Thermal Window
Temperature differential between the melting point of the solder alloy and the peak temperature reached by the component leads defines the safety margin of the process. A narrow liquidus delta increases the risk of cold solder joints if the oven temperature fluctuates. Most lead-free soldering processes aim for a delta that ensures complete wetting without damaging the board or the parts.
Reflow Soldering
Precise control of the peak temperature is necessary to keep the liquidus delta within the limits specified by the solder paste manufacturer. If the liquidus delta is too high, the flux may char or the component packages may crack due to excessive thermal stress. Thermocouples are used to measure this gap at various points on the assembly during a profile run.
Large components often have a smaller delta than smaller parts because they heat up more slowly.
Quality Risk
Inconsistent heating across the board surface can lead to a variable liquidus delta that causes different wetting behaviors. Brittle solder joints sometimes form if the peak temperature is only marginally above the melting point. Monitoring this value helps in maintaining a stable manufacturing process and reducing the rate of rework.
The maximum allowable temperature for the most sensitive component usually sets the upper boundary for this differential.
