What is Flux for Soldering?

Flux is a chemical compound that enables successful soldering by cleaning metal surfaces and facilitating solder flow. Without flux, creating reliable solder joints would be nearly impossible.

The Science Behind Flux

Metal surfaces oxidize when exposed to air, forming a thin oxide layer. This oxide layer prevents solder from bonding to the base metal. Flux works through several mechanisms:

Chemical Cleaning: Flux contains acids or other reactive compounds that dissolve metal oxides when heated.

Oxide Prevention: While active, flux creates a protective barrier preventing new oxidation during the soldering process.

Surface Tension Reduction: Flux reduces the surface tension of molten solder, allowing it to spread and wet the joint surfaces properly.

Types of Flux

Rosin-Based Flux

Derived from pine tree sap, rosin flux is the most common type for electronics.

Variants:

• R (Rosin): Minimal activity, cleanest residue
• RMA (Rosin Mildly Activated): Moderate cleaning power, most common
• RA (Rosin Activated): Strongest cleaning, requires thorough post-solder cleaning

No-Clean Flux

Formulated to leave non-corrosive residue that doesn't require removal. Widely used in modern electronics manufacturing.

Water-Soluble Flux

Highly active flux that dissolves easily in water for cleaning. Must be removed as residue is corrosive.

Organic Acid Flux

Stronger than rosin, used for difficult-to-solder materials. Requires careful cleaning.

Flux Forms

Liquid Flux: Applied with brush, dropper, or spray. Good for pre-treating surfaces.

Flux Paste: Thick consistency that stays in place. Ideal for SMD work and rework.

Flux Pen: Convenient for touch-up and precision application.

Cored Solder: Flux contained inside the solder wire—releases automatically during soldering. Most convenient for hand soldering.

Flux Gel: Medium viscosity between liquid and paste.

How to Use Flux

1. Prepare surfaces: Clean parts of heavy contamination before applying flux
2. Apply flux: Use appropriate method based on flux form
3. Heat promptly: Flux is most effective when fresh; don't let it dry before soldering
4. Solder: Apply heat and solder while flux is active
5. Clean if required: Remove residue based on flux type and application requirements

When Flux is Essential

• Soldering oxidized or tarnished surfaces
• Joining dissimilar metals
• SMD and fine-pitch component work
• Desoldering and rework operations
• Any situation where solder doesn't flow properly

Common Mistakes

Too little flux: Solder won't flow properly, joints will be dull and weak.

Too much flux: Excessive residue, potential for flux spattering.

Wrong type: Using plumbing flux on electronics will cause corrosion.

Old flux: Flux loses effectiveness over time, especially once exposed to air.

No cleaning when required: Water-soluble flux left on boards will cause corrosion.

Conclusion

Flux is essential for creating reliable solder joints. It cleans surfaces, prevents oxidation, and helps solder flow. For most electronics work, rosin-based or no-clean flux is appropriate. Always match your flux type to your application and cleaning capabilities.