Analysis of the Use of Temperature Zones in Reflow Welding Equipment

Reflow soldering temperature curve refers to the curve of the temperature on a certain pin on the component over time when the SMD component passes through the reflow soldering furnace. The temperature curve is a function of the temperature applied to the assembly component over time, Y=F (T), reflected as a curve of the temperature at a given point on the printed circuit board changing over time during the reflow process. The reflow soldering temperature curve you see is divided into four temperature ranges, so what are the functions of these four temperature ranges? JEENOCE shares it here.

1、 Effect of preheating section on reflow soldering temperature curve

The purpose of this temperature range is to heat the circuit board at room temperature as soon as possible, but rapid heating cannot be fast enough to cause damage to the board or parts and result in the loss of solvent in the flux. The usual heating rate is 1-3 ℃/second. In actual production, it is not necessary to ensure that the curves of each selected point achieve a relatively ideal situation. Sometimes, due to differences in component density, high temperature withstand, and significant differences in thermal characteristics, or due to differences in sheet metal and limitations in reflux furnace capacity, the temperature curve of some points cannot meet the requirements. In this case, it is necessary to consider the impact of each component on the entire circuit board function and select favorable reflux parameters.

2、 Effect of reflow soldering temperature curve insulation section

The boiling point of the solvent is between 125 and 150 ℃. Starting from the insulation section, the solvent will continue to evaporate, and the resin or rosin will begin to soften and flow at 70 to 100 ℃. Once melted, the resin or rosin can quickly diffuse on the welded surface. The active agent dissolved in it will flow and react with the surface oxide of the lead tin powder to ensure that the lead tin powder is clean during fusion welding in the welding section.

The main purpose of the insulation section is to ensure that all components on the circuit board reach the same temperature before entering the welding section. The heat absorption capacity of components on the circuit board usually varies greatly, and sometimes the insulation period needs to be extended. However, too long insulation period may lead to the loss of soldering flux, which can be fully combined and wetted in the fusion welding area, weakening the soldering ability of the solder paste. If the temperature rise rate is too fast, it can lead to rapid vaporization of the solvent, It may cause defects such as blowing holes and solder beads, and a short insulation period may not fully utilize the active agent. It may also cause an imbalance in the preheating temperature of the entire circuit board, resulting in defects such as no soldering, post soldering disconnection, and solder joint voids. Therefore, the length and temperature value of the insulation period should be determined based on the design of the circuit board and the convective heating capacity of the reflux furnace.

The temperature of the insulation section of the general reflow welding temperature curve is between 100 and 160 ℃, with an increase rate of less than 2 degrees per second. A platform of about 0.5 to 1 minute at around 150 ℃ helps to reduce the end area of the welding section to a small size.

3、 Effect of reflow zone on reflow soldering temperature curve

The high temperature in the reflux section is 245 degrees Celsius, and the low temperature is 200 degrees Celsius. The time to reach the value is approximately 35/S; The heating rate of the reflux zone is 45 degrees/35S=1.3 degrees/S. According to (how to set the temperature curve correctly), it can be seen that the time for this temperature curve to reach its value is too long. The entire reflux time is approximately 60 seconds.

4、 Reflow soldering temperature curve cooling section effect

The cooling time of the reflow temperature curve is about 100S, and the temperature drops from 245 degrees to about 45 degrees. The cooling speed is: 245 degrees to 45 degrees=200 degrees/100S=2 degrees/S.