Editing Reflow soldering (section)

==Reflow zone==
{{more citations needed section|date=September 2012}}
The third section, the reflow zone, is also referred to as the "time above reflow" or "temperature above liquidus" (TAL), and is the part of the process where the maximum temperature is reached. An important consideration is peak temperature, which is the maximum allowable temperature of the entire process. A common peak temperature is 20–40&nbsp;°C above liquidus.<ref name="indiumcorporation"/> This limit is determined by the component on the assembly with the lowest tolerance for high temperatures (the component most susceptible to thermal damage). A standard guideline is to subtract 5&nbsp;°C from the maximum temperature that the most vulnerable component can sustain to arrive at the maximum temperature for process. It is important to monitor the process temperature to keep it from exceeding this limit. Additionally, high temperatures (beyond 260&nbsp;°C) may cause damage to the internal [[die (integrated circuit)|die]]s of [[Surface-mount technology|SMT]] components as well as foster [[intermetallic]] growth. Conversely, a temperature that is not hot enough may prevent the paste from reflowing adequately.

[[File:Mark5 1826 Reflow Oven.jpg|thumb|An example of a commercial reflow oven.]]
<ref>{{cite web|last=Girouard|first=Roland|title=Mark5 Reflow Oven|url=https://hellerindustries.com/convection-reflow/|work=Heller Industries Website|publisher=Heller Industries Inc|accessdate=11 March 2024}}</ref>


Time above liquidus (TAL), or time above reflow, measures how long the solder is a liquid. The flux reduces surface tension at the juncture of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine. If the profile time exceeds the manufacturer's specification, the result may be premature flux activation or consumption, effectively "drying" the paste before formation of the solder joint. An insufficient time/temperature relationship causes a decrease in the flux's cleaning action, resulting in poor [[wetting]], inadequate removal of the solvent and flux, and possibly defective solder joints. Experts usually recommend the shortest TAL possible, however, most pastes specify a minimum TAL of 30 seconds, although there appears to be no clear reason for that specific time. One possibility is that there are places on the PCB that are not measured during profiling, and therefore, setting the minimum allowable time to 30 seconds reduces the chances of an unmeasured area not reflowing. A high minimum reflow time also provides a margin of safety against oven temperature changes. The wetting time ideally stays below 60&nbsp;seconds above liquidus. Additional time above liquidus may cause excessive intermetallic growth, which can lead to joint brittleness. The board and components may also be damaged at extended temperature over liquidus, and most components have a well-defined time limit for how long they may be exposed to temperatures over a given maximum. Too little time above liquidus may trap solvents and flux and create the potential for cold or dull joints as well as solder voids.