August 20, 2026
Preheat, Soak, Reflow and Cooling: What Each Phase Controls
Each phase serves a different purpose within a single temperature history. Its actual limits come from the paste, alloy, components and board rather than from the name of the phase.
- **Preheat/ramp:**raises the assembly while controlling gradients and volatile behavior.
- **Soak or activation region:**supports the flux system as specified by its supplier and reduces board differences where the process calls for it.
- **Reflow above liquidus:**melts the alloy long enough for the intended joint formation without exceeding package limits.
- **Cooling:**solidifies the joint under a controlled history and remains part of the process. Profile both hot and cold locations. A center curve that remains within the process window does not prove that the entire PCB is compliant. NXP notes that flux behavior determines important reflow settings and that every device must be considered. Use the four phases as a cause-and-effect model. If a joint fails, identify the phase and material requirement that were not met before changing the oven. Goldisgood IR zones can shape the heat input through the phases, but no emitter defines the material window. Related: building a profile and poor wetting diagnosis.