In the reflow soldering oven process, we often divide them into four stages: preheating, constant temperature, reflow, and cooling. Each stage has its important meaning. ETA will discuss the operation of the four temperature zones of reflow soldering with you. Method and specific role.
1. The role of reflow oven preheating zone
Preheating is to activate the solder paste, and to avoid the rapid high temperature heating during the immersion tin, which is the heating action performed to cause defective parts. It is to heat the PCB at room temperature as soon as possible to achieve the second specific goal, but the heating rate should be controlled within an appropriate range. If it is too fast, thermal shock will occur, and both the circuit board and components may be damaged; if it is too slow, the solvent will evaporate. Insufficient, affecting welding quality. Due to the faster heating rate, the temperature difference in the latter section of the SMA is larger. In order to prevent thermal shock from damaging the components, the maximum speed is generally specified as 4°C/s. However, the ascent rate is usually set to 1-3°C/s. The typical heating rate is 2°C/s.
2. The role of reflow ovens holding area
The main purpose of the heat preservation stage is to stabilize the temperature of the components in the reflow furnace and minimize the temperature difference. Give enough time in this area to make the temperature of the larger component catch up with the smaller component, and to ensure that the flux in the solder paste is fully volatilized. At the end of the heat preservation section, the oxides on the pads, solder balls and component pins are removed under the action of the flux, and the temperature of the entire circuit board is also balanced. It should be noted that all components on the SMA should have the same temperature at the end of this section, otherwise, entering the reflow section will cause various bad soldering phenomena due to the uneven temperature of each part.
3. The role of PCB reflow soldering oven zone
In this area, the heater temperature is set to the highest, so that the temperature of the component rises quickly to the peak temperature. In the reflow section, the peak soldering temperature varies depending on the solder paste used. Generally, the melting point temperature of the solder paste plus 20-40°C is recommended. For 63Sn/37Pb solder paste with a melting point of 183℃ and Sn62/Pb36/Ag2 solder paste with a melting point of 179℃, the peak temperature is generally 210-230℃, and the reflow time should not be too long to prevent adverse effects on SMA. The ideal temperature profile is the smallest area covered by the "tip area" that exceeds the melting point of the solder. In the reflow soldering area, pay special attention to the reflow time not to be too long, to prevent damage to the reflow furnace, it may also cause poor functions of the electronic components or cause the circuit board to be burnt.
4. The role of SMT reflow oven cooling zone
At this stage, the temperature is cooled to below the solid phase temperature to solidify the solder joints. The cooling rate will affect the strength of the solder joint. In this section, the lead-tin powder in the solder paste has melted and fully wetted the surface to be connected. It should be cooled as fast as possible, which will help to obtain bright solder joints with good appearance and low contact. angle. If the cooling rate is too slow, it will cause excessive eutectic metal compounds to be produced, and large grain structures are likely to occur at the solder joints, which will reduce the strength of the solder joints. The cooling rate in the cooling zone is generally about 4℃/S, and it is cooled to 75℃ That's it.