What must the flux and its residue be after soldering?

After soldering, the flux and its residue should ideally be water flushable. This characteristic is important because it allows for the easy cleanup of any leftover flux after the soldering process is complete, ensuring that no corrosive, acidic, or harmful substances remain that could affect the integrity of the solder joint or the components being soldered.

Water flushable fluxes typically are formulated to be non-corrosive to metals and can be easily cleaned with water, making them user-friendly and safe for electronic applications. This prevents potential issues such as corrosion over time or adverse reactions with other materials.

While other types of fluxes, such as acidic or solvent-soluble, can be effective in certain applications, they often require specific cleaning methods that might not be as easy or safe as simple water flushing. Acidic flux, for example, can leave corrosive residues that may jeopardize the longevity and reliability of the solder joint. Heat-resistant fluxes are important for high-temperature applications but do not inherently address the need for clean-up after soldering.