Water-Soluble Flux Composition for Lead-Free Soldering
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Solution Overview
Problem
Existing soldering fluxes fail to provide adequate joining capability for modern semiconductor packages, often resulting in non-wets, flux residue issues, and diffusion into substrate layers, especially with lead-free solders, leading to package failures and environmental concerns due to costly and harmful cleaning methods.
Innovation Solution
A flux composition comprising an activator, a medium-viscosity polymer solvent, and a high-viscosity copolymer solvent with specific monomers, which provides high tackiness and water solubility, preventing flux residue formation and allowing for effective cleaning with water, while maintaining stability above reflow temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing fluxes are used for soldering, then the soldering process can be performed, but non-wets occur at chip corners and flux residue remains on surfaces
Solution Approach 1:
The patent modifies the chemical composition parameters of the flux by incorporating specific activators (carboxylic acids with 2-10 carbon atoms), polymers (ethylene glycol derivatives with specified molecular weights), and additives to optimize the flux's chemical reactivity and physical properties, thereby achieving uniform wetting across the entire chip surface including corners
Solution Approach 2:
The flux comprises a composite formulation combining multiple components: carboxylic acid activators, polymeric solvents with controlled molecular weights, and optional additives. This composite structure enables the flux to simultaneously provide strong solder activation, uniform spreading, and complete residue solubility in water
2Ease of manufacture
If existing fluxes are used, then soldering can proceed, but flux residue cannot be cleaned with water and requires organic solvents
Solution Approach 1:
The patent designs the flux with water-soluble components by selecting polymers with hydroxyl groups (ethylene glycol, diethylene glycol, triethylene glycol) and carboxylic acid activators that form water-soluble salts with solder oxides. The molecular weight and chemical structure parameters are optimized to ensure complete solubility in water, eliminating the need for organic solvent cleaning
Solution Approach 2:
The flux is designed as a single-use consumable that leaves no persistent residue. The organic components are selected to be completely soluble in water, allowing the flux to perform its function and then be completely removed by water rinsing, eliminating the need for expensive and environmentally harmful organic solvent cleaning processes
3Reliability
If high tackiness is provided to prevent chip movement, then chip stability improves, but flux may diffuse into substrate outer layer
Solution Approach 1:
The patent carefully controls the molecular weight parameters of the polymeric components (ethylene glycol derivatives with MW 100-10,000) to achieve optimal tackiness. The molecular weight is high enough to provide strong adhesion and prevent chip movement during reflow, but low enough to prevent excessive diffusion into the substrate's porous solder mask layer
Solution Approach 2:
The flux formulation provides different functional properties at different stages: during application and reflow, the flux exhibits high tackiness to hold the chip in position; after cooling, the flux residue becomes water-soluble for easy removal. The polymer chain length and cross-linking density are optimized to provide localized adhesion at the chip-flux interface while preventing unwanted diffusion into the substrate
4Reliability
If flux provides adequate tackiness, then chip movement is prevented, but flux viscosity increases making application difficult
Solution Approach 1:
The patent optimizes the viscosity parameters by selecting polymers with specific molecular weights (100-10,000) and controlling the concentration of polymeric components in the flux formulation. This creates a viscosity window where the flux is thick enough to provide tackiness and chip holding capability, but thin enough to allow easy application by dispensing, spraying, or dipping methods
Solution Approach 2:
The flux exhibits dynamic viscosity characteristics where the viscosity changes with shear rate and temperature. At room temperature and low shear rates (during storage and application), the flux has higher viscosity providing tackiness. During application with mechanical agitation or heating, the viscosity decreases, facilitating easy dispensing and spreading
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 flux composition ensures reliable solder joining, prevents non-wets, and allows for efficient water-based cleaning, reducing environmental impact and improving the integrity of semiconductor packages.
Implementation Method 1
a medium-viscosity solvent being a polymer, and a high-viscosity solvent being a copolymer
Implementation Method 2
Solvent molecules of the flux, if they do not evaporate upon reflow due to their high boiling points
Implementation Method 3
the flux residue can be cleaned with water
Data Source
AI summary
The present invention is directed to a soldering method for joining objects is also provided, comprising the steps of applying a flux composition to at least a portion of one or more of the objects, and joining the objects.