New PCB Materials for Low Skew Close Phase Matching in 10Gbps Differential System Designs

A new process to weave, flatten and spread the glass reinforcement material has been developed to be used in high performance differentially routed printed circuit boards (PCB’s). In conventionally reinforced materials a granularity of dielectric constant is caused by the higher dielectric constant of the reinforcing glass fibers and the surrounding matrix of resin. These granularities are of the same order of magnitude as the differential trace parameters in terms of line spacing, trace width and trace pair separation. Consequently, one trace of the differential pair will have a different propagation velocity causing the different arrival times of the differential signal components. This asymmetry leads to common mode conversion of the signals and EMI as well as receiver detection difficulties. This paper describes the features and benefits of low cost processes for the modification of conventional PCB glass cloth reinforcing glass strands which mitigates the serious effects of intra-pair skew especially for 5, 6.25 and 10 Gigabit per second differential PCB trace designs.