Reasons for non melting and drying of reflow solder paste

After reflow soldering, the solder paste on the circuit board may sometimes experience incomplete melting, and there may be residual solder paste around all or partial solder joints that has not melted; There are also issues that are prone to dryness. JEENOCE is here to analyze with you.

1、 Reasons for solder paste not melting after reflow soldering

This problem may be caused by low reflow temperature or short soldering time, or it may be caused by excessive heat absorption or blocked heat conduction of components in the PCB board. At the same time, the quality of solder paste is also a major factor in the occurrence of this phenomenon. The better the quality of solder paste, the less likely it is to occur.

Solution

1. Adjust the temperature curve, the peak temperature should be 30-40 ℃ higher than the melting of the solder paste, and the reflux time should be 30-60 seconds;

2. Try to place the PCB board in the middle of the furnace for welding;

3. Do not use inferior solder paste and establish a management mechanism for solder paste use;

4. When designing on both sides, try to place large components on the same side of the PCB as much as possible. If it is determined that the layout cannot be separated, it should be staggered.

2、 After reflow soldering, the solder paste will dry and not melt

Solder paste is only used in a small area during the reflow soldering process, and it is easier to dry out than the solder paste in the solder paste box. At this point, the solder paste will not melt and the flux cannot cover the solder joint, resulting in poor soldering. At the same time, trace amounts of solder paste are easier to heat transfer, and high temperatures actually make the solder paste less likely to melt. So we can adjust the reflow temperature curve appropriately to solve this problem, or welding in a nitrogen environment is a good way to solve this problem.

On the other hand, the non melting of solder paste is also due to the presence of volatile flux in its own composition, which is also the reason why solder paste is prone to drying. Among them, the flux with the highest content of solder paste is rosin, which contains a large amount of rosin acid, which is prone to losing its activity at high temperatures. So, the temperature during the welding process should be controlled to ensure a temperature of around 200 ℃, neither too high nor too low is suitable. At the same time, the quality of the thixotropic agent can also lead to the solder paste being prone to drying. Poor quality of the thixotropic agent can affect the viscosity of the solder paste, and the solder paste with high viscosity is prone to drying. So, choosing high-quality solder paste can effectively solve the problem of solder paste drying easily.