What is welding flux?
Welding flux is a granular welding material that, when melted during welding, forms slag and gas. This substance plays a crucial role in protecting the molten metal and aiding metallurgical treatment.
The flux is usually a mixture composed mainly of rosin, an auxiliary material that ensures the smooth running of the welding process. Soldering is an important process in electronic assembly, and flux is an auxiliary material used during soldering.
The main function of the flux is to remove oxides from the weld and the surface of the base material to be welded, achieving the necessary surface cleaning.
Prevents surface reoxidation during welding, reduces weld surface tension and improves welding performance. The quality of the flow directly affects the quality of electronic products.
Flow Composition
Welding flux is made up of a mixture of minerals, including marble, quartz, fluorite and others, as well as chemicals such as titanium dioxide and cellulose.
The main application of welding flux is in submerged arc welding and electroslag welding.
In recent decades, in the welding process of electronic product production, rosin rosin flux mainly composed of rosin, rosin, halide-containing activators, additives and organic solvents is often used.
Although this type of flux has good solderability and low cost, it has high post-welding waste. These residues contain halide ions, which gradually cause problems such as decreased electrical insulation performance and short circuits.
To solve this problem, it is necessary to clean the residue rosin flux on the electronic printed board, which not only increases production costs, but also the solvent used to clean the residue rosin flux is mainly composed of fluorochlore. This compound is a substance that depletes the atmospheric ozone layer and is currently banned and gradually eliminated.
However, for various reasons, many companies still use the welding process with rosin resin flux and subsequent cleaning with fluorochlore cleaner, which has low efficiency and high cost, in addition to causing serious environmental pollution.
The no-clean flux, most used on the market and of highest quality, is composed of: organic solvents, natural resins and their derivatives, synthetic resin surfactants, organic acid activators, anticorrosive agents, co-solvents, and film-forming agents .
Simply put, it is a homogeneous transparent mixed solution formed by dissolving several solid components in several liquids, where each component has different proportions and functions.
Organic solvents:
A mixture of one or several types of ketones, alcohols, esters, commonly used ones include ethanol, propanol, butanol; acetone, toluene isobutyl ketone; ethyl acetate, butyl acetate, etc.
As a liquid component, its main function is to dissolve the solid components of the flux to form a homogeneous solution, making it easier to weld the components to evenly coat an adequate amount of flux components, at the same time it can clean light dirt and oil on the metal surface .
| Constituent Components | Primary function | |
| Volatile Components | Solvent | Regulation of viscosity and dispersion of solid components |
| Solid Composition | Resin | Primary ingredients, catalytic welding functions |
| Dispersant | Separation prevention, flow characteristics | |
| Activator | Deoxidation | |
Natural Resin and its Derivatives or Synthetic Resin Surfactants:
Halogen-containing surfactants have high activity and weldability, but because halogen ions are difficult to clean, ion residues are high, and halogen elements (mainly chlorides) have strong corrosive properties, they are not suitable for use as raw materials. Press for unclean products. flow.
Halogen-free surfactants have slightly weaker activity but fewer ionic residues. Surfactants are mainly non-ionic surfactants from the fatty acid family or aromatic family. Its main function is to reduce the surface tension generated when solder comes into contact with lead metal, increase the surface wetting force, increase the penetration of organic acid activators, and also play a role as a foaming agent.
Organic Acid Activator:
Composed of one or several types of dicarboxylic acids or aromatic acids, such as succinic acid, glutaric acid, itaconic acid, salicylic acid, fumaric acid, heptanoic acid, malic acid, succinic acid, etc., its main function is to remove oxides from the foot lead and the surface of molten solder, and is one of the main components of flux.
Anticorrosive Agent:
Reduces residues of solid components such as resins and activators after decomposition at high temperatures.
Co-solvent:
Prevents the tendency for solid components, such as activators, to desorb from the solution, avoiding poor uniform distribution of activators.
Film forming agent:
During the lead soldering process, the applied flux precipitates and crystallizes to form a uniform film. The residues after high-temperature decomposition can be quickly solidified, hardened, and have reduced viscosity due to the presence of film-forming agent.
Flow working principle
The working principle of flow is easy to understand. In short: during the entire welding process, the flux removes the oxide layer from the surface of the welding material through the action of its own active substances.
At the same time, it reduces the surface tension between the tin liquid and the soldering material, improving the flow and wetting properties of the tin liquid, thus helping to complete the soldering. Hence the name “flow”.
To fully analyze the working principle of flux, it involves using the flux activators to clean the oxides on the surface of the welding material, allowing the welding alloy to bond well with the welding material and form a weld spot. Substances that play an important role in this process are flux activators, which can quickly remove oxides from solder pads and component pins, and sometimes also protect the soldering material from further oxidation before soldering is completed. .
Furthermore, by removing the oxide film, the surfactants in the flux also begin to act. They can significantly reduce the surface tension of liquid solder on the surface of the soldering material, increase the fluidity and spreading ability of liquid solder, and ensure that the tin solder can penetrate every small brazing gap.
In the tin furnace welding process, at the moment the welded body leaves the surface of the tin liquid, due to the wetting action of the flux, the excess tin solder will flow along the pin, thus avoiding bad phenomena such as spikes welding and bridges. .
Functions of welding flux
Flow Functions:
(1) Remove oxides from the welding surface, reduce the melting point and surface tension of the solder, and reach the brazing temperature as quickly as possible.
(2) Protect the weld metal in liquid state from being affected by harmful gases in the surrounding atmosphere.
(3) Allow the liquid solder to have an appropriate flow rate to fill the brazing seam.
(4) Destroy the metal oxide film to clean the solder surface, which leads to solder wetting and the generation of solder joint alloys.
(5) It can cover the weld surface, preventing the weld or metal from further oxidizing.
(6) Increases the activity of the weld and the surface of the metal to be welded, reducing the surface tension of the weld.
(7) The solder and flux are fused, which can increase the fluidity of the solder and further improve the wettability.
(8) It can accelerate the heat transfer from the soldering iron head to the surface of the solder and the welded object.
(9) Appropriate flux can also improve the appearance of the solder joint.
Functions of flux in submerged arc welding:
(1) . Mechanical protection: Under the action of the electric arc, the flux melts into the surface slag, protecting the weld metal in liquid state from the intrusion of gases into the surrounding atmosphere, thus preventing gas inclusions in the weld seam.
(two) . Transfer the necessary metal elements to the melting pool.
(3) . Promote a smooth and straight weld seam surface with good formation. The melting point of flux should be 10-30 ℃ lower than the melting point of solder; Under special circumstances, the melting point of flux may be higher than that of solder.
If the melting point of the flux is too low compared to the solder, the flux will melt prematurely, causing the flux composition to lose activity when the solder melts due to evaporation and interaction with the parent material.
The choice of flux generally depends on the nature of the oxide film. Alkaline oxide films, such as Fe, Ni, Cu, etc., generally use acidic fluxes containing boron anhydride (B2O3), acidic oxide films, for example, to deal with cast iron containing high SiO2 oxide film, generally use alkaline flow of Na2CO3 to form easily melted Na2SiO3 and enter the slag. Some fluoride gases are also often used as fluxes, react evenly and leave no residue after welding.
BF3 is often mixed with N2 for brazing stainless steel at high temperatures. The fluxes used for brazing below 450 ℃ are soft fluxes, which are divided into two types, one is water-soluble, generally composed of simple or mixed saline solutions of hydrochloric acid and phosphoric acid, they have high activity and strong corrosiveness, and need be cleaned after welding.
The other is a water-insoluble organic flux, usually based on rosin or synthetic resin, with the addition of organic acid, organic amine or its salt HCl or HBr to increase film removal capacity and activity.
Conditions that commonly use flow
(1) The melting point must be lower than that of solder.
(2) The surface tension, viscosity and density should be lower than those of the weld.
(3) It should not corrode the original material and should increase the fluidity of the weld and remove the oxide film on the metal surface at the welding temperature.
(4) Flux residue is easy to remove.
(5) It should not produce toxic gases and odors to avoid harm to the human body and environmental pollution.
Types of Welding Flux
There are several ways to classify welding flux, including its use, manufacturing method, chemical composition, metallurgical properties during welding, and pH and particle size of the flux.
Regardless of the classification method used, it only highlights some aspects of the welding flux and does not fully cover all of its characteristics.
Common classification methods include:
1. Neutral welding flux
A neutral welding flux is one that does not significantly change the chemical composition of the weld metal or welding wire after welding.
This type of flux is often used for multi-pass welding, especially when the base metal is thicker than 25 mm.
The characteristics of a neutral welding flux are as follows:
The. The flux contains little or no oxides such as SiO 2 MnO and FeO.
B. Flux does not cause oxidation in the weld metal.
w. Welding a seriously oxidized base metal can result in porosity and cracks in the weld bead.
2. Active welding flux
An active welding flux is one that contains a small amount of deoxidizing agents such as Mn and Si. This type of flux can improve resistance to porosity and cracking.
Following are the characteristics of an active welding flux:
The. The presence of deoxidizers such as Mn and Si can cause changes in the chemical composition of the deposited metal as the arc voltage fluctuates. An increase in Mn and Si can increase the strength of the deposited metal, but decrease its impact resistance. Therefore, it is important to closely control the arc voltage during multipass welding.
B. Active welding flux has strong ability to prevent porosity.
3. Alloy welding flux
An alloy welding flux contains additional alloy components that serve as transition elements. Most alloy welding fluxes are sintered.
This type of flux is mainly used for welding low-alloy steel and for wear-resistant coatings.
4. Fusion welding flux
Fusion welding flux is produced by combining raw materials of various minerals in a specific proportion, heating them to more than 1300℃, melting and mixing them completely, and then cooling them in water to form granules .
The process continues with drying, crushing, sieving and packaging for use.
In China, a common brand of fusion welding flux is “HJ”. The first digit after the “HJ” designation indicates the MnO content, the second digit represents the SiO 2 and CaF 2 content, and the third digit distinguishes between different brands of the same type of welding flux.
5. Sintered welding flux
After dosing the ingredients, the dry mix is carried out and a binder (glass of water) is added to the wet mix. The mixture is then granulated.
It is then sent to a drying oven for curing and drying and finally sintered at approximately 500 degrees.
In China, a common brand of sintered welding flux is represented by “SJ”. The first digit after the “SJ” designation indicates the slag system, while the second and third digits distinguish between different brands of the same slag system flow type.
6. Other classification methods
Flux types can be broadly divided into organic, inorganic and resin series.
Resin flux is usually extracted from tree secretions. It is a natural product with little corrosiveness. Rosin is a representative of this type of flux, which is why it is also called rosin flux.
Since flux is typically used in combination with solder, it can be divided into soft flux and hard flux corresponding to solder.
Commonly used in the assembly and maintenance of electronic products are rosin, rosin mixed flux, solder paste and hydrochloric acid and other soft fluxes. They should be selected according to welding different parts in different occasions.
Welding Flow Control
- Flow drying control and heat preservation
Before using flux, it must first be cooked according to flux instructions. This drying standard is obtained through testing and process inspection control and is correct data with quality assurance. It is an enterprise standard and different companies have different requirements.
In the following, refer to the flux drying temperature and retention time recommended by JB4709-2000 “Welding Procedure for Steel Pressure Vessels”. Generally, when drying the flux, the pile height does not exceed 5cm. The welding material library generally dries more rather than less at once and prefers thickness to thickness in stacking. Strict management must be applied in this regard to ensure the quality of stream drying.
Avoid excessive stacking thickness and ensure complete drying of the flow by prolonging drying time.
- On-site flow management control and recycling disposal
The welding area should be clean and debris should not be mixed with the flux, including the flux used for filling the flux should be issued in accordance with regulations, preferably maintained at around 50 ℃, timely recycling of the flux to avoid contamination . The repeatedly used flux uses 8 and 40 mesh sieves to sift and remove impurities and fine powder, and then mixed with three times the new flux for use.
Before use, it must be dried at 250-350°C and kept warm for 2 hours. After drying, it is kept in an incubator at 100-150 ℃ for the next use, and outdoor storage is prohibited. In complex field conditions or high relative humidity, timely management of the site must be carried out, kept clean, carried out necessary tests for moisture resistance of flow and mechanical mixtures, control the rate of moisture absorption and mechanical inclusions, avoid disorderly stacking and mixing flow.
- Flow particle size and distribution requirements
Flux has certain particle size requirements, the particle size must be appropriate, so that the flux has a certain permeability, the welding process does not emit continuous arc light, and avoids air pollution in the molten pool to form pores. Fluxes are generally divided into two types, one with common particle size of 2.5-0.45mm (8-40 mesh) and the other with fine particle size of 1.43-0.28mm (10-mesh). 60).
Fine powder smaller than the specified particle size is generally no more than 5%, and coarse powder larger than the specified particle size is generally no more than 2%. Tests and control of the particle size distribution of the flux must be carried out to determine the welding current used.
- Control of flow particle size and pile dispersion height
If the flux layer is too thin or too thick, it will cause holes, spots and pores on the weld surface, forming an irregular shape of the weld path. The thickness of the flux layer should be strictly controlled in the range of 25-40 mm. When using sintered flux, due to its low density, the flux stack height is 20%-50% higher than that of molten flux. The larger the diameter of the welding wire, the greater the welding current and, consequently, the greater the thickness of the flux layer.
Due to the non-standard operation of the welding process and the unfair treatment of fine powder flux, there will be intermittent irregular pits on the surface of the weld, the non-destructive test is qualified, but the appearance quality is affected, and the shell thickness is locally weakened.
How to choose the right flow ?
It is impossible for users to test the composition of the flow. If you want to know whether the solvent in the flux evaporates, simply measure it from the specific gravity. If the specific gravity increases too much, you can determine that the solvent has evaporated.
When choosing the flow, there are some suggestions for users:
(1) Smell the odor to preliminarily determine what type of solvent is used. The smell of methanol is relatively small but pungent, the smell of isopropanol is a little heavier, and ethanol has a fragrant smell.
Although the supplier may also use a mixed solvent, the supplier is usually willing to provide a composition report upon request.
However, the price of isopropanol is about 3 to 4 times higher than methanol, so if you pressure the supplier for a lower price, the quality may be questionable.
(2) Determine the sample, this is also the most fundamental method for many companies to choose the flow. When confirming the sample, the supplier should be asked to provide the relevant parameter report and compare it with the sample.
If the sample is confirmed as OK, the subsequent delivery must be compared with the original parameters. If any abnormality appears, check the specific gravity, acidity value, etc.
(3) The current flow market is mixed. When choosing, you must have a clear understanding of the supplier's qualifications.
Welding Flux FAQ
- What is a flux in welding?
Flux in welding is a material used to promote, facilitate and protect the fusion of metals during the welding process. It is used to prevent the formation of oxides and other unwanted byproducts that can form due to heat. Flux can be in the form of a liquid, paste or solid material and helps create a better, cleaner and stronger weld.
- Do you need flux for electromagnetic welding?
Yes, flux is essential in electromagnetic welding. The electrode used in stick welding, often called a “stick,” is covered with flux. As the stick depletes, the flux creates a gas shield around the weld area, protecting the molten metal from the surrounding air, which can lead to contamination and weak welds if not protected.
- Is flux welding as strong as MIG?
The strength of a weld is determined primarily by the skill of the welder and the preparation of the materials, rather than the type of welding. That being said, MIG (Metal Inert Gas) welding tends to produce cleaner and sometimes stronger welds than flux core welding due to the use of a shielding gas. However, flux welding is more versatile and works better on thicker, dirtier, and rustier materials.
- What is flux welding best for?
Flux welding is best used in situations where it is difficult to control environmental conditions, such as outdoors or in drafty conditions. This is because the flux creates a protective barrier that shields the weld from atmospheric gases. It is also great for welding thicker, rougher materials and when welding out of position. Additionally, it is generally more economical and easier to learn than other types of welding.
