Reliable soldering can be achieved by soldering clean surfaces. Usually surfaces are cleaned with abrasives and solvents, but after surface is cleaned surface immediately oxides especially when heated. During oxidation surface is covered by thin oxide film which prevents solder from contacting to metal. This is why flux is used in soldering process. In order to remove/deoxidize surface flux must be applied during soldering. Flux chemically removes surface oxide when heated and makes good metal to solder contact.
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There are several categories of soldering fluxes:
Acid Flux (or commonly known fluxes: Zinc chloride, hydrochloric, ammoniac);
Each flux has their own specifics and may be used for different soldering technologies.
Acid Flux is most active of the group. It is effective on almos all metals excluding aluminum and magnesium. But Acid(chloride) flux has several significant disadvantages: it is highly corrosive, electrically conductive and it is difficult to remove from soldered joint.
You should never use acid fluxes in electronic device soldering and repair, as it will cause corrosion and even can short-circuit device where gaps between tracks are small.
Organic flux have almos same effect as acid flux but is less corrosive, isn’t conductive to electric current and is easier to remove from soldered joints. But again it has corrosive effect if not cleaned well.
Organic fluxes also are biodegradable if stored for longer time. So this flux also not acceptable for microelectronics.
Rosin flux is ideally suited for microelectronics as it has special molecular structure. It is usually pure rosin dissolved in suitable solvent. Rosin flux works well with almost all electronic contacts. Flux become very effective at soldering temperatures. It is non corrosive, non conductive.
Rosin flux is usually included in solder wire core. So when soldered with such solder, when heated flux is applied to the joint and flux flows on surface and removes oxide. Rosin fluxes may be different type of activity:
R- Rosin only;
RMA â€“ Rosin Mildly Activated;
RA- Rosin Activated.
Depending on surface cleanness it depends what activity Rosin to use. If surface is very clean use R rosin, for less clean surfaces use RMA or RA rosin flux, but they have additional ingredients like organic acids and other acids. So they leave inert residues after soldering.
Despite what flux is used it is always recommended to clean joints with flux remover in order to avoid corrosion, contamination and of course PCB looks much more professional when cleaned.