# Where Exactly is the Signal?

When I first got involved in printed circuit board design in the early 1990s, fast rise/fall times were just starting to become an issue. Prior to that we had been pretty much a “connect-the-dots” kind of technology. But as rise times got faster, it became necessary to worry about (electromagnetic) fields. One manifestation of that was EMI, and the increasing need to pass FTC compliance testing.

So, a new type of engineer came on the scene: the electromagnetic compliance/compatibility engineer. Until that time, we understood that electrical current on a copper trace was the “flow” (movement) of electrons along the trace1. In fact, the definition of an amp of current on a copper conductor is 6.25*1018 electrons crossing a surface in one second2.

But this new breed of engineers came along and many of them started saying things like3:

• “No, current isn’t electron flow. Electrons can’t flow at the same speed signals flow.” (My response: But they can transfer energy between themselves at the speed of light, which is how they “flow.”)
• “Maxwell and Maxwell’s equations tell us that the signal is in the field around the trace, not on the trace itself.”
• Stop worrying about traces; ignore them. Just control the fields and you will be fine.”

So, before we answer the questions about where the current and signal truly are, let’s look at the fundamental principles behind Maxwell’s equations and see what they say4. The following discussion heavily paraphrases these principles and simplifies them for issues relative to this article. No, calculus will not be necessary.

The Principles

Charles-Augustin de Coulomb (1736–1806) was a French physicist. He is best known (at least to us) for developing a couple of laws in the 1700s. One was:

“There are two types of charge, positive and negative. ‘Unlike’ charges attract and ‘like’ charges repel each other (Figure 1) with a force that is proportional to the product of their charge and inversely proportional to the square of the distance between them.”

Coulomb also gave us another law related to magnetism: Every magnetic pole is a dipole with an equal and opposite pole. That is the same thing as saying that a magnetic “north” pole cannot exist without there also being a magnetic “south” pole. Even if you cut a magnet in half (see Figure 2) the individual poles would not be preserved; new poles would appear to preserve the dipole nature of the magnet.

To read this entire article, which appeared in the December 2022 issue of Design007 Magazine, click here.

## Optimizing Communication Between Fabricators and Designers

03/21/2023 | Andy Shaughnessy, Design007 Magazine
During DesignCon, I spoke with James Hofer from Accurate Circuit Engineering about some of his customers' biggest challenges. We discussed various ways to increase the level—and quality—of communication between designers and fabricators. James also offered some interesting observations about bridging the gap between designer and fabricator. How often do you communicate with your fabricator?

## DFM 101: Final Finishes: OSP

03/09/2023 | Anaya Vardya, American Standard Circuits
One of the biggest challenges facing PCB designers is not understanding the cost drivers in the PCB manufacturing process. The next final finishes to discuss in this series is OSP. As with all surface finishes there are pros and cons with the decision of which to use. It is a combination of application, cost, and the properties of the finish. OSP is RoHS-compliant as there is zero lead content in the finish.

## DFM 101: Final Finishes—HASL

02/14/2023 | Anaya Vardya, American Standard Circuits
One of the biggest challenges facing PCB designers is not understanding the cost drivers in the PCB manufacturing process. This article is the latest in a series that will discuss these cost drivers (from the PCB manufacturer's perspective) and the design decisions that will impact product reliability.