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Design007-Nov2018

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84 DESIGN007 MAGAZINE I NOVEMBER 2018 by Chang Fei Yee KEYSIGHT TECHNOLOGIES This article studies the effect of PCB fiber weave on signal integrity in terms of mode conversion and differential channel loss due to intra-pair skew. The study used Keysight ADS 2DEM simulation to observe s-parameter (i.e., insertion loss and differential to common- mode conversion) and eye diagram for signal transmission at 1 Gbps and 10 Gbps. Introduction A PCB dielectric substrate is composed of woven fiberglass that is strengthened by epoxy resin. The microscopic top view of PCB sub- strates of fiber weave 106 and 3313 are illus- trated in Figures 1a and 1b. The thick lines in light brown color are fiberglass, while the square columns in black color are an epoxy resin. A higher-numbered configuration (e.g., 3313) denotes denser fiberglass weave. Fiberglass material features dielectric prop- erties that differ very much from the proper- ties of the epoxy resin. For instance, NE-glass fiber has a dielectric constant (Dk) and loss tangent (Df) of 4.4 and 0.0006, respectively; meanwhile, E-glass fiber has a Dk and Df of 6.6 and 0.0012. Epoxy resin has a Dk of 3.2, which is very different than that of fiberglass. When a substrate with sparse fiber weaving is used, PCB traces could cross different regions of resin and fiberglass more frequently. As a result, the speed or propagation delay of the signal changes frequently along the trace from transmitting to receiving end. The relationship between them is governed by Equation 1. Equation 1 v = signal's speed on PCB (in unit inches/ns) c = speed of light (12 inches/ns) D k = dielectric constant This phenomenon poses a critical challenge to multi-gigabit serial signal transmission. In the worst-case scenario, for example, the trace of a non-inverting signal could be routed on fiberglass without crossing the resin region Effects of PCB Fiber Weave on High-Speed Signal Integrity Figure 1a: Fiber weave with a 106 configuration. Figure 1b: Fiber weave with a 3313 configuration.

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