What is reflow soldering? The temperature profile explained, leaded vs. lead-free, common defects and why reflow is the standard SMD soldering method.
What is Reflow Soldering?
Reflow soldering is the standard method for SMD assembly. Solder paste is applied to the PCB, components are placed, and the board passes through a reflow oven where the paste melts to create permanent solder joints. All SMD components are soldered simultaneously.
The Temperature Profile
The reflow profile has four zones: Preheat (25→150°C, slow ramp), Soak (150→200°C, temperature equalization), Reflow (peak 235-245°C for lead-free), and Cooling (controlled cool-down max 4°C/s).
Leaded vs. Lead-Free
Lead-free (SAC305) soldering is standard in Europe due to RoHS regulations. It requires higher peak temperatures (235-245°C vs. 210-225°C) and has a slightly higher defect rate compared to leaded solder.
Common Reflow Defects
Typical issues: tombstoning, solder bridges, cold joints, voids, warpage, and head-in-pillow defects. These can be prevented with optimized profiles, regular oven maintenance, and SPI inspection.
Best Practices for Reflow Soldering
Modern EMS providers use reflow ovens with nitrogen atmosphere. This prevents oxidation and improves wetting – especially important for lead-free soldering.
- Custom profiles: Each product receives a tailored temperature profile
- Profile recording: Complete documentation of every oven run ensures traceability
- SPI before the oven: 3D solder paste inspection ensures only correctly pasted boards enter the oven
- AOI after the oven: Every solder joint is automatically optically inspected
When choosing your manufacturing partner, make sure these quality measures are implemented as standard.
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