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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 an eye diagram for signal transmission at 1 Gbps and 10 Gbps.
A PCB dielectric substrate is composed of woven fiberglass that is strengthened by epoxy resin. The microscopic top view of PCB substrates of fiber weave 106 and 3313 are illustrated 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 properties that differ very much from the properties 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.
To read this entire article, which appeared in the November 2018 Design007 Magazine, click here.