Vacuum Reflow Soldering With Oxide Removal and No Flux Residue
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Solution Overview
Problem
The existing soldering method using residue-free solder paste results in flux scattering during reflow due to solvent components, and oxide films on solder or electrodes are not thoroughly removed, leading to potential solder scattering and incomplete bonding.
Innovation Solution
A soldering method involving the application of solder paste mixed with residue-free flux, followed by heating in a vacuum to volatilize the flux, then in a reducing atmosphere to remove oxide films, and finally in a vacuum at a higher temperature to melt the solder, preventing flux residue and oxide film issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If residue-free solder paste with solvent components is used, then cleaning step becomes unnecessary, but flux scatters during melting causing solder scattering
Solution Approach 1:
The patent divides the heating process into multiple stages with different atmospheric conditions: first pre-heating in vacuum to remove solvent, then main heating in reducing atmosphere for soldering. This segmentation prevents flux scattering by eliminating solvent before melting occurs.
Solution Approach 2:
The patent performs preliminary removal of solvent components from the solder paste during the pre-heating stage in vacuum atmosphere before the main soldering process. This preliminary action eliminates the cause of flux scattering before it can occur during melting.
2Productivity
If heating is performed in atmospheric pressure or nitrogen atmosphere, then soldering can be conducted, but oxide films remain on solder and electrode surfaces
Solution Approach 1:
The patent uses a reducing atmosphere (such as nitrogen or hydrogen) during the main heating stage to prevent oxide film formation on solder and electrode surfaces. This inert environment ensures reliable bonding by maintaining metal surfaces in a reduced state during soldering.
Solution Approach 2:
The patent changes the atmospheric parameters from vacuum (pre-heating) to reducing atmosphere (main heating) to achieve different objectives: solvent removal first, then oxide prevention during soldering. This parameter change optimizes both cleaning and soldering quality.
3Reliability
If multiple heating stages with different atmospheres are used, then flux scattering and oxide films are prevented, but process complexity increases
Solution Approach 1:
The patent implements a multi-functional heating process where the same heating device performs multiple functions: vacuum pre-heating for solvent removal and reducing atmosphere main heating for oxide prevention and soldering. This universal approach achieves multiple objectives without requiring separate equipment.
Solution Approach 2:
The patent maintains continuous heating action through the transition from pre-heating to main heating stages, with the atmosphere changing but the heating process continuing without interruption. This continuous useful action ensures consistent soldering quality while streamlining the process.
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
This method effectively prevents solder scattering and thoroughly removes oxide films, ensuring reliable bonding without flux residue and voids, even with challenging solder alloys like Sn-Sb or Bi-Sn, and allows for flexible solder paste design.
Implementation Method 1
a second step of volatilizing the residue-free flux contained in the solder paste by heating the circuit board in a furnace set to be a vacuum state
Implementation Method 2
a third step of removing an oxide film on at least the soldering portion by heating the circuit board in the furnace set to be a reducing atmospheric state
Implementation Method 3
a fourth step of melting solder contained in the solder paste by heating the circuit board in the furnace set to be a vacuum state and a third temperature which is higher than the second temperature
Data Source
Figure 1A~1C
Figure 1D~2
AI summary
A solder scattering is prevented at the time of reflow and the oxide films formed on the surfaces of solder or electrodes are thoroughly removed. The soldering method according to the present invention contains the steps of: applying solder paste to the electrode on a printed circuit board and mounting an electronic part on the solder paste, volatilizing the residue-free flux contained in the solder paste by heating the printed circuit board in a chamber set to be a vacuum state and approximately 180 degree C at the time of pre-heating (interval A), removing oxide films formed on the electrode and the like by heating the printed circuit board in the chamber set to be a formic acid atmospheric state and the temperature of approximately 200 degree C at the time of reducing (interval B), and melting solder powder contained in the solder paste by heating the printed circuit board in the chamber set to be a vacuum state and the temperature of 250 degree C at the time of main heating (interval C).