Effect of Surface Treatment of Transmission Line on Frequency Characteristics

Because of the considerable increase in internet traffic, the bitrate of optical transceivers has increased. In recent years, 56 Gbps (28 Gbaud PAM4) interfaces have already been deployed for commercial use, and 112 Gbps (56 Gbaud PAM4) signals have begun to be discussed by several standard development organizations. With increase in the bitrate, technologies for accurately analyzing the effects of the surface treatment of transmission lines on printed circuit boards are becoming more important. This is due to the fact that at high frequencies, the skin effect, in which the current concentrates on the surface of the pattern, becomes relatively significant. This study fabricated test boards for measuring transmission lines with three types of surface treatments, which were plated with only Cu foil, ENEPIG, and IGEPIG. The transmission lines were measured using a VNA and high-frequency probes. The validity of results was confirmed using an electromagnetic field simulator based on the finite-difference time-domain method.