Time above Liquidus

Dwell Duration

Optimization of the time spent in the molten state requires a precise adjustment of the conveyor speed and the temperature of the reflow zones. The time above liquidus must be long enough to allow for the uniform distribution of heat across the entire assembly, including large components with high thermal mass. During this period, the flux remains active and cleans the surfaces to be joined, while the molten metal flows into the vias and around the component leads.

If the board moves too fast through the oven, the dwell time will be insufficient, often causing “cold” solder joints or incomplete wetting on the larger pads. If the conveyor is too slow, the dwell time increases, which can lead to the overheating of sensitive components and the degradation of the printed circuit board substrate. Process engineers use multi-channel thermal data loggers to monitor the actual temperature at different points on the board to ensure that every joint stays within the specified time window.

Intermetallic Control

Reaction rate between the liquid tin and the solid copper is extremely high, and the growth of the intermetallic layer is directly proportional to the time above liquidus. As the solder melts, copper atoms dissolve into the liquid and react to form the Cu6Sn5 phase. A longer dwell time allows more copper to dissolve, leading to a thicker and more irregular intermetallic layer.

While some growth is necessary for a strong metallurgical bond, an overly thick layer is a major reliability risk because it is brittle and prone to cracking under mechanical shock. Most high quality manufacturing specifications limit the initial intermetallic thickness to less than three micrometers, which is difficult to achieve if the time above liquidus is not tightly controlled. Furthermore, an extended liquid state can lead to the formation of large, needle-like crystals that weaken the bulk solder and reduce its fatigue life.

Keeping the dwell time toward the lower end of the recommended range is generally preferred for high reliability applications.

Reflow Window

Selection of the ideal profile depends on the complexity of the board and the requirements of the components. A wide reflow window for the time above liquidus provides more flexibility in the manufacturing process, but modern high-density boards often have a very narrow window due to the presence of both tiny resistors and large thermal sinks. If the difference in temperature between the hottest and coldest parts of the board is too large, it becomes impossible to keep all joints within the target time range.

This is often solved by increasing the soak time or using a more sophisticated reflow oven with better airflow control. After the profile is established, it is locked into the production system and monitored daily to ensure that no drift occurs. Any change in the paste manufacturer or the component supplier may require a re-evaluation of this parameter to maintain the same level of quality.

The final verification of the time above liquidus is a key part of the process validation record, providing the proof that the assembly was manufactured under optimal conditions.