voids at a bonded interface. The solder alloy has an alloy composition that includes, by mass %, Ag: 1.0 to 4.0%, Cu: 0.1 to 1.0%, Ni: 0.005 to 0.3%, Co: 0.003 to 0.1%, Ge: 0.001 to 0.015%, optionally one or more of Mn, Pd, Au, Pt, Cr, V, Mo, and Nb each with an upper limit of 0.01% by mass %, and the balance being Sn. The alloy composition satisfies the following relation (1):0.0003<(Ni/Co)×(1/Ag)×Ge<0.05(1)Ni, Co, Ag, and Ge in the relation (1) each represent the contents (mass %) in the alloy composition.