A study of high-density differential transmission line of the package board based on crosstalk reduction

In recent years, with improvement in the speed of digital equipment, differential transmission lines are used abundantly in the circuit substrate. However, by using the differential lines, the number of signal wires doubles as compared with the conventional single-ended. In this paper, the authors investigated the cross-sectional shape of the trace of printed circuit board, which can reduce the cross-talk interference or improve the wiring density. The three patterns of the cross-sectional shape of the trace are examined in this paper. The one is conventional rectangular shape, which the width of the trace is longer than the thickness (low aspect ratio), and the second pattern is a shape of high aspect ratio (the thickness of the trace is longer than the width), and the last trace pattern is a shape of square. The characteristic impedances of the all three trace patterns are designed to be 100 ohm in differential mode. Also, the cross sectional area is arranged to be same among the three trace patterns in order to make the transmission loss conditions are same. As a result, we discovered that the amount of cross talks become minimum, when the cross sectional form of the trace is square. In verification of this idea, the authors simulated for three patterns of cross-sectional shapes of signal traces by the HSPICE of circuit analysis software and the HFSS of three-dimensional electromagnetic-field analysis software.