How To Use Flux When Soldering Electronics

When electronic segments are soldered to a circuit board, if the metal surface of the board isn’t perfect and is ready for soldering, you will not get a decent metallurgical connection between the surfaces. The way to getting a proper solder joint is utilizing a compound cleaning specialist known as Flux previously and during the soldering process. Here’s a more intensive glance at all of this and how to use Flux when soldering electronics.

What is Flux?


Flux is a chemical cleaning liquid or pastes used for removing and preventing the formation of the oxides before, during, and after the soldering process of electronic components onto circuit boards and other materials.

What is the Primary Purpose of Flux?

Oxides are formed when metal is exposed to air. The layer of these oxides prevents the formation of a good solder joint. The primary purpose of the flux is to remove and clean the oxides layer and prepare the metal surface for soldering. The flux also protects the metal surface from reoxidation and corrosion.

What is the chemical composition of the flux?

Flux comprises the following four essential components: Activators, Vehicles, Solvent, and Additives.

  • Activators: It may be Organic or Inorganic but Its primary function is to dissolve the metal oxides and expose the unoxidized metal. 

Activators are of two types: Highly Active & Milder Activator.

ActivatorChemical Composition
Highly Active ZnCl2, NH4Cl, HCl, PCl3,  HOC(CO2H)(CH2CO2H)2, HBr, and some also contain Fluorides.
Milder Activator.CH3COOH, HO2C−R−CO2H, Amino acid, and also contain halides or organohalides.
Activator Composition
  • Vehicles: These are non volatile liquid or solid its primary function is to prevent the hot metal surface from the chemical reaction with oxygen. 

These are primarily natural or modified rosin. 

It’s composed of Abietic acid, aromatic acid, glycols, diethylene glycol, glycerol, and some surfactants that lower the surface tension liquid/liquid and liquid/gas.

  • Solvents: These are the liquid that acts as a solvent to make a mixture of different components. Its principal function is to facilitate and usually quickly evaporates.
  • Additives:  These are nonionic stabilizers or antioxidants or dyes that protect the solder joints for the future.

Inorganic fluxes are primarily used in brazing and high-temperature area due to the thermal instability of the organic fluxes.

Composition: Borax, Borates, fluorides, and Chlorides these chemicals act both as vehicles and activators.

How To Apply Flux in Different Types Of Soldering?

For soldering electronics, there are three following categories of flux.

  • Rosin and Rosin Substitutes Flux
  • Water-soluble Flux
  • No-Clean Flux

These categories are having different types of chemical compositions. Which type of flux to be used depends on the needs of the components and boards to be soldered. You can apply these fluxes in the following methods.

  • Wave Soldering
  • Solder Reflow
  • Selective Soldering
  • Manual Solder
Type Of SolderingSoldering ProcessHow To apply
Wave Soldering It is used in the manufacture of printed, surface mount, and through-hole circuited boards. The circuit board is passed through the molten solder during this process, and the component is soldered to the board.Flux is sprayed on before being soldered.
Reflow Soldering During this process, a solder paste of solder and flux is temporarily attached to one or thousands of tiny electrical parts to their contact pad. The whole assembly is subjected to prescribed heat. The solder paste reflows in a molten state, creating stable solder joints.Solder paste is the mixture of the solder and flux
Selective Soldering This process follows an SMT oven reflow process; parts to be selectively soldered are usually surrounded by components previously soldered in a surface-mount reflow process. The selective soldering process must be sufficiently precise to avoid damaging them.Flux is sprayed, or a few drops are added to the part which is to be soldered.
Manual Solder Soldering process in which the soldering iron is used to repair the specific part of the component.Liquid flux or solder wire are used having flux in the core.
How To Apply Flux

How Do You Clean Flux & Its Residues?

The most common method to clean flux residues from a replacement area is to soak a cotton or foam swab with isopropyl alcohol or another cleaning solvent and rub it around the repair area. While this may be satisfactory for no-clean flux, where the goal is a visually clean PCB, this may not be clean enough when more heavily activated fluxes are involved, like RA or aqueous.

The dirty little secret is that flux deposits will not evaporate along with the organic solvent. You may dissolve the flux and its residues, and some of the dissolve residues will soak into the swab, but most of the insoluble residues will settle back onto the board surface. Many times these white residues are more challenging to remove than the flux.

One quick and easy addition to this process is to flush the board after swabbing around the repair area. While the solvent is still wet, spray over the whole board with an aerosol flux cleanser. Hold PCB at an angle to allow the solvent to flow over the board and runoff, along with any residues that are picked up.

The straw attachment that comes with aerosol flux removers is the best way to improve the spray efficiency and penetrate the components.

Some aerosol flux removers come with a brush accessory. The cleaning solvent sprays through the brush, so agitation can increase by cleaning while spraying. 

A lint-free poly-cellulose wiper can be placed over the repair area to absorb the inorganic flux residues, and spraying and scrubbing can occur over the material. Then remove the wipe and brush attachment, and spray over the board for the final rinse.

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