During the whole reflow soldering process, the reflow soldering machine can be divided into 5 temperature zone processes. That is: 1. heating zone; 2. 2. Constant temperature zone; 3. Flux area; 4. Welding area; cooling zone. Here is a brief introduction to the specific functions of each temperature zone of the reflow soldering machine.
The function of reflow soldering heating zone: the heating zone is the first process, which is to make the temperature of each point on the PCBA enter the working state as soon as possible without damaging the product. The so-called working state starts to volatilize the components of the solder paste that are not helpful for soldering.
The role of reflow soldering constant temperature zone: the constant temperature zone plays two roles. One is constant temperature, which is to provide enough time for the temperature of the cold spot to "catch up" with the hot spot. When the temperature of the solder joint is closer to the hot air temperature, the heating rate will be slower. We use this phenomenon to make the temperature of the cold spot gradually approach the temperature of the hot spot. The purpose of making the temperature of the hot and cold spots close is to reduce the amplitude of the peak temperature difference when entering the fluxing and welding areas, so as to control the quality of each solder joint and ensure consistency. The second function of the constant temperature zone is to volatilize the unused chemical components in the solder paste.
The role of the reflow soldering flux zone: The fluxing process is when the active material (flux) in the solder paste comes into play. The temperature and time at this point provide the activation conditions required for the flux to clean the oxide.
The role of the soldering area of reflow soldering: when the temperature enters the soldering area, the heat provided is sufficient to melt the metal particles of the solder paste. Generally, the materials used for device soldering terminals and PCB pads have higher melting points than solder paste, so the starting temperature of this area is determined by the characteristics of solder paste. For example, for 63Sn37 solder paste, the temperature is 183oC. After the temperature rise exceeds this temperature, the temperature must continue to rise and maintain for a sufficient time to make the molten solder paste have sufficient wettability and be able to form an IMC with the solder terminals of each device and the PCB pads.
Reflow cooling zone function: The final cooling zone function, in addition to returning the PCBA to room temperature to facilitate the operation of the subsequent process, the cooling rate can also control the microcrystalline structure inside the solder joint. This affects the life of the solder joints.
