Non-Cyanide Au-Sn Alloy Plating via Tetravalent Tin Stabilization
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Solution Overview
Problem
Conventional cyanide-based Au—Sn alloy plating solutions face environmental toxicity issues and instability due to oxidation of divalent Sn compounds, leading to insoluble compound formation and disproportionation reactions, making it difficult to achieve stable non-cyanide based Au—Sn plating solutions suitable for industrial applications.
Innovation Solution
A non-cyanide based Au—Sn alloy plating solution is developed, comprising a non-cyanide soluble gold salt, a tetravalent Sn compound, and a thiocarboxylic acid-based compound, such as thioglycolic acid, which stabilizes tetravalent Sn and adjusts precipitation potential, along with sugar alcohols and dithioalkyl compounds to enhance stability and precipitation efficiency, maintaining a neutral pH and avoiding cyanide toxicity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a cyanide-based Au—Sn alloy plating solution is used, then good liquid stability and reliable Au—Sn alloy deposition are achieved, but environmental toxicity and cyanide safety issues occur
Solution Approach 1:
The invention changes the chemical composition parameters by replacing cyanide-based gold salts with non-cyanide alternatives (such as gold sulfite salts, gold thiosulfate salts, or chloroauric acid salts) while adjusting the auxiliary agent system to include sulfur-containing compounds like thiosulfate, sulfite, or dithiocarbamate. This parameter change eliminates cyanide toxicity while maintaining solution stability through the new complexing system.
2Object-affected harmful factors
If a non-cyanide soluble gold salt is used, then environmental safety is improved, but Au deposits by disproportionation reaction due to low stability
Solution Approach 1:
The invention introduces sulfur-containing auxiliary agents (thiosulfate, sulfite, dithiocarbamate, or their compounds) as intermediaries that form stable complexes with gold ions. These intermediaries prevent the disproportionation reaction of non-cyanide gold salts by controlling the redox potential and maintaining gold in a soluble ionic state, thereby ensuring solution stability without cyanide.
3Productivity
If a divalent Sn compound is used in cyanide-based plating solution, then Au—Sn alloy plating is achieved, but insoluble compounds form due to oxidation of Sn and deposition occurs
Solution Approach 1:
The invention performs preliminary protection of tin compounds by using tetravalent Sn compounds (such as stannic chloride, sodium stannate, or tin(IV) acetate) that are inherently more stable and resistant to oxidation. This preliminary stabilization prevents the formation of insoluble Sn oxides or hydroxides before plating occurs, maintaining solution stability throughout the plating process.
4Stability of the object's composition
If tetravalent Sn is used to avoid oxidation problems, then liquid stability improves, but the large difference in precipitation potentials between Au(I) and Sn(IV) makes it difficult to obtain constant eutectoid composition
Solution Approach 1:
The invention changes the chemical environment parameters by introducing sulfur-containing complexing agents that selectively interact with tetravalent Sn ions. These agents form stable Sn-S complexes that modify the precipitation potential of Sn, reducing the potential difference between Au and Sn. This enables simultaneous controlled deposition of both metals in the desired eutectoid ratio (such as Au-10wt%Sn or Au-20wt%Sn) while maintaining solution stability.
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 solution reduces environmental impact, stabilizes the plating liquid, and enables effective Au—Sn alloy plating on semiconductor wafers by controlling precipitation characteristics and maintaining solution stability over time, achieving high industrial practicality and efficient Au—Sn alloy deposition.
Implementation Method 1
a thiocarboxylic acid-based compound which is used as a complexing agent that stabilizes tetravalent Sn
Implementation Method 2
as a precipitation accelerating agent that changes a precipitation potential of tetravalent Sn to allow precipitation of an alloy with Au
Implementation Method 3
a method of using a Au—Sn alloy plating solution
Data Source
AI summary
The present invention provides a non-cyanide based Au—Sn alloy plating solution capable of performing a Au—Sn alloy plating treatment by a plating solution composition that is neutral and does not contain cyanide. In the present invention, a non-cyanide soluble gold salt, a Sn compound composed of tetravalent Sn, and a thiocarboxylic acid-based compound are contained. The non-cyanide based Au—Sn alloy plating solution of the present invention can further contain sugar alcohols, and, in addition, can further contain a dithioalkyl compound.