Simulation of differential skew considering fiber kink effects

Abstract

As a result of increasing signal transmission rates to as high band levels as several tens of Gbps, skew induced by the difference in dielectric constant between the glass-cloth and the resin is posing a huge problem. However, due to the extremely difficult comparison of this skew between actual measurements and simulations, few studies on such comparison have been reported to date. We developed a new analysis technique to clarify the transmission delay time difference (skew) in differential signal transmission lines, depending on the positional relationship between the glass-threads and the conductor lines in a circuit board. This analysis technique has the following four characteristics. The first is that the angle of the lines to the glass-cloth is expressed by cascade connections of multiple analytical models that are different in the positional relationship between the glass-cloth and the lines. The second is that analytical models are combined in cascade connection assuming that the positional relationship between the lines and the glass-cloth appears randomly and is distributed uniformly. The third is that analytical models are prepared assuming that the angle between the lines and the glass-cloth is distributed uniformly within a certain range due to fiber kink effects. The fourth is that the assumption that skew in stripline structure, which has two glass-cloth insulating layers right above and below the lines, can be calculated by the sum of two sets of the skew induced by one glass-cloth-containing insulating layer. As a result of comparison between analysis and actual measurement results, it was confirmed that this analysis technique could reproduce skew distributions observed in real circuit boards with a high degree of accuracy.As a result of comparison between analysis and actual measurement results, it was confirmed that this analysis technique could reproduce skew distributions observed in real circuit boards with a high degree of accuracy.