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Electromagnetic field solvers have traditionally been used by a small slice of engineers—the full-time signal integrity experts—and their use has been limited to the most demanding designs. But as design speeds increase, problems requiring field solvers are becoming more mainstream and field solvers are popping up all over the place, including free or inexpensive solvers available through an internet search. Many companies are claiming that their field solvers can be used by hardware design engineers without the SI background that solvers once required.
We asked Todd Westerhoff, product marketing manager for Siemens EDA, and Bill Hargin, CEO of Z-zero, to cut through the fog of field solvers and explain how engineers can ensure they avoid a “garbage in, garbage out” (GIGO) scenario when using field solvers to analyze their designs.
Andy Shaughnessy: Let’s start by defining a field solver. What exactly does a field solver do?
Bill Hargin: I think the simplest answer, Andy, is that a field solver is a black box that converts mechanical design parameters and material properties into a simulatable electrical model using Maxwell’s equations. You give a field solver a precise dimensional description of a structure you plan to build, along with the materials it will be built from, and it gives you back a model that you can either examine directly or use as part of a system simulation.
There are two broad classes of field solvers used in PCB design: 2D solvers, which are used to predict transmission line (signal trace) behavior under specific conditions, and 3D solvers, which are more complicated to use, but provide accurate answers under general-case conditions. 2D solvers have become mature enough that designers performing simulations may be using them without realizing it; they’re embedded parts of simulation processes in some tools.
Todd Westerhoff: Let’s take a step back. Why do we simulate our designs? Why do we simulate anything?
Hargin: So that we can predict the electrical behavior of our designs—the performance of our designs in advance.
Westerhoff: Precisely. We use simulation to predict whether something will work before we build it, so that we can debug and optimize our designs before we commit them to prototype fab. There’s really only one question, or two, that we ever care about: Will it pass or fail against requirements, and by how much? The “by how much” part is operating margin and it’s critically important. No simulation is ever 100% accurate; no matter how careful we are, we can’t model and predict everything. Every simulation and every simulation result has a margin of error associated with it. When we simulate, we’re looking to prove that the operating margin at the system level is substantially more than the margin of error in the simulation we performed. That’s when we can say the design will work with room to spare.
When we talk about field solvers, we need to remember it’s the margin at the system level that we care about. We need to model the structure in question (a transmission line, a package breakout, a differential via, etc.) as accurately as required to ensure adequate margin at the system level. But we don’t want to model things more accurately than required, because that takes additional time and effort that isn’t really moving the design forward; it’s just analytical overkill.
To read this entire conversation, which appeared in the July 2022 issue of Design007 Magazine, click here.