Reflow soldering is the backbone of SMT assembly. Getting the thermal profile right determines whether your boards have reliable solder joints or chronic field failures. Most solder defects trace back to incorrect profiles.
The four zones
- Preheat: 25-150C at 1-3C/s — gradual temperature rise to avoid thermal shock
- Soak: 150-200C for 60-120s — activates flux, equalizes board temperature
- Reflow: peak 235-250C — solder melts and wets the pads
- Cooling: controlled descent at 1-4C/s — grain structure forms here
Critical parameters
- Peak temperature: typically 235-250C for SAC305 solder
- Time above liquidus (TAL): 45-90 seconds
- Ramp rate: 1-3C/s maximum to prevent component damage
- Cooling rate: 1-4C/s for fine grain structure
Common profile mistakes
- Too fast ramp — causes tombstoning on small passives
- Insufficient soak — cold solder joints on large components
- Excessive peak temperature — component damage, board warpage
- Too fast cooling — brittle solder joints, cracking under thermal cycling
Profile for lead-free vs leaded
SAC305 (lead-free) requires higher peak temperatures than Sn63Pb37. The entire profile shifts up 20-30C. Components and board materials must be rated for the higher temperatures. Most modern PCB assembly has transitioned to lead-free.
Profiling best practices
- Profile with a loaded board, not bare PCB
- Measure at the largest component and the smallest
- Verify profile after any oven maintenance
- Document profiles and attach to work instructions
- Re-verify when changing solder paste or board thickness
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