玻纤效应对高速信号的影响

The impact of fiberglass weave on high-speed signal transmission

PCB信号传输的高频和高速化发展对印制电路板材料的选择、设计及制作提出了更高的要求，尤其是100G骨干网的发展，印制电路板上差分阻抗线要实现25Gbps的传输速率。当系统总线上的信号速率提升到Gbps级别时，之前电子工业界认为的PcB介质层是均匀的假设不再适用，PcB介质层是由嵌在环氧树脂中的玻纤束交织混合而成，玻璃纤维束之间的间隙会导致介质层相对介电常数的局部变化（玻纤效应），在高速传输时会对信号产生不可忽视的影响。文章分析探讨了不同规格玻纤布的玻纤效应对传输线阻抗波动的影响及其对差分信号Skew（偏斜失真）值的影响程度，对比了扁平玻纤布及NE-Glass玻纤布对阻抗波动及差分Skew的改善效果，并分析了不同布线方式对Skew4~的改善，提出了减少差分Skew的处理方法，可为高速信号传输的选材及设计优化提供依据。

The development of PCB signal transmission towards high frequency and high speed has raised higher requirements for the selection of PCB materials, design and manufacturing, specially with the development of 100G backbone network, the transmission rate of differential traces on printed circuit board up to 25 Gbps. When the system bus signal transmission rate reach Gbps level, the homogeneous medium theory which was recognized in low data rate field could be no more applicable. The dielectric layer of PCB is composed of epoxy resin and fiberglass weave, and the relative dielectric constant of dielectric layer would be fluctuant local, which was caused by the gap between fiberglass weave（fiberglass weave effect）. So the medium for signal propagation is non- homogeneous, and the influence on signal cannot be ignored in high-speed transmission. In this paper, the effect of fiberglass weave on the impedance of transmission line was investigated, and the effect on differential Skew value was also discussed. Besides, the effect of flat fiberglass cloth and NE-Glass cloth on the improvement of impedance fluctuation and Skew value were analyzed, and the impact of wiring mode on the Skew value was studied. The methods used to quantify the effect of fiberglass and strategies to mitigate it were proposed, which offer foundation for the selection of high-speed materials and design optimization.