# Electromagnetic Analysis Design Insight: Effects of Meshed Reference Planes on Interconnects

Interconnects in rigid and flexible boards can be formalized and simulated as transmission lines: strip, microstrip, coplanar, single-ended, or differential. The number of conductors in transmission line models is two for single-ended and three for differential lines. The signal conductors in the electrical model correspond to the interconnect traces, and the reference conductors correspond to one or two reference planes. Static or quasi-static field solvers are usually used to extract modal (impedance, attenuation, and phase delay) and per-unit-length (RLGC) parameters for analyzing data links. These models may be accurate up to very high frequencies depending on the geometry and material models. Reference planes in such models are assumed to be solid. That is usually correct assumption for most of the rigid-board interconnects (except BGA fields in some cases).

But most of the flexible interconnects have meshed or hatched planes; the reference conductors have periodic cutouts. The question is how do you build accurate models for such structures? It turns out that the traces over conductors with periodic cutouts can be effectively simulated as periodic structures; that requires a 3D electromagnetic analysis of a small segment instead of the analysis of a cross-section or complete link. Similar to the regular transmission lines, periodic structures can be characterized with per-unit-length impedance, admittance, and the modal parameters: attenuation, phase delay, characteristic impedance. That is very convenient for building models for interconnects with arbitrary length. This article shows what one can learn from this analysis using a particular example. Simbeor software was used for the analysis of interconnects with meshed planes.

Electromagnetic Waves in Traces Over Planes With Periodic Cutouts

In general, a periodic structure is made up of repetitions of a unit cell in one, two, or three dimensions. For instance, Figure 1 provides a simple example of the periodic repetition of a rectangular cutout in transmission line reference plane. The cutouts are repeated in two dimensions, but for the wave propagating along the trace, it is essentially a one-dimensional periodic structure (it becomes 2D for leaky-mode investigation). To extract the per-unit-length and modal parameters of such a structure, 3D electromagnetic analysis is needed due to the non-TEM structure of the waves propagating along the traces.

Most of the transmission lines in PCB or packaging interconnects have so-called quasi-TEM waves with the electric and magnetic fields mostly perpendicular or transverse to the propagation direction. Parameters of such transmission lines can be accurately approximated with the analysis of a single cross-section in a 2D field solver. With the cutouts in the reference plane, the waves become non-TEM and not even quasi-TEM due to the presence of the longitudinal components in the electric and magnetic fields.

To read this entire article, which appeared in the January 2019 issue of Design007 Magazine, click here.

## Bridging the Simulation Tool Divide

04/12/2021 | I-Connect007 Editorial Team
Todd Westerhoff of Siemens EDA recently spoke with the I-Connect007 Editorial Team about the divide between users of high-powered enterprise simulation tools and those who need a more practical tool for everyday use, and how Siemens is working to bridge the gap. Todd also shared his views on why so many engineers do not use simulation, as well as advice for engineers just getting started with simulation tools.

## Cadence’s Celsius: Don’t End up Holding the Hot Potato!

12/17/2020 | Clive "Max" Maxfield, Maxfield High-Tech Consulting
I was just thinking about the party game Hot Potato. It reminded me of today’s increasingly competitive marketplace: Accurate thermal analysis must be performed, and any potential issues have to be identified and addressed as early as possible in the design cycle. Otherwise the system will run into problems, market windows will be missed, and someone will be left holding the hot potato. Trust me, you do not want to be that someone.

## This Month in Design007 Magazine: Thermal Fundamentals With Mike Jouppi

09/09/2020 | I-Connect007 Editorial Team
The I-Connect007 team recently interviewed Mike Jouppi, one of the champions of thermal management in PCBs. Mike spent decades working on updating the old IPC current-carrying data, which dated back to the 1950s, and he is the primary architect behind IPC-2152— the standard for determining current-carrying capacity in printed board design. As Mike explained in this wide-ranging interview, even if you’re using the latest thermal design software, you still need to have a firm understanding of the fundamentals.