When copper foil and substrate is not fully bonding together, air bubbles can appear between them, which may cause short circuits or broken circuit.

Stainless steel plate with blow holes
There will be a lack of pressure between copper foil and substrate because of the existing blow holes, finally leading to the formation of air bubbles. Steel plate must be replaced when such a problem is found.

Stainless steel plate with long scratches
Stainless steel plate with long scratches on the surface will cause air bubbles by strip heaving on the scratch location of steel plate. Minor surface abrasions are easily buffed-out with sandpaper, and treated steel plate has no effect on the CCL surface, but steel plate with deep scratches must be replaced.

Self-detecting of adhesive sheet
Adhesive sheet with local creases will cause a reduction of resin power, thus a non-uniform flow of resin leads to poor air elimination. When separating from the copper foil, the substrate shows white where the air bubbles are. In this case, the adhesive sheet with creases should be put in the inner layer so that resin from both-sides can provide for copper foil in the pressing process.

Adhesive sheet with high volatility
When heating adhesive sheet, melting resin will prevent the evolution of volatiles, and finally air bubbles are formed. This problem can be solved with air eliminating by a non-vacuum pressing machine, reducing the press force properly at first, and increase pressing slightly over a period, repeating several times.