Gauss Stack – Differential Skew Simulation

What is Skew?

To put it very simply, skew (also known as “timing skew” or “clock skew”) is a phenomenon where two signals arrive at their destinations at different intervals. This becomes of serious concern anytime you’re working on signals that are supposed to be synchronous, such as anytime you’re working with differential pairs. Fiber weave related skew occurs in Printed Circuit Boards because the dielectric materials are typically woven-glass composites, within which the Dielectric Constant (Dk) varies spatially. Because the Dk dictates the speed of electromagnetic waves through a medium, variation in Dk means variation in propagation speed of a signal, which is why 2 signals, even if they travel the same distance, may arrive at different times, based on the particular paths they took. This becomes a critical issue when dealing with very high speed signals, where the tolerance for skew is a percentage of the unit interval.

Fiber Weave Effect

Essentially, the problem of skew in PCB differential transmission lines, by and large boils down, to the Fiber Weave Effect. The weave of the glass fibers in the dielectric materials used in a PCB is responsible for the spatial variations in Dk in each dielectric layer, because the dielectric constants of the resin and glass are typically dissimilar (glass Dk is higher than that of the resin). In a plain weave, the percentage of the glass at the knuckles, which are where the warp threads cross over the weft or vice-versa, is highest and, as a result, the Dk is also at a high point. The lowest Dk in a weave would be in open areas, just filled with resin (no glass). Spread glass mitigates this phenomenon by closing the open gaps, but the concentration still cannot be uniform, as long as the glass and resin have different Dk values. Use of materials with low Dk glass, as well as the use of multiple plies, also helps mitigate the fiber weave effect.

The Skew / Fiber Weave Effect Problem

A key problem when working on high frequency, high data rate designs is that your skew budgets become very small, due to the reduced duration of a unit interval, and you don’t have any means to predict or design for skew. Being a stochastic phenomenon (by nature, due to the alignment of fiber weave and traces), even measuring skew doesn’t give you a meaningful picture, since skew can only really be understood in terms of a distribution. So, what can you do to characterize skew? Build & measure hundreds of differential pairs for each new design you work on? Of course, that’s not feasible, so you’re left with some rule-of-thumb skew mitigation techniques and hoping that your skew looks more like the distribution on the left than the one below. Or at least that’s how it used to be…

Stochastic Skew Simulation in Gauss Stack

In Gauss Stack, you can simply specify your stackup (choose your materials and constructions), pick your copper layer and specify the trace width/gap and the orientation of the trace with regard to the warp and weft directions of the glass weave and, with the click of a button, through a stochastic Monte Carlo simulation, build a distribution and confidence intervals for the Maximum Skew on your differential pair. Now, you can simulate and design for skew at the stackup design stage to find something that works for your design! Some applications may still require additional skew mitigation approaches, like the use of rotation and/or re-timers, but the power to predict skew can help you find the solution that either works out the gate or gets as close as possible independent of the use of these additional techniques.

Predict Skew with Gauss Stack Today!