SMT reflow soldering is generally divided into four stages: preheating zone, insulation (soaking) zone, reflow zone, and cooling zone. The passing time of the entire reflow soldering process is usually controlled within 3-7 minutes. However, this is not an absolute fixed value and may vary due to various factors.
1、 Each stage is determined by time and function
1.1 Preheating zone:
Time: The preheating zone usually takes 60-120 seconds.
Function: The purpose of this stage is to slowly raise the temperature of the PCB board from room temperature to the temperature at which the flux begins to activate, usually reaching 135 ℃ -150 ℃. The heating rate during this stage should be controlled at 1-3 ℃/s. By slowly increasing the temperature, volatile solvents in the solder paste can be effectively removed, reducing thermal impact on components, and activating the flux initially. At the same time, the temperature difference between different parts of the PCB board can be reduced.
1.2 Insulation (uniform heat) zone:
Time: The duration is usually 60-120 seconds.
Function: At this stage, maintain the PCB board within a specific temperature range, typically between 150 ℃ and 180 ℃ (adjusted according to the type of solder paste). This allows the components in various areas of the PCB board to reach the same temperature, further reducing the relative temperature difference, ensuring that the flux is fully utilized, removing oxides from the surface of the component electrodes and solder pads, laying a good foundation for subsequent soldering, and preventing the PCB board from suddenly entering the high-temperature soldering area and being damaged.
1.3 Reflux zone:
Time: The time for the reflow zone to be above the melting point of the solder is generally between 30-60 seconds, but there are also data showing 10-30 seconds or 45-90 seconds. The duration of peak temperature is crucial and requires strict control.
Function: In this stage, the temperature of the PCB board needs to be raised above the melting point of the solder paste. The peak temperature of lead solder paste is 210 ℃ -230 ℃, and that of lead-free solder paste is 235 ℃ -260 ℃. At this temperature, the solder powder in the solder paste melts, and the liquid solder infiltrates, diffuses, and flows between the PCB pads and component pins, forming solder joints and completing the soldering of component electrodes and pads.
1.4 Cooling zone:
Time: The cooling time is usually between 70-150 seconds.
Function: In this stage, the PCB board temperature needs to be quickly reduced to below 100 ℃, and the cooling rate is usually set at 3-4 ℃/second. By rapid cooling, the solder joints solidify, forming a sturdy welding joint and improving welding quality. But the cooling rate should not be too fast, otherwise the solder joints may crack; It should not be too slow to avoid aggravating the oxidation of solder joints. The ideal cooling curve has a mirror relationship with the reflow zone curve, and the closer it is to this relationship, the tighter the solid structure of the solder joint and the higher the welding quality.
2、 Factors affecting the passage of time
1. Solder paste types: Solder pastes with different compositions and characteristics have different recommended reflow soldering temperature curves and stage times. For example, certain special solder pastes may require longer preheating or reflow times to ensure sufficient melting and chemical reactions.
2. PCB board material and size: The material of the PCB board determines its thermal conductivity, and the size affects its thermal capacity. PCB boards with good thermal conductivity and small size may have relatively fast heating and cooling rates, and may require shorter reflow soldering time; PCB boards with poor thermal conductivity and large dimensions may require longer time to achieve uniform temperature and complete the soldering process.
3. Component type and layout: Small components such as surface mount resistors and capacitors have small thermal capacity and relatively short soldering time, which may take 2-3 seconds; Large components such as electrolytic capacitors, inductors, or large-sized BGA chips have a large thermal capacity and require longer soldering time, possibly 5-8 seconds or even longer. If the layout of components on the PCB board is dense and heat transfer is affected, it may also be necessary to extend the reflow soldering time appropriately.
4. Reflow soldering equipment: Reflow soldering furnaces of different brands and models have differences in heating methods, number of temperature zones, length of temperature zones, and heating efficiency. Equipment with high heating efficiency, multiple and reasonable temperature zones may be able to complete high-quality welding in a shorter period of time; Some old or poorly performing equipment may require longer time to ensure welding effectiveness.