Soldering terminology

Flux is used in soldering, with the exception of special cases (e.g. abrasion soldering, soldering in reductive atmosphere, mechanical joining), to clean the solder joint in order to prevent oxidation of the metal, to reduce the surface tension of the molten solder and to increase wettability. For this purpose, it must both be in a position to remove the surface oxides of metals and to bring the solder into direct contact with the base material. To achieve this effect, mostly rosin-containing flux is used, which is also called flux R (R stands for rosin). If it does not provide high working efficiency in itself, flux activators are usually added in industrial applications. It is extremely complicated to evaluate the effectiveness of a flux because a wide range of factors must be considered in addition to the solder material: the material, type and quality of the parts to be soldered, conditions of the soldering process, requirements for the solder joint, etc. need to be considered, as well as the type and amount of chemicals added to the rosin. In some cases, more than 20 different activators are added to flux used in solder paste. The exact composition is a trade secret of the manufacturer and is not published. To categorize the flux based on its performance, the U.S. Military Specification (MIL) uses the abbreviations R, RMA and RA, where RMA stands for Rosin Mildly Activated and RA stands for Rosin Activated. Activators are usually halides, used in different quantities in RA and RMA fluxes. Halogen content of RMA flux residues is comparatively low, meaning that no cleaning is required (no-clean flux).

Depending on the type and proportion of activators, fluxes can be water-soluble or water-insoluble. Fluxes typically contain rosin or resin, in which the products release the reaction between oxides contained in the solder and in the base and the activator, as well as eventual binders. The concentration and viscosity is adjusted to match the soldering conditions by mixing with a diluent. Since water-soluble flux is corrosive, the soldering process must be followed by a purification step of rinsing with water and neutralization. Water-insoluble fluxes may contain resin or rosin and are widely used for soldering electronic devices. The R, RMA and RA fluxes presented in the table below belong to this category. RMA fluxes are subject to more stringent requirements relating to the use of halogen-containing activators and may therefore leave residues on the assembly. The second table contains an overview of the activators for water-soluble fluxes and the associated water purification process. Suitable binders for such fluxes are glycerol, polyethylene glycol, polypropylene glycol, and polyoxyethylene phenyl ether while some commonly used diluents are water, alcohols with a number of carbon between 1 and 3 or alcohol ethers with a high boiling point. As shown in Table 3, water-insoluble fluxes consist of rosin or resin with activators. Cleaning with water is allowed for such fluxes with solids content (rosin content) of 5% or less. For this purpose, thinners such as terpene, alcohols with a number of carbon between 1 and 4 etc. are used. These fluxes are particularly suitable for pre-flux or post-flux use while solder paste fluxes have a different composition. Typical fluxes of this type are shown in Table 4. Test specifications for such fluxes can be found in the Federal Spec. (QQ-S-571E).