Tin Electroplating Uniformity via Surfactant Additives
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Solution Overview
Problem
Conventional tin electroplating methods face challenges in achieving uniformity and smooth surface morphology for lead-free solder deposits on copper pillars, leading to issues with void formation and reliability in solder joints.
Innovation Solution
A method involving an electroplating composition with a source of tin ions, an acid electrolyte, a grain refiner of specific formula, and nonionic surfactants is used to deposit a tin-containing layer on copper pillars, ensuring within-die uniformity of ≤ 5% and absence of voids, with a surface roughness of ≤ 200 nm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional solder electroplating baths are used, then lead-free solder can be deposited, but the deposits have relatively rough surface morphology (Ra ≥ 800 nm) and poor within-die uniformity (>30% WID)
Solution Approach 1:
The patent changes the chemical parameters of the plating bath by introducing specific additives: a sulfopropylated anionic surfactant (0.01-5 g/L) and a grain refiner (10-30 ppm). These parameter changes transform the deposition process, reducing surface roughness from ≥800 nm to ≤200 nm and improving within-die uniformity from >30% to <10% WID.
Solution Approach 2:
The patent uses surfactants and grain refiners as intermediary substances that mediate between the electrolyte and the substrate. The sulfopropylated anionic surfactant acts as a mediator to control nucleation and growth, while the grain refiner (such as benzylidene acetone) further refines the crystal structure, achieving smooth morphology and uniform deposits.
2Shape
If conventional brighteners and grain refiners are used in tin electroplating baths, then lustrous surfaces can be achieved, but solder bumps deposited therefrom suffer from poor morphology and very poor within-die uniformity
Solution Approach 1:
The patent extracts or removes conventional brighteners and grain refiners from the plating bath formulation. By eliminating these traditional additives that cause poor within-die uniformity, the patent creates a cleaner deposition environment that enables both lustrous surfaces and uniform morphology when combined with the new sulfopropylated anionic surfactant system.
3Quantity of substance
If small amounts of lead-free solder are deposited on copper pillars, then capping layer applications are achieved, but uniform height of solder composition is difficult to obtain both within a die and across the wafer
Solution Approach 1:
The patent achieves local quality control by using the sulfopropylated anionic surfactant and grain refiner to control nucleation and growth at the local level. This enables uniform height deposition even for small amounts of solder on copper pillars, with within-die uniformity improved from >30% to <10% WID, ensuring consistent capping layer quality across the entire wafer surface.
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 method achieves high within-die uniformity and smooth surface morphology, preventing void formation and enhancing solder joint reliability by ensuring uniform tin-containing layer deposition on copper pillars.
Implementation Method 1
The present invention provides an electroplating composition comprising: a source of tin ions; an acid electrolyte; 0.0001 to 0.075 g/L of a grain refiner
Implementation Method 2
applying sufficient current density to deposit a tin-containing layer on the conductive bonding features
Data Source
Figure 1
Figure 2
AI summary
Tin electroplating baths having certain brightening agents and nonionic surfactants provide tin-containing solder deposits having good morphology, reduced void formation and improved within-die uniformity.