New Copper Surface Treatment for Polyimide Multilayer Printed Wiring Boards

A new treatment method for the copper innerlayers of polyimide multilayer printed wiring boards has been developed. Conventional oxide coatings experience acid penetration through the bonding interface during through‐hole plating pretreatment. This problem was eliminated by substitution of metallic copper for the surface oxide. Promotion of the copper innerlayer adhesion to the prepreg by the oxide coating was based upon a mechanical interlocking effect caused by the minute roughness of the oxide crystals. Reduction treatment of the surface oxide layer was found to give a metallic copper surface with no changes in its morphology. Adhesion strength of polyimide prepregs to copper foils after the reduction treatment was equivalent to that of the original brown oxide coating. Acid resistance was enhanced by elimination of the oxide layer from the bonding interface. The reduction treatment, combined with the conventional oxide coating technique, can realise high density multilayer printed wiring boards with greater reliability and performance.