What is a Flux Remover?

What is a flux remover? What are the different types? How do you select a flux remover?

Solder Soldering 
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Editorial Team - PCB Directory

May 18, 2021

A Flux Remover is a chemical solvent that is used to clean or remove flux residue, white residue, and other impurities from the surface of a PCB. Typically, the isopropyl alcohol or water-based solvents are used as flux removers.

Flux residue is usually found on the Printed Circuit Board after the soldering process, during desoldering, when repairing the PCB or in the rework process. This residue needs to be cleaned. If the flux residue is not properly clean, then it will affect the aesthetic appearance of PCB, reduce the reliability, reduce the adhesion of the conformal coating, cause leakage current or even short circuit due to the ionic contamination left from flux residue.

Flux Remover as a Spray

In the PCB world, the three types of flux - Rosin, Water-soluble, and no-clean flux. The residue of Rosin and water-soluble flux should be cleaned by using the flux remover. No-clean flux usually does not need any/much cleaning. However, some sort of cleaning technique is recommended because the residue of no-clean flux may affect the adhesive of conformal coating.

Flux remover for the residue of Rosin Type Flux:

Isopropyl alcohol & methyl alcohol are the flux removers or solvents used to clean the residues of rosin type flux. These solvents are low cost, effective, and quick to clean. Also, the Semi-aqueous and saponified aqueous flux removers or cleaners can also be used to clean the residues of rosin type flux.

Rosin type flux is the popular, older, but still used in the soldering process. The activators of rosin flux are abietic acid and plicatic acid. There are three types of rosin flux - Rosin (R), Rosin Mildly Activated (RMA), and Rosin Activated (RA). After the soldering process, the RA and RMA type of rosin flux will leave more residue than rosin (R). Actually, the rosin (R) flux does not leave any residue because there is no activator present in the rosin (R). However, the residue left by rosin flux should be cleaned because it can affect the reliability, adhesion of the conformal coating, and aesthetics of the board.

Flux removers for the residue of Water-Soluble Flux:

After the soldering process, the water-soluble flux leaves a residue that is highly corrosive in nature and contains ions, which can be conductive. If the residue of water-soluble flux is not cleaned, it can create the electrical short. Hot water is used to clean the residues of water-soluble flux; in addition, Saponifiers are added to the cleaning process. Saponifiers are alkalines that reacts with the residue on the board to form soap and glycerin to ease the cleaning process.

Other flux removers for water soluble flux are deionized water, and detergents.

Flux removers for no-clean flux:

No-clean flux is mostly used in reflow and wave soldering. A proper thermal profile will not form the flux residue. The residue of no-clean flux may cause poor adhesion of the conformal coating. Hence, it is recommended to clean the residues of no-clean flux. The residues can be cleaned by using a polar solvent or a solution of water and saponifier.

So how do you choose a flux remover or solvent?

The selection of a flux remover would depend on the type of flux being used. It would also depend on the type of cleaning method that can be used. For example, Aerosol spray, Brush cleaning, Hand cleaning, Vapour Degreasing, Ultrasonic, immersion, Bench top spraying system, and orbital aqueous cleaner. Different removers are available in different formats.

Another factor would be the properties required like those that dry fast, or are non-flammable, non-ozone depleting, or have some other property.

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