The inner layer process is to form inner layer circuit of PCBs. Before starting, a surface pre-treatment is carried out to remove impurities and increase surface roughness of the copper foil. Inner layer process includes dry film lamination, exposure and DES. First, the dry film is applied to prevent plating and etching in the subsequent processes. Next, the process of inner layer image exposure is to transfer the designed circuit image to the required copper surface. After exposure, the board goes through an integrated system that develops, etches, and strips the inner layers.