Copper Plating Method on Flat Surface for High Frequency Signal Transfer

A copper plating process for printed wiring boards operating in the GHz frequency range has been developed that improves the adhesion strength between the copper interconnects and the resin substrate by using an imidazole compound to form coordinate bonds. The coordinate bonds are prepared by impregnating the resin substrate with an aqueous solution of amino-group-containing imidazole (AI). The adhesion strength is measured by a tensile test with various concentrations of AI solution and different treatment times of impregnation. As a result, the highest adhesion strength of 4.2–5.1 N·cm-1 can be obtained on the entire surface of a 340×340 mm2 substrate, where the flatness is down to 58.7 nm. This result is due to the optimization of the impregnating condition for AI and of the removal condition for weak boundary layers. The signal transmission characteristics of the interconnects on the substrates fabricated by this processes are compared with the conventional process by S 21 parameter extraction in the GHz frequency range. The transmission characteristics of this process are markedly improved compared with those of conventional processes.