Impact of surface treatment on high frequency signal loss characteristics

In multi-layer printed circuit board (PCB) manufacturing, adhesion promoters are applied between the inner layer copper surface and prepreg resin to ensure superior bonding reliability. Traditional adhesion promoters used over the last decade include processes such as black oxide and oxide replacement processes. Both inner layer bonding enhancement processes apply a coating or chemically etch the copper surface to create certain amount of roughness and provide mechanical bonding. Most oxide replacement processes also cover the roughened surface with organic coatings that further improves the inner layer adhesion through chemical bonding as well. While the conventional inner layer adhesion promoters pose no major functionality issues to the manufacturing of commodity PCBs, the impact on the electrical performance for high frequency application is a concern. Over the past decade, the remarkable advances in the mobile phone industry and wireless networks have pushed the high frequency industry to uncharted territories. With the emergence of 3G cellular network in China, the demand for high frequency multilayer boards will continue to increase. As the amount of data transfer increases, the signal frequency also increases accordingly. It is also known that as operating frequency approach 1 GHz, electrical signals no longer pass through the bulk of the copper track but instead travel through the skin that closely follows the track contour. As the "skin effect" phenomenon is observed, any small changes in surface area or roughness can result in significant impact on the impedance and consequently in signal loss. By maintaining a smooth and consistent track surface, non-etching adhesion promoter type processes can adhere to a more stringent track impedance requirement and provide a reduced signal loss at higher operating frequencies. This paper discusses a number of simulation and modeling investigations in which the effect of a non-etching adhesion promoter is directly com- pared with an etch-based process. The paper explores specifically the influence of surface roughness on the high frequency signal loss characteristics of a variety of track geometries. The study clearly shows clear advantages of non-etching bonding technologies over the conventional surface roughening process when operating at higher frequencies.