Flux plays an important role in processes like soldering and brazing, serving as a chemical compound that prepares metal surfaces for joining. It removes oxidation and prevents re-oxidation during heating. This promotes wetting, allowing molten solder to spread uniformly for strong, reliable connections. Understanding its composition is key to appreciating its applications.
Fundamental Ingredients
Flux formulations consist of several key components. Activators chemically clean metal surfaces by dissolving oxide layers. They react with metal oxides to form salts and water, exposing clean metal for bonding.
Vehicles transport activators and other components. They ensure uniform application and distribution, often evaporating during preheat. Common vehicles include natural resins like rosin, synthetic resins, or liquids such as isopropyl alcohol or glycols.
Binders and rheology modifiers control the consistency and flow properties of the flux, influencing how it adheres and spreads. These components maintain the flux’s physical form (paste, liquid, or solder wire core). They also affect residue behavior after joining.
Diverse Flux Formulations
Fluxes are formulated in various ways, each suited to different applications based on their chemical makeup. Rosin-based fluxes originate from natural resins, primarily from pine trees, containing organic acids like abietic acid. These fluxes are non-corrosive at room temperature but become mildly acidic when heated, dissolving thin oxide layers.
Rosin fluxes are often categorized by their activity level, ranging from unactivated (R type) to mildly activated (RMA) and activated (RA), with activators such as organic acids or halogenated compounds added to increase their cleaning power. Unactivated rosin fluxes leave non-conductive residues, while activated versions may leave corrosive residues requiring cleaning.
Water-soluble fluxes primarily use organic acids or inorganic acids as their active components. Examples of organic acids include citric acid, lactic acid, and glycolic acid. These fluxes are highly active in removing oxides, and their residues are easily cleaned with water after soldering.
No-clean fluxes are formulated to leave minimal, non-conductive, and non-corrosive residues after heating, eliminating the need for post-soldering cleaning in many applications. They typically feature a low solid content and incorporate synthetic resins and milder activators, often weak organic acids.
Inorganic acid fluxes are highly aggressive and contain strong acids or salts like hydrochloric acid, phosphoric acid, or zinc chloride. They effectively remove heavy oxidation from difficult-to-solder metals, but their corrosive residues require immediate, thorough cleaning to prevent damage.
Matching Flux to Application
Flux composition dictates its properties and suitability for different metal joining tasks. In electronics soldering, the main concern is electrical reliability, preferring no-clean or water-soluble fluxes that leave minimal or easily removable residues. Rosin-based fluxes are also common due to their non-corrosive nature at room temperature and non-conductive residues.
In plumbing applications, where stronger cleaning action is needed for copper pipes, more aggressive acid fluxes are used. These highly corrosive fluxes effectively strip away oxidation layers for strong, watertight seals. Their potent nature means residues require careful removal.
Brazing, a high-temperature joining process, employs fluxes with alkaline salts (e.g., borax-based compounds or fluoroborates). These formulations activate at elevated temperatures, dissolving oxides and protecting metal surfaces from atmospheric oxygen during brazing. The specific flux type depends on the base and filler metals, and the working temperature.