Cause analysis and solution of wicking problem in reflow soldering

1: The positive temperature of reflow soldering is higher than the back temperature

During the reflow process, the reflow temperature shall be set according to the PCB material, size, component specification and component density Of course, it also depends on whether it is a double-sided soldering process If the PCB is thin and small in size, 128pin or larger QFP must be attached during the double-sided soldering process This problem should be considered when setting the temperature

This is because during the double-sided soldering process, it is necessary to consider that the back components may have high parts or even drop parts due to the secondary reflow Therefore, the back temperature setting should be slightly lower than the positive temperature However, if the setting is improper and the back temperature is too much lower than the positive temperature, the result is that the PCB is heated less and the component is heated more. Due to the fluidity of the solder after melting, the solder will not adhere to the pad, but climb to the pins of the QFP with high temperature. The phenomenon is that the pad has less tin and the pins of the QFP are bridged with tin

2: Pad oxidation

If the OSP PCB is not put into production on the BOT surface in time (24 hours) after the top surface is completed, the pad will be oxidized, and the tin eating capacity of the pad will be seriously reduced during soldering Of course, the soldering tin will climb onto the pins of the components, causing the pad to connect the pins of the components with less tin

The above two countermeasures:

1: When setting the reflow temperature, the difference between the positive temperature and the back temperature shall be controlled at 15 degrees, and the measured temperature difference shall be controlled at 5 degrees

2: Two production lines are used, and the BOT surface is put into production immediately after the top surface is completed If two lines cannot be opened, the PCB produced shall be subject to moisture-proof control