Fluxes

Fluxes are chemical compositions used in soldering, brazing and welding. They clean the metal’s surface, assist with heat transfer, and remove unwanted oxide films. According to J-STD-004, a popular flux classification system from the IPC and EIA, there are four broad categories of products: rosin (RO), resin (RE), organic (OR) and inorganic (IN) fluxes. Rosin fluxes contain abietic acid, a chemical extracted from pine-tree sap. They may also contain activators such as halides. There are two subcategories of RO fluxes: high-solids and low-solids. High-solids resin fluxes contain greater amounts of halide activators which, if left in place, will convert metal surfaces into metal salts. Low-solids RO fluxes contain lower levels of halide activators, but may still corrode solder wires. Resin fluxes are based on synthetic or modified organic resins. RE fluxes provide better thermal stability than RO fluxes and are often easier to clean. In addition, resin fluxes can be modified to flash-off from the heat of a solder tip.

Flux classification systems include ANSI/IPC-SF-818 and MIL-F-14256E. MIL QQS is an older standard which defined fluxes as rosin (R), rosin-activated (RA), rosin mildly activated (RMA), and water soluble (WS). Type R fluxes are the least active rosin-based products. They are designed for use with very clean surfaces. RA fluxes are the most active rosin fluxes; however, they leave the most residues. RMA fluxes contain a small amount of an additional activator, typically a halide. These fluxes leave small amounts of an inert residue that is usually non-corrosive and tack-free. Water soluble fluxes (WSF) can be used in electronics applications, but strip oxides aggressively. Incomplete WSF removal may result in problems such as electrical leakage currents and metal migration. Most water soluble fluxes can be divided into two subtypes: organic and inorganic. Organic or organic activated (OA) fluxes are more active than RA fluxes. Inorganic fluxes are the most active fluxes of all.

For electronics manufacturers, selecting fluxes requires an analysis of printed circuit board (PCB) types and connection density. Simpler PCBs can use fluxes with lower solid contents. Denser boards require fluxes with more solids. Mounting method is also an important consideration. For example, because surface mount technology (SMT) leaves hard-to-remove residues underneath components, flux manufacturers have developed products that do not require cleaning. Additional considerations when selecting fluxes include soldering method and material compatibility with the activator. As a rule, metals such as stainless steel require the strongest or most active fluxes. Halogen-free fluxes do not contain substances such as chlorides or fluorides. Ammonia-free fluxes are also available.

• Brazing Processes
  Brazing joins parts by heating them to more than 840°F and applying a filler metal that has a melting temperature below that of the base metal. Filler metal flows into the joint by capillary...
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• Flux Cored Arc Welding (FCAW)
  In the Flux Cored Arc Welding (FCAW) process, metal parts are joined by melting base and filler metals with an arc struck between a consumable filler metal wire and the base alloy work piece. Flux...
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• Flux Coated / Stick
  Flux coating is commonly applied to stick welding electrodes as well brazing rods or strips. Stick electrode welding or shielded metal arc welding is among the most widely used welding processes. The...
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Part Numbers for Fluxes