PCB preheating temperature is too low or the preheating time is too short. The solvent and moisture in the solder do not evaporate.Raise preheat temperature or extend preheat time.
Cause of formation:
（1）PCB dampened during manufacturing or storage.
（2）High humidity and moisture condense on the multi-slit PCB, and no measures for moisture detection were taken in the plant.
（3）Coating and flux do not dissolve in each other.
Leakage flux or coating areas, flux moisture absorption and water.
（4）Poor soldering layer, coated with solder residue.
Poor processing of substrate, and accumulation of tank liquid caused by rough hole wall. No analysis was made in PCB design.
（5）Improper preheating temperature.
（6）Silver-plated pieces are dense.
（7）Improper selection of wave crest of solder.
（1）Change PCB storage conditions to reduce dampness.
（2）Choose proper flux.
（3）Spray the flux evenly and increase the preheating temperature.
（4）Change the PCB design to analyze the uniformity of heating.
（5）Open wave shaping PCB spot.