Quantifying Microstrip Phase Delay Variance Driven by Prepreg Glass Yarn Geometry
Prepreg glass bundle spacing causes local permittivity shifts that introduce picosecond phase delay variance in high-speed microstrip traces.
Transmission Line Propagation Velocity
Signal Speed
Rate at which an electromagnetic signal travels through a conductor on a circuit board determines circuit timing. The transmission line propagation velocity depends on the dielectric properties of the surrounding substrate. This speed is always less than the speed of light in a vacuum.
Physical Determination
Effective permittivity of the dielectric medium surrounding the conductor is the primary factor governing this signal speed. When the signal propagates along a microstrip on the outer layer, it travels through both the board laminate and the surrounding air, resulting in an effective dielectric constant that increases the transmission line propagation velocity. Stripline traces on inner layers are fully embedded in the laminate, experiencing a higher effective permittivity and therefore a slower signal speed.
Calculating this velocity precisely is necessary for matching the trace lengths of parallel buses.
System Consequence
Timing margin and signal synchronization in high-speed digital buses are directly affected by the propagation speed of the traces. If two lines in a parallel memory bus have different lengths or run through different dielectric materials, the varying transmission line propagation velocity causes the signals to arrive at different times. This skew can lead to data setup or hold time violations, causing errors in high-speed memory interfaces like DDR.
Designers employ trace length matching and choose homogeneous laminate materials to ensure that all signals in a bus arrive at their destination simultaneously. High-speed processor interfaces require this level of timing control.