Solder flux and solder paste are two different materials used in soldering processes, each serving a specific purpose in electronics assembly. Here are the critical differences between them:

  1. Composition of Solder Flux and Solder Paste
    • Solder Flux: It is a chemical compound or mixture typically in liquid or gel form. It contains active ingredients designed to clean and prepare the surface of the soldered components. Flux helps remove oxides, contaminants, and other impurities from the metal surfaces, promoting proper wetting and adhesion of the solder.
    • Solder Paste: On the other hand, is a mixture of solder alloy powder and flux. It combines solder material and flux in a single substance, typically semi-solid. This material contains tiny solder particles suspended in a flux medium.
  2. Function of Solder Flux and Solder Paste
    • Solder Flux: The primary function is to clean and prepare the metal surfaces for soldering. It helps prevent oxidation, promotes good solder wetting, and enhances solder joint quality. Flux can be applied separately to the components or integrated into soldering wire or paste.
    • Solder Paste: Paste serves as the solder material and the flux. It attaches electronic components (surface-mount devices or SMDs) to printed circuit boards (PCBs). When heated during the reflow soldering process, the material melts, and the flux within it cleans the surfaces and facilitates solder bonding between the components and the PCB.
  3. Application of Solder Flux and Solder Paste
    • Solder Flux: Flux is applied separately to the soldering joints or components before soldering. It can be in the form of liquid, gel, or even flux pens for precision application. There are different types of flux, including rosin, water-soluble, no-clean, and more, each suited to specific soldering applications.
    • Solder Paste: Paste is applied directly onto the PCB’s solder pads using a stencil or dispenser. It is commonly used in surface-mount technology (SMT) and reflow soldering processes for mounting SMD components. During the reflow process, the solder in the paste reflows and forms solder joints between the components and the PCB.

In summary, soldering flux is primarily used to clean and prepare soldering surfaces. In contrast, holding paste combines solder material and flux and attaches components to PCBs, especially in SMT processes. The choice between flux and paste depends on the specific soldering application and the type of components being used.

Solder Flux Residue Around Solder Joint
Residue from Flux Material is Most Often Cleaned