In solder paste, the three primary flux types are rosin-based flux, water-soluble flux, and no-clean flux. Each is engineered for specific soldering scenarios, with distinct activation strengths, residue properties, and post-soldering requirements. Below is a concise breakdown of their core characteristics:
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Rosin-Based Flux
- Core Components: Natural pine resin (abietic acid as the key activator) + organic acids + solvents.
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Classification (per IPC standards):
- R (Non-activated): Low activity, minimal residue, for clean, oxide-free surfaces.
- RA (Rosin Activated): Mildly activated with weak acids, suitable for general consumer electronics.
- RMA (Rosin Mildly Activated): The most widely used grade, balanced activation for THT (through-hole technology) and SMT (surface mount technology) assembly.
- RS (Rosin Super Activated): High activation with strong acids, for heavily oxidized or hard-to-solder surfaces.
- Pros: Excellent wetting ability, stable soldering performance, low cost.
- Cons: Leaves sticky, non-water-soluble rosin residue. Mandatory solvent cleaning (isopropyl alcohol, chlorinated solvents) is required for high-reliability applications (aerospace, medical devices) to avoid moisture/tcontaminant buildup.
- Typical Uses: Low-cost PCB assembly, consumer electronics, through-hole soldering.
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Water-Soluble Flux
- Core Components: Synthetic organic acids/amines (no rosin) + halide salts + water-miscible solvents.
- Key Trait: Fully soluble in deionized water, no rosin residue.
- Pros: Strong activation (ideal for lead-free solder alloys like SAC305), environmentally friendly, easy to clean with water (no harsh solvents needed).
- Cons: Slightly higher cost than rosin-based flux; requires strict control of water purity during cleaning to avoid residue spots.
- Typical Uses: High-reliability electronics (medical devices, aerospace), lead-free soldering, automated production lines with water-based cleaning stations.
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No-Clean Flux
- Core Components: Modified rosin/synthetic activators + low-residue solvents (formulated to minimize residue volume).
- Key Trait: Residue is non-corrosive, non-sticky, and requires no post-soldering cleaning for most applications.
- Pros: Eliminates cleaning steps (saves time and cost), compatible with SMT automated placement machines, residue is visually transparent (no impact on PCB appearance).
- Cons: Weaker activation than rosin-based/water-soluble flux; not suitable for heavily oxidized surfaces or high-reliability applications.
- Typical Uses: SMT surface mount assembly, high-volume consumer electronics (phones, laptops), PCB with dense component layouts where cleaning is impractical.