SMAThe main reason for the appearance of fingernail-sized bubbles after welding is that water vapor is entrained in the PCB substrate, especially the processing of multi-layer boards. Because the multi-layer board is made of multi-layer epoxy resin prepreg and then hot-pressed, if the epoxy resin prepreg storage period is too short, the resin content is not enough, and the pre-drying to remove the water vapor is not clean, it is easy to be hot-pressed. Entrained water vapor. It will also leave air bubbles due to insufficient glue content in the semi-solid sheet itself and insufficient bonding force between layers. In addition, after the PCB is purchased，      

     Because the storage period is too long, the storage environment is humid, and the patch is not pre-baked in time before the patch production, and the damped PCB patch is also prone to foaming.。

     Solution: After the PCB is purchased, it should be checked and accepted before it can be put into storage; the PCB should be pre-baked at (120±5) ℃ for 4 hours before the patch.