The Purpose of Soldering Flux? A Simple Guide

Soldering relies on one essential material: soldering flux. Whether you fix broken wires, assemble electronics, or create intricate jewellery, understanding flux is key. You’ve probably seen flux in your soldering process, but do you know what it does and why it matters? Let’s take a closer look at this often-overlooked component.

What is Soldering Flux?

Soldering flux is a chemical compound that helps the solder to flow smoothly and adhere to the surfaces being joined. It plays a crucial role in strengthening the connection between materials and making it more reliable. Without flux, solder can’t bond properly to metal, which leads to weak joints and possible circuit failures.

Flux typically comes in the form of a gel, paste, or liquid, and it is used on metal parts that will be soldered together, such as copper wires or circuit board components.

How Does Flux Help in Soldering?

  1. Prevents Oxidation Oxidation forms when metal meets air, creating oxides that stop solder from sticking well. Flux breaks down these oxides and keeps metal from oxidizing during soldering.
  2. Improves Wetting of the Solder When you apply solder to a joint, the goal is for it to flow smoothly across the surfaces and create a strong bond. However, metal surfaces can often be a little resistant to solder adhesion. Flux improves the “wetting” properties of the metal, meaning the solder will spread and bond more effectively. This is especially important for creating reliable connections on delicate components like PCBs (Printed Circuit Boards).
  3. Cleans the Surface In addition to preventing oxidation, flux helps clean the metal surfaces by removing dirt, grease, and other contaminants. These contaminants can create a barrier that stops the solder from sticking, leading to weak connections. Flux effectively “cleans” the surfaces, ensuring a solid bond.
  4. Reduces Surface Tension Surface tension can cause solder to bead, rather than spread evenly. Flux lowers tension so solder flows smoothly, making precise joints easier. This is especially important for precision work, such as soldering small, intricate parts.
  5. Ensures Better Heat Transfer For the solder to melt properly and form a good joint, you need to apply heat evenly to the surfaces you join. Flux acts as a heat conductor, helping you transfer heat more efficiently. It also spreads the heat evenly across the components, reducing the chance of overheating and damage to sensitive parts.

Types of Soldering Flux

There are several types of flux, each suited for different applications. Here are the most common types you’ll encounter:

  1. Rosin Flux Rosin flux is one of the most commonly used flux types, particularly for electronics. It’s derived from pine sap and works well with most metals. Rosin flux is available in different grades (such as “rosin activated” or “no-clean”) and is generally safe to use, as it’s non-corrosive and leaves minimal residue.
  2. Acid Flux Acid flux is used primarily for soldering metals that have a higher tendency to oxidize, like brass or steel. However, acid flux is much more corrosive and can damage electronic components if not cleaned off thoroughly after soldering.
  3. Water-Soluble Flux As the name suggests, water-soluble flux can be easily cleaned up with water. It’s highly effective for cleaning and leaves no residue once removed. However, it requires more care during cleaning, as leftover flux can lead to corrosion if not properly washed off.
  4. No-Clean Flux No-clean flux is ideal for situations where cleaning the flux residue is either impractical or unnecessary. It leaves behind minimal residue, which is generally safe for most electronic components, and does not need to be cleaned off after soldering.

How to Use Flux in Soldering

Using flux correctly is easy once you understand the basics. Here’s a step-by-step guide to using soldering flux:

  1. Prepare the Surface: Start by cleaning the parts you’re going to solder. Ensure the metal is free from dirt, grease, or any oxidation.
  2. Apply the Flux: Using a brush or syringe, apply a thin layer of flux to the metal surfaces that you intend to solder. You don’t need a lot, just enough to cover the area where the solder will be applied.
  3. Heat the Joint: Heat the joint with a soldering iron until it’s hot enough to melt the solder. The flux will start to work by preventing oxidation and allowing the solder to flow smoothly.
  4. Apply the Solder: Once the joint is heated, apply the solder. The flux will help the solder to flow evenly and create a strong bond. If necessary, you can add more flux to ensure smooth flow.
  5. Clean Up: After soldering, especially with acid or water-soluble flux, make sure to clean the area to remove any excess flux residue. This is especially important for long-term durability and reliability.

Conclusion

Soldering flux is an essential tool in the soldering process. Whether you’re working on electronics, jewellery, or plumbing, flux ensures that your soldering job is smooth, efficient, and reliable. It prevents oxidation, enhances the flow of solder, cleans surfaces, and improves heat transfer. By understanding and using flux properly, you’ll be able to create strong, durable solder joints every time.

So next time you’re soldering, don’t forget about flux—it’s your secret weapon to achieving the perfect connection!

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Learn More!

Wikipedia – Flux
DKL Metals – Clearflow 7000

Frequently Asked Questions: Soldering Flux & Industrial Applications

What is the primary function of flux in the soldering process?

The primary function of soldering flux is to chemically clean metal surfaces by removing oxidisation. This ensures the molten solder can “wet” the surface effectively, creating a reliable metallurgical bond. In industrial manufacturing, using a high-quality flux is essential to prevent dry joints and ensure long-term conductivity.

Which soldering flux is best for lead-free electronics?

For lead-free soldering, which requires higher thermal profiles, we recommend active no-clean fluxes or water-soluble organic acid fluxes. Because lead-free alloys (like SAC305) have poorer wetting characteristics than traditional tin-lead, specialised formulations like the DKL Clearflow range are engineered to remain stable at higher temperatures.

What is the difference between no-clean and water-soluble flux?

No-clean flux leaves behind a non-conductive, non-corrosive residue that does not typically require removal, making it ideal for high-volume PCB assembly. Water-soluble flux is more aggressive and excellent for heavily oxidised components, but the residues must be washed off with deionised water to prevent circuit board corrosion over time.

Is DKL soldering flux RoHS and REACH compliant?

Yes, all modern DKL Metals flux products are fully RoHS and REACH compliant. We prioritise environmental safety and sustainable manufacturing, ensuring our fluxes are free from restricted hazardous substances, making them suitable for use in all UK and EU regulated industries.

How should I store soldering flux to ensure maximum shelf life?

To maintain performance, liquid flux and solder paste should be stored in a cool, dry place away from direct sunlight. Liquid fluxes typically have a shelf life of 12–24 months, while solder pastes often require refrigeration (between 2°C and 10°C) to prevent the flux and alloy from separating or drying out.

Do I need specialized flux for soldering aluminium or stainless steel?

Yes, standard electronics flux will not work on aluminium or stainless steel due to their tough, rapidly reforming oxide layers. You require a highly active, specialised inorganic acid flux designed specifically for these metals to achieve a successful bond.