The residue after remelting, the residue remaining after electronic soldering, is a complex mixture of various components, mainly influenced by the type of flux used.

The main components are:

• Resin: natural or synthetic often used in fluxes. It normally forms an insulating film that protects the joint, but potentially attracts moisture (hygroscopic)
• Activators: depending on the type of flux, these can be organic acids ((e.g., abietic acid, benzoic acid), amines (e.g., imidazoles, triazoles) or halide salts (e.g., alkylammonium halides).
• Metal salts: traces of tin complex salts (Sn) may be present from the solder itself
• Other additives: thickeners, solvents and wetting agents used in the flux that may be present in trace amounts.

Factors influencing the composition of the residue after remelting can thus be:

1- the type of flux

• “No-Clean” fluxes: designed to leave minimal residue, typically containing organic acids and specific activators
• “Water-soluble” fluxes: designed for cleaning with water, leaving minimal residue, but requiring cleaning
• “Halide-activated” fluxes: highly active, but leave corrosive residues that require cleaning;

2- the soldering process: during the remelting process, the thermal profile can result in more or less flux evaporation, consequently leaving more or less flux residue on the board. Prolonged exposure to high temperatures can cause flux decomposition with the formation of unwanted residue, or reduced exposure can equally leave potentially harmful products over time.

Impact of the residue:

Corrosion: residue in the presence of moisture and electric current can trigger corrosion, electro-corrosion or electro-migration processes, compromising the functionality of the electronic board.
Contamination: excess flux residue can compromise the quality of any coating processes (conformal coating) if not removed.

Cleaning:

Residue removal is critical in high-reliability environments, and still remains a guarantee of reliability.

Cleaning methods include water-based cleaning (PROMOCLEAN™), solvent-based cleaning (TOPKLEAN™, QUICKSOLV™), and vapor-phase cleaning (PROMOSOLV™).