Understanding Electronics Soldering Flux/Paste: Types, Applications

In the world of electronics manufacturing and repair, soldering is a foundational process and soldering flux/paste is a critical component for ensuring durable, high-quality joints. Whether you're working in a commercial environment or undertaking technical repairs, selecting the appropriate flux/paste can make all the difference.

In this article, we’ll explore the different types of soldering flux/paste used in electronics, their applications, how flux compares to solder paste, and important considerations like matching flux to your soldering process. 

See our range of QUALITEK Fluxes and Paste HERE

What Is Soldering Flux?

Soldering flux is a chemical compound applied to metal surfaces prior to and during soldering. Its primary functions include:

• Removing oxides from metal surfaces

• Preventing further oxidation during soldering

• Improving wetting to help solder flow more easily

• Enhancing mechanical and electrical integrity of solder joints

By keeping surfaces clean and oxidation-free, flux helps produce stronger, more reliable solder joints—critical in all electronic assemblies.

Types of Soldering Flux for Electronics

1. Rosin-Based Flux (Natural Resin)

Derived from refined pine resin, rosin-based flux is ideal for general electronics assembly and repair.

• Form: Wire core, liquid, or paste

• Residue: Non-corrosive but sticky—typically cleaned with isopropyl alcohol

• Use Case: Manual soldering, through-hole components, legacy repairs

2. No-Clean Flux

A low-residue option designed for efficiency, especially in high-volume production.

• Form: Wire, liquid, or pen

• Residue: Minimal—usually safe to leave on the board

• Use Case: Surface mount technology (SMT), automated soldering

3. Water-Soluble Flux (Organic Acid-Based)

A highly active flux for critical applications where absolute joint reliability is required.

• Form: Liquid or paste

• Residue: Must be cleaned thoroughly with deionised water

• Use Case: High-reliability industries such as medical, aerospace, and automotive electronics

Matching Flux to Your Soldering Process

When selecting a flux, it’s important to consider whether your process involves leaded or lead-free (unleaded) solder, as this impacts the thermal profile and flux activity required. Lead-free soldering operates at higher temperatures, increasing oxidation. Therefore, fluxes designed for lead-free applications are more active, promoting strong wetting and clean joints in tougher conditions. In contrast, leaded solder, which melts at lower temperatures, typically pairs well with standard rosin or no-clean fluxes that require less aggressive activity. Brands like Qualitek offer flux solutions optimised for both environments, helping to ensure soldering reliability across all processes.

Choosing the Right Flux for Your Project

Flux vs. Solder Paste: What's the Difference?

While flux and solder paste are both essential in electronics soldering, they serve different roles—and are used in different stages of the process.

🔹 Solder Flux

• Contains no metal content

• Used to clean and prepare metal surfaces for soldering

• Applied separately in wire soldering or rework processes

• Available in liquid, gel, or pen form

🔸 Solder Paste

• A mixture of flux and powdered solder alloy

• Used primarily in surface mount (SMT) and reflow soldering

• Allows precise application via stencil printing

• Melts during the reflow process, forming the joint while the flux cleans the surface

Essentially, solder paste combines the cleaning action of flux with the metal needed to form the joint, making it ideal for mass production environments.

Qualitek Solder Paste – Performance and Precision

Qualitek offers a full range of high-performance solder pastes designed for modern SMT and reflow processes. Their pastes are available in both leaded and lead-free formulations, including:

• SAC305 (Lead-Free) – Popular for RoHS-compliant production

• Sn63/Pb37 (Leaded) – Eutectic alloy for consistent flow and minimal voiding

• Low-Residue and No-Clean Paste Options – Minimises post-process cleaning

• Available in multiple particle sizes – Type 3, 4, 5 depending on application needs

Each formulation is engineered to deliver:

• Excellent stencil release

• Stable viscosity and long shelf life

• Consistent solder volume and wetting

• Minimal solder balling or voids

Qualitek’s solder pastes work seamlessly with their flux technologies, ensuring smooth integration into automated workflows.

Trusted UK Supplier: Grove Sales Ltd

For UK-based professionals and businesses, Grove Sales Ltd is the go-to authorised distributor for Qualitek soldering products. Their extensive offering includes:

• Solder wire, liquid fluxes, gels, pens, and solder paste

• Lead-free and leaded products for all process types

• Expert advice on product compatibility and best practices

• Fast, reliable delivery across the UK

• ISO-certified customer support

Explore the full range at www.grovesales.co.uk or contact the team directly for tailored recommendations.

Final Thoughts

Soldering flux is essential for achieving reliable, high-quality joints in any electronics application. Whether you're working with hand soldering, SMT rework, or automated reflow, selecting the right flux—or solder paste—is crucial for long-term performance.

By sourcing high-quality materials from trusted brands like Qualitek, and working with experienced UK suppliers like Grove Sales Ltd, you’ll ensure your soldering process is efficient, compliant, and built for success.

See our range of QUALITEK Fluxes and Paste HERE