In electronic manufacturing, solder paste and flux are often used together, which may lead to confusion about their functions. Although closely related, they serve different purposes. Choosing the right materials plays a vital role in ensuring high-quality SMT and PCBA production.
This guide clearly explains their differences, functions, applications, storage conditions, and how to choose the right solder paste.
1. Solder Paste vs. Flux
Many people assume solder paste and flux are the same, but they are not:
- Solder Paste: A mixture of solder powder, flux, and additives. It melts during reflow and forms permanent solder joints.
- Flux: A chemical cleaner added to solder paste or used separately to remove oxides and improve wetting during soldering.
In short:
Flux helps clean and improve soldering, while solder paste forms the actual solder joint.
2. What Is Solder Paste?
Solder paste (also called solder cream or tin paste) consists of:
- Solder powder
- Flux
- Various additives
It has a sticky texture that allows it to remain on PCB pads until reflow.
Key features:
- Accurate deposition on PCB pads
- Ensures strong, permanent connections
- Essential material in SMT assembly
3. Main Functions of Solder Paste
- Creates permanent metal-to-metal bonding
- Ensures reliable electrical connections
- Allows precise component placement in stencil printing
- Supports fine-pitch and high-density SMT components
4. Storage and Usage Guidelines
Storage
- Temperature: 1–10°C
- Shelf life: 6 months unopened
- Keep away from sunlight
Before Opening
- Allow 3–4 hours to reach room temperature (25 ± 2°C)
- Do not heat artificially
- Mix thoroughly for 1–3 minutes
During Use
- Apply only 2/3 jar quantity on stencil
- Add paste gradually based on printing speed
- Do not mix used solder paste with new paste
- Use printed PCB within 4–6 hours for best results
5. Types of Solder Paste
1. Rosin-Based (Requires Cleaning)
- Excellent wetting and soldering
- Leaves visible residues
- Requires cleaning with chemicals
2. No-Clean Type
- Minimal residue
- No cleaning required
- Faster production speed
3. Water-Soluble Paste
- Residues easily cleaned with water
- Environment-friendly
- Often used in high-reliability products
6. How to Choose the Right Solder Paste
Selection depends on desired performance and PCB design needs:
Key criteria:
- With Lead / Lead-Free
- No-Clean / Water-Soluble
- Alloy composition
- Melting point
- Powder granularity
- Viscosity
- Storage stability
- Printing stability
Good solder paste improves yield rate, reduces defects, and ensures long-term reliability.
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