According to current knowledge, it is not possible to remove flux residues by simply washing or dissolving them by a simple cleaning method - i.e. there is no suitable solvent or chemical solution - because this cannot be done without affecting (corrosion) the substrate material.
Mechanical Cleaning
Generally, cleaning of flux residues can only be done by mechanical methods. From solid surfaces and solid joints, as well as stainless steel fixings and connections, flux residues can be mechanically removed by sand or slag blasting. Wire brushing is a second alternative for flux residue removal. We recommend using stainless steel wire brushes for cleaning. Rotating stainless steel wire brushes may also be suitable - if their surface area can be accessed. Brushes made of copper and brass should be used if the surfaces being cleaned will not come into contact with any subsequent welding or brazing cycles, as traces of copper (even dust particles) in contact with aluminum can cause serious erosion problems.
Chemical Cleaning
Flux residues have a slightly higher solubility in some acids and in some acids. However, in most cases some chemicals (aluminum or stainless steel) will also be affected (corroded) by these chemicals.
Warm borsic acid solution (10 to 15%, 75 to 80°C) can be used to remove flux residue from some soldered assemblies. The dissolution of aluminum by borsic acid is relatively mild. The soaking time required to remove most of the debris varies from 10 to 30 minutes. However, flux residue cleaning will not be 100% successful.
The processing (preparation and use) and discharge (waste disposal) of such chemical solutions can be problematic and expensive, depending on their corrosion properties and the subsequent wastewater treatment required. Considerations for health, safety and the environment must comply with the Safety Data Sheets.
Ultrasonic Cleaning
Ultrasonic treatment can be effective in removing flux residue, but the parts that need to be cleaned must be suitable for ultrasonic immersion. A detergent (cleaning agent) can be added to the solution to increase cleaning effectiveness. Commercial solutions are available for ultrasonic cleaning. However, if any other additional chemicals (especially when adding acids or bases) are added to the flux-containing cleaning solution, their compatibility should be verified.
Summary
Flux residues can only be removed by mechanical methods, i.e. using wire brushes or sand/slag blasting. This is a very difficult and laborious process - and a very dirty one (dust generation!). Local exhaust and ventilation are required in the work area where parts are being cleaned. Since there is no suitable solvent, removing flux residues will not corrode the substrate materials.
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