Soldering Flux Basic Information

What is Soldering Flux?

IPC categorizes a flux as a chemical and physical agent that enhances the ability of molten solder to wet a metal surface by eliminating oxide or other surface films from the base metals and the solder. Additionally, the flux shields the surfaces from re-oxidation during soldering and modifies the surface tension of both the molten solder and the base metal.

What are Fluxes used for?

Fluxes serve multiple purposes:

  • They aid in the wetting process by eliminating oxidized layers from metal surfaces and adjusting surface tensions
  • Shield both solder and the components to be soldered from oxidation during the soldering process
  • Facilitate heat transfer between the parts being soldered to ensure their temperatures are balanced.

Flux Components

A flux may be solid, pasty or liquid, depending on how it is to be used.

  • A flux base, together with widely variable quantities of
  • Activator and
  • Solvents

Fluxes used in wave soldering are typically low in solids, consisting of only a small percentage of flux and the rest being solvents. This results in thin and easily movable materials. On the other hand, when incorporated into a paste for reflow soldering, the flux will include additional substances to enhance the paste’s flow and stickiness, as well as to prevent solder particles from settling too quickly. In numerous instances, the components of the paste serve multiple purposes.

Some typical features:

  • The presence of solvent helps with the application of flux or paste, but it evaporates during the initial heating stages of soldering.
  • The flux base exhibits fluidity when exposed to soldering temperatures and possesses excellent heat transfer characteristics.
  • Another crucial necessity for electronic soldering is that any leftover substances must be firm, relatively inactive, and possess effective insulating characteristics.

Flux Categories

Traditionally there were three main ways of categorizing a flux:

  • By its main active ingredients
  • Such as organic liquids or water, the kind of solvent employed to eliminate flux residues
  • By its effectiveness as a flux, often linked to its level of chemical reactivity, and directly related to its ability to erode the surfaces it comes into contact with.

The subsequent sections provide information about two primary categories of flux base material, based on their active ingredients.

  • Fluxes based on Rosin and Rosin substitutes
  • Water-washable organic (non-rosin) fluxes

Although inorganic fluxes are utilized for metal-working, they are typically not employed in advanced printed circuit assembly due to their tendency to cause corrosion.