How to Remove Solder Flux Residue
How to remove solder flux residue depends on the type of flux used in the soldering process.
Updated: April 08, 2021
Flux residues should be removed as soon as possible after the soldering. The longer the residues are left on the circuit board the higher is the possibility for corrosive damage on solder joints later in service and therefore poor electrical performance of the circuit board.
The two most common ways to remove solder flux residues from a circuit board are:
A perfect soldering joint can only be achieved with completely cleaned surfaces. Using soap and water, alcohol and other solvents to clean surfaces before soldering is an important step to accomplish a good and successful soldering connection, but it is not sufficient.
During soldering, oxide forms very quickly on the surface of heated metals creating films that prevent proper soldering operation. To overcome thin oxide films, it becomes necessary to use flux whenever soldering electronic components to a circuit board.
Fluxes can be divided into active and passive fluxes. No chemical reactions occur when the passive flux is heated while some chemical reactions occur when active fluxes are heated.
Active fluxes are not very popular for general use since it is not always possible to clean the corrosive residues formed during soldering with this flux. Active fluxes are mostly used on difficult surfaces for soldering where the use of passive fluxes doesn’t provide satisfactory results.
Active flux residues must be cleaned with hot soapy water and a wire brush. Acid fluxes should never be used in electronics.
The recommendation is to use passive fluxes wherever is possible. Passive fluxes usually come in the form of a paste. The most popular passive flux is rosin flux.
Rosin fluxes consist of natural and synthetic rosin and chemical additives called activators.
The function of the rosin flux is to remove thin oxide films and keep them removed the entire time during soldering.
Rosin flux is non-conductive and non-corrosive. At room temperature, rosin flux is in a solid state.
A melting temperature of flux must be lower than the melting temperature of solder to allow flux to do its function before the soldering. When heated with a soldering iron tip, the flux will rapidly change from solid to liquid state. Molten flux will flow onto the soldering surfaces to provide the full benefits of fluxing function.
The only fluxes allowed for use in electronics are pure rosin fluxes and RMA fluxes. RMA fluxes are rosins combined with mild activator. Activators in RMA fluxes accelerate the rosin’s fluxing function.
How to Remove Flux Residue
Consider some general practices when establishing the “how to remove solder flux residue” method. Determine whether the flux removal process leaves undesirable stains and other contamination.
Which cleaning fluid will be used as residue remover depends on particular flux residue.
Residues left after using rosin-based fluxes are the most common flux residues in electronics.
Residues left on the circuit board after the use of rosin flux are relatively harmless since rosin flux is very mild in its activity.
Solder flux residue can be removed in two steps. In the first step, organic solvents are used to remove flux residue. In the second step, another fluid is used to rinse away the solvent's leftovers.
Isopropyl alcohol is generally used for the removal of flux residues on circuit boards. It is used for cleaning circuit boards from other contamination as well.
Don’t use denatured alcohol and acetone because they can leave residues.