What is Solder flux and why is it needed for soldering?
With the rapid advancement of modern industrial technology, welding has become a crucial method for joining materials and is widely used across various industries. Flux, an indispensable auxiliary material in the welding process, has grown increasingly significant. This article will provide a detailed introduction to the concept, classification, functions, and applications of flux. The specific content is as follows:
What is flux?
Flux is a chemical substance used in the welding process to aid and facilitate the welding process while providing protection and preventing oxidation reactions. It is typically a mixture with rosin as its main component, but it may also contain various other chemicals such as activators, solvents, surfactants, corrosion inhibitors, antioxidants, and film formers. These components work together to ensure the smooth progress of the welding process.
Main components of flux:
- Rosin: As one of the primary components of flux, rosin plays a critical role in the welding process.
- Activators: Such as hydrogen, inorganic salts, organic acids, and organic halides, primarily function to remove oxides from the surfaces of solder pads and solder at welding temperatures and form a protective layer to prevent further oxidation of the substrate.
- Solvents: Dissolve the components within the flux, acting as carriers for each component, creating a uniform, viscous liquid.
- Surfactants: Reduce the surface tension of the flux, enhancing its wetting ability on the solder powder and pads.
- Corrosion inhibitors: Like benzotriazole (BTA), inhibit the corrosive effects of activators on metals such as copper in the flux.
- Antioxidants: Such as phenolic compounds, prevent the oxidation of solder during the welding process.
- Film formers: Made from substances like hydrocarbons, alcohols, and esters, these become active at welding temperatures and form a protective film.
Types of Flux
There are three main types of flux:
Inorganic Acid Flux
This type of flux is highly effective for handling stronger metals such as brass, copper, and stainless steel. When using inorganic acid flux, thorough cleaning is essential afterward. Any corrosive residues left on the surface can compromise the weld joint. If cleaned properly, the result is a perfect connection.
Organic Acid Flux
Also known as water-soluble flux, this type is made by mixing organic acids such as citric acid, lactic acid, and stearic acid with solvents like isopropanol and water. Organic acid flux quickly removes oxides, making it superior to rosin flux in this regard. It also ensures good welding with high flux activity. However, any residual flux must be removed after welding; otherwise, its conductivity can impair the performance of the PCB.
Rosin Flux
Rosin flux is one of the oldest types, primarily composed of natural resins derived from pine trees. Modern formulations mix natural rosin with other flux agents to optimize its performance. Rosin flux is versatile—it’s excellent for removing oxides and foreign substances from metal surfaces. While liquid rosin flux is acidic, it becomes inert upon cooling, meaning it won’t damage your circuit board unless reheated to a liquid state. For added safety, it’s recommended to remove any rosin flux residue, especially if you plan to apply a conformal coating or are concerned about the PCB’s appearance.
What Are the Functions of Flux?
Flux offers the following advantages:
- To improve bonding at the joint: It ensures that the two areas you want to join are firmly bonded together.
- To clean the surface before welding: This is especially important for areas that are difficult to clean, such as metals that are oxidized or contaminated with oil.
- To enhance the wettability of the solder: It ensures that the solder flows evenly across the joint, which is essential for a strong and reliable electrical connection.
- To prevent oxidation: It prevents the formation of oxides, which can weaken the bond between surfaces.
Why Is Flux Used in Welding?
Removal of Oxides: When metal surfaces come into contact with air, an oxide film can easily form, preventing solder from wetting and spreading on the metal surface. Upon heating, flux releases surfactants that react with the ionized oxides on the metal surface, removing the oxide layer and ensuring smooth welding.
Enhancing Wettability: The surfactants in flux significantly reduce the surface tension of liquid solder on the metal surface, enhancing the flowability of the liquid solder. This ensures the solder can penetrate every fine solder joint, improving the overall quality of the weld.
Preventing Reoxidation: At high temperatures, metal surfaces oxidize more rapidly. Flux covers the high-temperature solder and the metal surface, isolating them from air, thereby preventing the weld from oxidizing further at high temperatures and protecting the welded area from oxidative damage.
Aiding Heat Conduction: Flux spreads over the surfaces of the solder and the metal being welded, helping to distribute heat evenly across the entire welding area, which contributes to a more stable welding process.
Improving Appearance of Welds: Flux helps to shape the solder joints and maintain the gloss of the joint surface, preventing defects such as sharp points or bridging, thereby enhancing the aesthetic and reliability of the weld.
Flux plays a crucial role in welding: it not only improves welding conditions and quality but also protects the welded surface from oxidative damage, making the welds more aesthetically pleasing and reliable. Therefore, the use of flux in welding is essential.
Steps for Manually Using Flux
Select the Appropriate Flux: Choose the right flux based on the welding material, welding method, and process requirements. The choice of flux should consider factors such as its deoxidizing ability, stability, corrosiveness to the welding materials, and environmental impact.
Prepare Tools and Materials: In addition to flux, prepare solder wire, a soldering iron, solder pads, tweezers, desoldering braid, cleaning agents, and cleaning cloths. Ensure that all tools and materials are in good condition and meet welding requirements.
Clean the Solder Pads: Before applying flux, clean the solder pads to remove any oxides and dirt. This can be done using a cleaning agent and a cleaning cloth.
Apply the Flux: Use an appropriate tool (such as a brush, dropper, or spray) to apply the flux evenly to the solder pads. Be careful not to pour too much flux at once to avoid waste and ensure welding quality. Replenish the flux as needed based on the amount of production.
Preheat the Soldering Iron: Preheat the soldering iron to the appropriate temperature, typically determined by the welding material and flux requirements.
Place the Solder Wire: Position the solder wire near the component leads or solder points on the solder pads, ready for welding.
Weld: Gently touch the preheated soldering iron tip to the solder wire and solder pad, allowing the solder wire to melt and wet the solder pad and component leads. During the welding process, the flux will act to deoxidize, reduce surface tension, and protect the welded area.
Check the Welding Quality: After welding, inspect the quality of the solder joints to ensure they are secure and free from defects such as cold solder joints, dry joints, or solder bridges.
Clean the Solder Pads: After welding, clean the solder pads with a cleaning agent and cloth to remove any excess solder and flux residues.
Summary
With the rapid development of smart manufacturing and the industrial internet, the production and application of flux will become more intelligent and automated, enhancing production efficiency and product quality. As an essential auxiliary material in welding processes, the importance and application prospects of flux cannot be overlooked. In the future, with continuous technological advancements and market expansion, flux will play a vital role in more fields, contributing significantly to industrial development.
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