Thermal Duration
Heat exposure intervals within a solder reflow oven determine the formation of the intermetallic bond between the component and the pad. Convective reflow dwell time specifies the number of seconds a circuit assembly remains at or above the liquidus temperature of the solder alloy. This duration ensures that all joints across the board reach a sufficient state of fusion regardless of their thermal mass.
Process Window
Maintaining a specific convective reflow dwell time is a requirement for creating reliable connections in surface mount technology. If the interval is too short, the solder may not fully wet the surfaces, leading to cold joints or poor mechanical strength. Conversely, an excessive period above liquidus encourages the growth of brittle intermetallic layers or damages heat-sensitive components like electrolytic capacitors.
Engineers use a thermal profiler with thermocouples attached to various points on a test board to measure this time. The data collected allows for the adjustment of conveyor speed and zone temperatures to keep the process within the limits defined by the solder paste manufacturer.
Performance Limit
The window for success is narrow and depends heavily on the specific alloy used. For leaded solder, the dwell is typically shorter than for lead free alternatives due to the lower melting point.
