Flexible circuit board and process for producing the same

A method of manufacturing a flexible circuit board, comprising:

• providing a substrate made of polyimide having an upper surface and a lower surface;
• forming a polyamic acid layer on the surface of the substrate, and conducting a first pre-curing process on the polyamic acid layer such that the polyamic acid layer is semi-cured;
• depositing a photoresist on the polyamic acid layer, and conducting a second pre-curing process on the photoresist and the polyamic acid layer;
• exposing and developing the photoresist according to a circuit configuration diagram to partially remove the photoresist and the polyamic acid layer thereunder, partially revealing the surface of the substrate and leaving a remaining photoresist and a remaining polyamic acid layer thereunder, wherein the remaining polyamic acid layer and the remaining photoresist jointly defines a compartment having an inner wall surface including at least a side wall and a bottom wall;
• curing the polyamic acid layer which becomes a polyimide layer, forming a precursor substrate;
• using a basic agent with a pH value between 11 and 12 to treat the bottom wall of the substrate for the side wall of the polyimide layer to include a chemical group O═C—N—C═O and one carbon-nitrogen bond of the chemical group is cleaved, forming a ring-opened polyimide with a carboxyl group in the substrate;
• forming a palladium adhesion enhancing layer on the side wall and the bottom wall by using a palladium adhesion enhancer, wherein the palladium adhesion enhancer of the palladium adhesion enhancing layer is chemically bonded with carboxyl group of the ring-opened polyimide;
• forming a first electrically conducting layer on the palladium adhesion enhancing layer to fix the first electrically conducting layer on the bottom wall and the side wall;
• removing the remaining photoresist and revealing the polyamic acid layer thereunder;
• plating a second electrically conducting layer on the surface of the precursor substrate, selectively forming an electric circuit with the first electrically conducting layer and the palladium adhesion enhancing layer in the compartment of the precursor substrate;
• and
• forming an electrically insulating layer on the surface of the precursor substrate to cover the electric circuit and the polyimide on the surface of the precursor substrate.