Harmonizing Circuit Design and EMC DesignPart 9 Evaluation Circuits and Boards (3) Electromagnetic Interference (EMI) and Electromagnetic Susceptibility (EMS)

Hello! I’m Inagaki, at ROHM.

This ninth column is my third on evaluation circuits and boards. This time, I’d like to talk about methods for creating evaluation boards for semiconductor integrated circuits (LSIs and ICs), PCB layouts, and related topics.

A sudden question here: do you enjoy creating circuit boards?

I myself like it quite a lot. A PCB layout has puzzle-like elements, such as arrangements of components and signal lines, and I enjoy pondering how to obtain a visually beautiful layout.

As this is an engineering field, an aesthetic perspective focusing on “beauty” may seem out of place, but a broad-ranging view of a board is related to its performance. The placement of components is the first factor that decides how the overall board is created, and is extremely important in its own right. Once the components and their placement are determined, wiring between them is consequently determined. When deciding on the arrangement of components, one should bear in mind how important signal lines and wiring are to be arranged between them. And, care must be exercised to ensure that component positions do not result in exceptionally long wires or the occurrence of wires requiring detours. In these and other ways, the designer must exercise his imaginative abilities to the fullest extent.

For board wiring to be “beautiful” means….

Put differently, it means that signal lines and wires are orderly. More specifically, this means that wires are short in length, with no odd bending, and with wiring directly connecting components. Of late there have been EDA tools (CAD tools) that, upon input of a circuit diagram, automatically generate PCB layouts; I have never used one. It may even be that leaving everything up to such a tool results in a higher degree of perfection, but I enjoy this work.

The PCB layout is in fact closely related to electromagnetic compatibility (EMC), and so is vitally important.

Electromagnetic compatibility (EMC) is broadly divided into electromagnetic interference (EMI, emission), which generates electromagnetic noise, and electromagnetic susceptibility (EMS, immunity), in which malfunctions and failure are caused by external electromagnetic noise. Each of these may be further classified, according to the path of transmission, as conducted and radiated. In circuit board creation, these four combinations–conducted emission (CE), radiated emission (RE), conducted immunity (CI), and radiated immunity (RI)–must be considered. This is quite a bit of terminology, but the following diagram may serve as a reference.

In board creation, the greatest attention must be paid to parasitic inductances (L) and parasitic capacitances (C) in the board. This is because, at series resonance frequencies and parallel resonance frequencies in high-frequency bands caused by these parasitic components, the electromagnetic compatibility (EMC) characteristics can be worsened.

Well then, what can we do to improve the electromagnetic compatibility (EMC) characteristics of a PCB layout?

There are products called “EMC components” on the market; using these is one option, but some believe that these products alone are not sufficient.

Where electromagnetic interference (producing electromagnetic noise) is concerned, the board wiring should be thought of as a “transmitting antenna” for electromagnetic noise. In the case of electromagnetic susceptibility (in which electromagnetic noise causes malfunctions), the board wiring should be considered a “receiving antenna” for electromagnetic noise. With this understanding, the kind of wiring required will become clearer.

Recently, multilayer boards such as four-layer boards have come into general use. In many multilayer boards, the board top layer and board bottom layer are made solid grounding planes to the extent possible. This is also one kind of metal shielding of the board; signal wiring and control wiring are fabricated in the inner layers.

And, board dimensions are made as small as possible. When the board is made small, wire lengths are shorter as a result. This in turn results in reduced parasitic inductances and capacitances. From a similar perspective, surface-mounted components with dimensions as small as possible are also ideal. In fact, a “beautiful” PCB layout is none other than a layout with smaller parasitic elements.

Thank you very much for your attention.