Flux Types Explained: Rosin, No Clean, Water Soluble & Acid
Flux Types Explained: Rosin, No Clean, Water Soluble & Acid
Flux is one of the most critical — and most misunderstood — materials in electronics assembly. Choosing the wrong flux type can result in poor solder joints, residue-related reliability failures, cleaning headaches, or process incompatibilities. This guide breaks down the four primary flux chemistries used in electronics manufacturing, how they work, and how to choose the right one for your process.
What Does Flux Do?
Flux serves two essential functions during soldering. First, it chemically removes metal oxides from the surfaces being joined — oxides prevent solder from wetting and bonding properly. Second, it protects those cleaned surfaces from re-oxidizing during the heat of the soldering process. Without flux, reliable solder joints are nearly impossible to achieve consistently.
Flux is available in liquid form (for wave soldering and hand soldering), as a core inside solder wire, as a paste (in solder paste for SMT), and in pen applicators for rework and touch-up.
Rosin Flux (R, RMA, RA)
Rosin flux is derived from pine tree resin and has been the foundation of electronics soldering for decades. It is available in three activity levels:
R (Rosin) — The mildest formulation, with minimal activators. Suitable for very clean, easily solderable surfaces. Leaves a clear, hard residue that is generally considered non-corrosive and can be left on the board in many applications.
RMA (Rosin Mildly Activated) — The most widely used rosin flux in electronics assembly. Mild activators improve wetting on moderately oxidized surfaces while keeping residues benign enough to leave in place on most assemblies. The industry standard for general-purpose electronics soldering.
RA (Rosin Activated) — Higher activator content for difficult-to-solder surfaces. Residues are more aggressive and should be cleaned after soldering, particularly on assemblies with high-impedance circuits or in humid environments.
When to use rosin flux: General electronics assembly, hand soldering, wave soldering, and rework where a proven, well-characterized chemistry is preferred. RMA is the default choice for most applications.
Cleaning: Rosin residues can be left in place (R and RMA in most applications) or cleaned with rosin-compatible solvents or saponifiers.
No Clean Flux
No clean flux formulations are engineered to leave residues that are electrically benign and do not require removal after soldering. They use low-halide or halide-free activator systems that are fully consumed or rendered inert during the soldering process.
Standard No Clean — Suitable for most commercial and industrial electronics. Residues are minimal, clear or slightly amber, and non-corrosive under normal operating conditions.
No Clean VOC Free — Water-based no clean formulations that eliminate volatile organic compounds. Required in facilities with VOC emission restrictions and preferred in environmentally conscious manufacturing programs.
No Clean Halogen Free — Formulated without halogens for assemblies where halogen content must be minimized — common in automotive, medical, and high-reliability applications.
When to use no clean flux: High-volume SMT and wave soldering where cleaning is not practical or economical. Also preferred when assemblies will be conformally coated, as long as residues are confirmed compatible with the coating system.
Important caveat: "No clean" does not mean "no residue." If residues will be under low-standoff components or in high-humidity environments, cleaning may still be advisable even with no clean flux.
Water Soluble Flux
Water soluble flux uses organic acid (OA) activators that are highly aggressive — providing excellent wetting even on heavily oxidized or difficult surfaces — but must be thoroughly cleaned with deionized water after soldering. Residues left on the board are corrosive and will cause reliability failures over time.
Standard Water Soluble — High activity, excellent wetting performance. Requires aqueous cleaning immediately after soldering.
Water Soluble VOC Free — Water-based carrier eliminates VOC concerns while maintaining the cleaning requirement.
When to use water soluble flux: Applications requiring the highest solder joint quality and where an inline aqueous cleaning system is available. Common in military, aerospace, medical, and high-reliability commercial electronics.
Cleaning requirement: Water soluble flux residues must be removed with deionized or RO water, typically in an inline or batch aqueous cleaning system. Cleaning should occur as soon as practical after soldering.
Acid Flux
Acid flux uses strong inorganic acid activators that provide extremely aggressive oxide removal. These fluxes are not suitable for electronics assembly. Residues are highly corrosive and will rapidly destroy circuit boards and components.
When to use acid flux: Plumbing, HVAC, sheet metal work, and other non-electronic applications. Never use acid flux on electronics. Even trace residues will cause corrosion and catastrophic circuit failure.
Flux Activity Standards: IPC J-STD-004
The IPC J-STD-004 standard classifies flux by chemistry and activity level. Chemistry designators include RO (Rosin), RE (Resin), OR (Organic), and IN (Inorganic). Activity levels are L (Low), M (Medium), and H (High), with halide content indicated by 0 (halide free) or 1 (halide containing). For example, ROL0 is a rosin-based, low-activity, halide-free flux — equivalent to a traditional R or RMA chemistry.
Flux Format: Liquid, Cored Wire, Paste & Pen
Liquid flux — Applied by wave, spray, foam, or selective fluxer in automated soldering, or by brush in hand soldering and rework.
Cored solder wire — Flux is contained in cores inside the solder wire, releasing during soldering.
Solder paste — A mixture of solder powder and flux paste, printed onto PCB pads for SMT reflow soldering.
Flux pens — Convenient applicators for rework, touch-up, and selective fluxing. Available in all major flux chemistries.
Choosing the Right Flux for Your Process
Use RMA rosin for general-purpose hand soldering and wave soldering where a proven chemistry is preferred and light cleaning or no cleaning is acceptable. Use no clean for high-volume SMT and wave soldering where eliminating the cleaning step reduces cost and cycle time. Use water soluble when maximum wetting performance is required and an aqueous cleaning system is in place. Use acid flux only for non-electronic applications.
KEM-TRON stocks a comprehensive range of Kester flux products across all chemistries and formats — including liquid flux, flux pens, cored solder wire, and solder paste in rosin (R, RMA), no clean, no clean VOC free, no clean halogen free, water soluble, water soluble VOC free, and acid flux formulations.