PCB EMC Design: Module Partitioning and Special Component Layout

Any serious discussion of EMC in PCB design eventually arrives at the same two decisions: how the board is divided into functional blocks, and where the critical components physically sit. Certain devices — frequency-generating circuits, drivers, power modules, filter components — exert a disproportionate influence on EMC. Their relative position and orientation on the board determine how strongly the board radiates electromagnetic fields and how readily it picks up fields from elsewhere. The same layout decisions also set the ceiling on routing quality: a poor placement cannot be rescued by good routing, while a sound placement makes short, clean, well-separated traces achievable in the first place. This article covers the partitioning methods used to organize a board, the placement sequence that follows from them, and the specific handling required for power supplies, clocks, inductors, bus drivers, and filter components.

Highlights:

Partition the board by function, by frequency, and by signal type before any routing begins, keeping the resulting blocks non-interleaved so that signal paths stay short and cross-block coupling stays low.
Place the power section at the board's power entry point, keep clock circuits far from board edges and I/O connectors, and keep bus drivers away from the front-panel edge.
Filter components deliver their rated performance only when placed by the proximity rule: decoupling capacitors at the IC power pins, power filters at the power entry, and interface filters at the connectors.

Why Partitioning and Placement Drive EMC Performance

PCB EMC design cannot be separated from module partitioning and key-component placement. There are two reasons. First, the relative position and direction of frequency-generating devices, drivers, power modules, and filter components on the board have a large effect on both the emission and the reception of electromagnetic fields. A clock circuit tucked into a corner next to a cable connector behaves very differently from the same circuit placed deep inside the board. Second, the quality of the placement directly determines the quality of the routing that follows. Once the blocks are fixed and the devices are pinned down, the routing options are largely constrained; a placement that forces long signal returns or forces a clock line to cross the whole board cannot be corrected later by careful trace drawing.

Three Ways to Partition a Board

Partitioning by Function

Every circuit block on a board implements a distinct function: clock circuits, amplifier circuits, driver circuits, A/D and D/A conversion circuits, I/O circuits, switching power supplies, filter circuits, and so on. A complete design may contain several of these functional blocks at once. When laying out the PCB, the whole circuit can be divided into blocks according to the direction of signal flow. This keeps the overall placement rational, produces short overall routing paths, prevents the blocks from interleaving with one another, and reduces the likelihood of interference between blocks.

Partitioning by Frequency

The circuit can also be partitioned according to the operating frequency and rate of the signals it carries. Blocks are laid out progressively from high frequency to medium to low, with no interleaving between them. Arranging the frequency domains this way keeps fast, radiating circuitry from sitting directly alongside slow, sensitive circuitry.

Partitioning by Signal Type

By signal type, the board splits into a digital section and an analog section. To reduce interference from digital circuitry into analog circuitry and let the two coexist in a compatible state, the PCB layout must assign them separate regions and provide the necessary spatial isolation, which lowers the coupling between them. For data conversion circuits such as A/D and D/A converters, the device should be placed at the boundary between the digital and analog regions. The orientation of the device follows the direction of signal flow, so that signal leads are as short as possible, with the analog-side pins sitting above the analog ground and the digital-side pins sitting above the digital ground.

The Combined Placement Process

One underlying principle governs the whole layout: follow the signal flow relationship and make the traces carrying critical high-speed signals as short as possible. Board neatness and visual appeal are secondary considerations, not the starting point. Clock signals in particular should be as short as possible; if the clock trace cannot be shortened, shield ground traces should be added on both sides of it. Sensitive signal lines should likewise be given shielding treatment.

Clock circuits radiate strongly and will have a significant impact on sensitive circuits, especially analog ones. For this reason the clock circuit should be kept away from unrelated circuits during placement. To limit the outward radiation of the clock signal, clock circuitry should generally be kept away from I/O circuits and cable connectors.

Low-frequency digital I/O circuits and analog I/O circuits should be placed close to the connectors. Clock circuits, high-speed circuits, and devices such as memory are usually placed in the innermost part of the board, as far as possible from the card handle. Medium- and low-speed logic circuits generally go in the middle of the board. If A/D and D/A circuits are present, they are usually placed in the very center of the board.

The essential points of this process can be summarized as follows:

Divide the board into regions. Circuits of different functional types should occupy different regions, separating digital, analog, interface, clock, and power circuitry from one another.
Place data conversion circuits at the junction between the digital region and the analog region.
Place clock circuits, high-speed circuits, and memory circuits in the innermost part of the board, away from the card handle; place low-frequency I/O circuits and analog I/O circuits close to the header.
Use signal-flow-based placement so that the traces for critical and high-frequency signals are the shortest, rather than giving priority to board neatness and appearance.
Keep the power amplifier and the control/driver section away from local openings in the shield, and route them off the board as quickly as possible.
Place crystals and crystal oscillators adjacent to their corresponding ICs.
Keep the reference voltage source, including analog voltage signal input lines and the A/D conversion reference supply, as far as possible from digital signals.

Layout of Special Components

The Power Supply Section

Every board using distributed power carries one or more DC/DC power modules, together with the associated circuitry such as filtering and protection, forming the board's power input section. Modern switching power supplies are a major source of EMI, and the interference band can reach beyond 300MHz. Several boards in a system may each have their own independent supply, yet the interference can still propagate to other boards through the backplane or through space, and the longer the board's supply lines, the larger the problem becomes. The power section must therefore be mounted at the board's power entry point. If the power section occupies a large area, it should also be grouped uniformly on one side of the board. A preferred arrangement, and the one most boards adopt.

The orientation of the power section is chosen mainly so that the input and output lines run smoothly and do not cross. In addition, because the power section on a board tends to be relatively self-contained yet frequently generates EMI problems, it is recommended to isolate the power section from the rest of the circuitry using a via fence or a split line.

The Clock Section

The clock is often the largest interference source on a board and the area that most needs special treatment during PCB design. On the placement side, two things matter: the clock source should be as far as possible from the board edge, especially from the card handle, and the trace from the clock output to its load should be as short as possible. On the routing side, clock traces should be given priority for inner-layer routing, together with the necessary matching and shielding.

Inductive Coils

Coils, including relays, are the most effective devices for both receiving and emitting magnetic fields; where a relay is required, a solid-state relay should be considered wherever possible. Coils should be placed as far as possible from EMI sources, which may include switching power supplies, clock outputs, and bus drivers. High-speed traces and sensitive control lines must not run on the PCB beneath a coil. If this cannot be avoided, the orientation of the coil must be considered so that the field strength direction is parallel to the plane of the coil, keeping the number of magnetic field lines passing through the coil to a minimum.

The Bus Driver Section

As system capacity grows, bus rates rise and the required bus drive capability rises with them, while the number of bus lines increases substantially at the same time. Bus matching is difficult to make perfect, so the radiated field strength around bus drivers such as the 16244 is typically very high. In measurements taken on some boards, the bus driver section proved to be the other major EMI source besides the clock.

In placement terms, the bus driver section should be as far as possible from the card handle to reduce radiation escaping the system, while the distance from the driven signal to its destination should be as short as possible.

Where necessary, local shielding can be added over large bus driver sections.

Filter Components

Filtering is an indispensable and the most commonly used countermeasure. Schematic design frequently specifies a great deal of filtering: decoupling capacitors, three-terminal capacitors, ferrite beads, power supply filtering, interface filtering, and more. Yet during PCB design, if the filter components are placed improperly, the filtering effect is greatly reduced and may even disappear entirely.

The general consideration for mounting filter components is the proximity rule. For example:

Decoupling capacitors should be as close as possible to the power pins of the IC.
Power supply filters should be as close as possible to the power input or the power output.
Filters for a local functional block should be close to that block's entry point.
Filters for external interfaces, such as ferrite beads, should be as close as possible to the connectors.

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