1. The occurrence of bubbles during the reflow soldering process of a four-layer PCB at a temperature of 240°C may be attributed to several factors.2. One possible cause is the lack of pre-baking of the PCB material before reflow soldering, which can lead to the expansion of volatile substances within the material and the formation of bubbles during the soldering process.3. Issues with the laminating structure, such as inadequate adhesion between layers or uneven pressure distribution, may also result in delamination and bubble formation at high temperatures.4. If the PCB is shipped immediately after completion without adequate baking, it may be prone to bubbling during transportation or usage due to temperature changes.Preventive and control measures:1. Ensure that the PCB material undergoes appropriate pre-baking to remove volatile substances before reflow soldering, thus preventing bubble formation during the process.2. Optimize the laminating process to ensure strong and uniform adhesion between layers, reducing the risk of delamination at high temperatures.3. After PCB fabrication, thorough baking should be conducted to eliminate potential bubbling hazards, ensuring the product is free from quality issues upon shipment.