Soldering Paste vs Flux: What's the Difference?

Understanding the difference between solder paste and flux is essential for choosing the right materials for your soldering application. While they're related, they serve different purposes.

What is Flux?

Flux is a chemical cleaning agent used during soldering. Its primary functions are:

Remove oxides from metal surfaces
Prevent oxidation during heating
Improve solder wetting and flow
Reduce surface tension between solder and metal

Flux comes in various forms: liquid, paste, gel, and as a core inside solder wire.

What is Solder Paste?

Solder paste is a mixture of two components:

Tiny solder spheres (metal alloy particles, typically 25-45 microns)
Flux medium (provides the same benefits as standalone flux)

The paste consistency allows it to be deposited precisely using stencils or dispensers, then melted during reflow to form solder joints.

Key Differences

Property	Flux	Solder Paste
Contains solder	No	Yes
Primary use	Assist hand soldering	SMT reflow assembly
Application method	Brush, pen, spray	Stencil, dispenser
Additional solder needed	Yes	No
Storage requirements	Room temperature	Refrigerated
Shelf life	1-2 years	3-6 months

When to Use Flux

Hand Soldering: Apply flux before using solder wire to improve joint quality.

Rework and Repair: Clean and prepare surfaces for resoldering.

Desoldering: Helps remove components and clean pads.

Improving Difficult Joints: Additional flux aids wetting on oxidized or difficult surfaces.

Touch-up Work: Quick application with flux pens for minor corrections.

When to Use Solder Paste

SMT Assembly: The standard method for surface-mount production.

Reflow Soldering: Paste is applied, components placed, then reflowed in an oven.

BGA and Fine-Pitch: Essential for components where hand soldering isn't practical.

Prototyping SMD: Even hobbyists use paste with hot air or hot plates.

Solder Paste Composition

Typical solder paste contains:

Metal content: 85-92% by weight (SAC305, Sn63Pb37, etc.)
Flux content: 8-15% by weight
Particle size: Classified by Type (Type 3, 4, 5)

Finer particle sizes (higher type numbers) allow smaller feature printing.

Application Methods

Flux Application

Brush or bottle applicator for liquid flux
Flux pen for precision touch-up
Spray for coverage over larger areas
Syringe for controlled dispensing

Solder Paste Application

Stencil printing (production)
Syringe dispensing (prototype, repair)
Pneumatic dispensers (automated)

Storage and Handling

Flux: Most fluxes store at room temperature for 1-2 years. Keep containers closed to prevent drying.

Solder Paste: Requires refrigeration (typically 0-10°C). Short shelf life of 3-6 months. Must warm to room temperature before use.

Conclusion

Flux is a cleaning and wetting agent used alongside separate solder wire, primarily for hand soldering and rework. Solder paste combines flux with solder particles for SMT reflow assembly. Choose based on your soldering method: flux for hand work, solder paste for surface-mount reflow.