Why is there a hole in the board after PCB pressing?

Resin cavities with the vacuum of the press has a relationship with the air is not pumped out residue in the resin, there is a reason is the flow of resin, this problem should take into account many factors, PP sheet glue content, PP sheet whether the expiration date, the pressure of the temperature changes in the press, the pressure of the press, and other reasons is that you have no impurities inside the PP sheet ah, anyway, there is a problem with a lot of factors, the first to seize! The main problem to see, first of all go to see whether the vacuum of the press is abnormal, and then test the press temperature rate is normal, and then look at the PP sheet there is no problem ......