Sometimes considered an accessory element, flux actually performs an important chemical and physical function: preparing metal surfaces for joining, protecting them from oxidation during heating and ensuring proper adhesion of the filler metal.
Consequently, understanding how flux acts, the difference compared to solder paste, and why correct application is important means laying the foundations for a reliable, repeatable, and defect-free brazing process.
We discuss this in this in-depth look, also presenting SEIT Elettronica’s solutions for controlled and effective automatic fluxing.
Brazing Flux: What It Is and How Does It Works
The term fluxing indicates the application of a chemical substance called flux onto the metal surfaces to be joined. This application precedes the brazing process
The role of fluxing
Its action has three functions:
- Removing oxides and surface impurities: During storage and handling, metals develop surface oxides or contamination from oils, dust, and processing residues—impurities that prevent proper interaction between the base metal and filler metal, compromising the quality of the joint. Flux dissolves and removes these oxides, restoring the surface to ideal conditions for brazing.
- Protecting surfaces during heating: During the thermal cycle, the increase in temperature accelerates the formation of new oxides even on previously cleaned surfaces. Flux creates temporary protection that limits this phenomenon until the joining is complete, keeping the surfaces reactive and clean throughout the process.
- Promoting the flow of filler metal: To obtain a uniform and continuous joint, the filler metal must distribute correctly across the surface without retreating or creating discontinuities. Flux reduces surface interference, facilitates capillarity (also referred to as “wettability”), and improves alloy diffusion, directly contributing to the quality and repeatability of the brazing bead.
Flux is applicable to various brazing technologies, from traditional brazing to induction brazing.
Difference between Flux and Solder Paste
One of the most frequent questions in brazing and brazing processes concerns the difference between flux and solder paste. The two terms are often used interchangeably, but they actually indicate distinct elements with different functions within the process.
Flux is a chemical substance applied before brazing, with a preparatory and protective function: it removes oxides and contamination, prevents new oxidation, and creates optimal conditions for the filler metal to distribute uniformly.
Solder paste, on the other hand, intervenes during the brazing process: once heated, it directly contributes to the joint by keeping the components to be welded in a stable position.
In SEIT automated and modular systems, fluxing and brazing operations can be controlled and integrated flexibly and effectively within the production cycle.
What is the best flux?
The question “What is the best flux?” does not have a single answer. The choice depends on a series of process variables that must be evaluated case by case:
- The type of material to be joined: Copper, brass, stainless steel, and special alloys develop oxides with different chemical characteristics; the flux must be compatible with the nature of the treated material.
- The process temperature: Every flux has an optimal activation range; outside of these temperatures, its effectiveness is significantly reduced.
- The heating technology used: The heating method—flame, induction, or furnace—affects flux behavior, as heat transfer times and methods vary considerably between processes.
In an automated industrial context, the flux must guarantee stability, uniformity, and repeatability cycle after cycle.
Why Correct Flux Application is Important
While the chemical function of the flux is crucial, so is the way it’s applied. Uneven distribution, too much, or too little product can compromise the entire joint.
How Flux Affects Brazing Quality
The quality of a weld or braze joint does not depend exclusively on the alloy or the temperature reached, but also on the correct management of the flux.
When flux is applied uniformly and in the appropriate quantity, it ensures that the filler metal can flow correctly, adhering continuously without interruptions. This translates into a stable joint without internal porosity or imperfections.
Good management creates effective protection during heating and avoids the formation of residual oxides that could generate points of structural weakness. Conversely, imprecise management can generate unprotected zones and discontinuities in the joint.
Common Errors in Fluxing
In industrial flow, errors are not always immediately visible but can have significant effects on the final result.
- Excessive flux dosage can leave residues that are difficult to remove, with both aesthetic and functional implications. Aesthetically, streaks, stains, or surface deposits may appear, requiring additional cleaning or compromising the part’s finish. Functionally, however, unremoved residues can promote corrosion or interfere with subsequent processes, such as painting, assembly, or marking.
- Insufficient quantity: Does not guarantee adequate protection, favoring the formation of oxides and adhesion defects.
- Non-uniform distribution: Causes the quality of the joint to vary even within the same production batch.
- Incorrect drying times: Residual moisture or solvents can react uncontrollably during heating, altering the behavior of the filler metal.
Finally, improperly managing drying times before brazing can compromise the stability of the thermal cycle. Residual moisture or incompletely evaporated solvents can react uncontrollably during heating, altering the behavior of the filler metal and the regularity of the joint.
In highly repetitive production, these critical issues can impact entire batches, directly impacting scrap, rework, and process costs.
Manual vs Automatic application of brazing flux
Manual application may be sufficient in limited production runs or during prototyping.
However, in structured industrial contexts with high volumes, the variability introduced by the operator affects the quality and repeatability of the joint in a way that is difficult to control.
Inaccurate fluxing due to insufficient quantities, uneven distribution, or failure to respect drying times can lead to even serious defects in the joint: internal porosity, poor wetting of the filler metal, cold joints, or localized oxidation that reduces the mechanical strength and sealing of the component.
The automation of fluxing, on the other hand, allows for precise control of all critical parameters: immersion times and uniformity, quantity of product applied, and drying conditions.
A key aspect is the control of the flux film thickness; unlike manual application, an automatic fluxing system like Autoflux by SEIT Elettronica guarantees a uniform and repeatable film cycle after cycle.
SEIT Automatic Fluxing Systems: Autoflux
Autoflux is designed for the controlled application of flux onto metal components. The system involves immersion in a flux tank with controlled movements followed by a hot air drying phase to ensure uniformity of treatment and optimization of cycle times.
Main features include:
- Digital control of all functions
Precisely manages parameters like immersion times and drying phrases to reduce variabilit and gurantee constant qualities from one production lot to another. - PLC control on programs
The Programmable Logic Controller (PLC) allows for storing and recalling personalized fluxing programs specific to each component. This feature ensures cycle repeatability and facilitates switching between different production runs without the need for manual reconfigurations. - Efficient drying process
The hot air drying phase is designed to ensure controlled stabilization of the flux before brazing, avoiding residual moisture or solvents that could compromise the quality of the joint during heating. The system integrates a heat recovery unit that reintroduces hot air into the drying cycle, increasing the system’s energy efficiency and reducing consumption. - Compliance with safety standards (ATEX)
Autoflux is designed to comply with ATEX directives, for the safe management of potentially flammable or volatile substances and to ensure high standards of protection for operators and the production environment. - Controlled component movement
The automated basket movement system ensures uniform and repeatable immersions, avoiding differences in flux exposure between pieces and contributing to homogeneous coverage of the treated surfaces.
Fluxing with Autoflux therefore occurs under controlled and repeatable conditions, reducing the variability typical of manual applications. Furthermore, automated cycle management increases productivity while maintaining high quality standards.
It is important to underline that Autoflux works in a complementary way to the subsequent brazing phases: the automatic fluxing of the body of the piece does not exclude the possibility of applying further flux to the specific joint area during brazing, but significantly reduces the quantity required.
In fact, the component arrives at the brazing station already protected over its entire surface, and the integration of additional flux can be limited to the joint area only, with advantages in terms of process cleanliness, reduction of residues and aesthetic quality of the finished component.
Our approach isn’t limited to a single fluxing step. In more complex production processes, fluxing can be integrated into modular platforms like Platinum Hub or Platinum TT, which can be configured to create multiple processing stations for fluxing, applying solder paste or preformed alloys, brazing, cooling, and other process operations.
In this way, fluxing fits effectively and efficiently into an integrated and highly customizable production flow.
Optimize Fluxing to Improve the Entire Process
Proper management of soldering flux or brazing flux, as we’ve seen, isn’t a secondary detail, but an important element in ensuring quality, efficiency, and reducing rework costs. Investing in automatic fluxing solutions means controlling a critical phase of the process and enhancing the value of the entire production chain.
Do you want to optimize the flow within your production lines or are you looking for a customized solution? Contact SEIT Elettronica Team for a dedicated consultation.
