Chengyuan reflow soldering temperature zone is mainly divided into four temperature zones: preheating zone, constant temperature zone, soldering zone, and cooling zone.
1. Preheating zone
Preheating is the first stage of the reflow soldering process. During this reflow phase, the entire circuit board assembly is continuously heated towards the target temperature. The main purpose of the preheat phase is to bring the entire board assembly safely to pre-reflow temperature. Preheating is also an opportunity to degas the volatile solvents in the solder paste. In order for the pasty solvent to drain properly and the assembly to safely reach pre-reflow temperatures, the PCB must be heated in a consistent, linear fashion. An important indicator of the first stage of the reflow process is the temperature slope or temperature ramp time. This is usually measured in degrees Celsius per second C/s. Many variables can affect this figure, including: target processing time, solder paste volatility, and component considerations. It is important to consider all of these process variables, but in most cases the consideration of sensitive components is critical. “Many components will crack if the temperature changes too quickly. The maximum rate of thermal change that the most sensitive components can withstand becomes the maximum allowable slope.” However, the slope can be adjusted to improve processing time if thermally sensitive elements are not used and to maximize throughput. Therefore, many manufacturers increase these slopes to a maximum universal allowable rate of 3.0°C/sec. Conversely, if you are using a solder paste that contains a particularly strong solvent, heating the component too quickly can easily create a runaway process. As volatile solvents outgas, they may splash solder from pads and boards. Solder balls are the main problem for violent outgassing during the warm-up phase. Once the board is brought up to temperature during the preheat phase, it should enter the constant temperature phase or pre-reflow phase.
2. Constant temperature zone
The reflow constant temperature zone is typically a 60 to 120 second exposure for removal of solder paste volatiles and activation of the flux, where the flux group begins redox on the component leads and pads. Excessive temperatures can cause spattering or balling of the solder and oxidation of the solder paste attached pads and component terminals. Also, if the temperature is too low, the flux may not fully activate.
3. Welding area
Common peak temperatures are 20-40°C above liquidus. [1] This limit is determined by the part with the lowest high temperature resistance (the part most susceptible to heat damage) on the assembly. The standard guideline is to subtract 5°C from the maximum temperature the most delicate component can withstand to arrive at the maximum process temperature. It is important to monitor the process temperature to prevent exceeding this limit. In addition, high temperatures (over 260°C) may damage the internal chips of SMT components and promote the growth of intermetallic compounds. Conversely, a temperature that is not hot enough may prevent the slurry from reflowing sufficiently.
4. Cooling zone
The last zone is a cooling zone to gradually cool the processed board and solidify the solder joints. Proper cooling suppresses unwanted intermetallic compound formation or thermal shock to components. Typical temperatures in the cooling zone range from 30-100°C. A cooling rate of 4°C/s is generally recommended. This is the parameter to consider when analyzing the results of the process.
For more knowledge of reflow soldering technology, please view other articles of Chengyuan Industrial Automation Equipment
Post time: Jun-09-2023