Strategies for Laying Out Printed Circuit Boards.

Most board designers employ a list of guidelines to help place components and route traces. For example, a typical guideline might be "minimize the length of all traces carrying a digital clock signal." Often, a designer is not familiar with the reason for the guideline or does not fully understand the consequences of violating the guideline for a particular application.

Quiz Question

Suppose you're laying out a high-speed multi-layer printed circuit board and you need to route a trace carrying a high-frequency signal from a digital component to an analog amplifier. You want to minimize the chance of having an electromagnetic compatibility (EMC) problem, so you search the web for EMC design guidelines and you find three guidelines that seem to pertain to your situation:

1.       minimize the length of high-speed traces;

2.       always gap any solid planes between analog and digital circuits; and

3.       never let a high-speed trace cross over a gap in the signal return plane.

You envision the three possible routing strategies illustrated in Figure 2. The first routing strategy routes the trace directly between the two components, but leaves the plane between them solid. The second routing strategy gaps the plane, but routes the trace over the gap. The third routing strategy routes the trace around the gap. Each of these alternatives violates one of the guidelines. Which is the best choice?

Figure 2: Which is the best trace routing alternative?

Is each alternative equally good because it satisfies 2 of 3 guidelines? Are they all bad because they all violate at least one guideline? These are the types of questions that circuit board designers are faced with every day. Making the right choice can be the difference between a board that meets all requirements and a board that has severe radiated emissions or susceptibility problems. In this case, one of the choices is much better than the other two. However before we reveal the correct answer, let's develop a strategy for evaluating printed circuit board layouts. With a proper strategy, the correct answer to this quiz question should become apparent.

 4 steps that every EMC engineer should apply when laying out a printed circuit board or reviewing an existing board design. These steps are:

·         Identify potential EMI sources and victims

·         Identify critical current paths

·         Identify potential antenna parts

·         Explore possible coupling mechanisms.

By taking the steps outlined above first, component placement and trace routing decisions will become clearer. It should also be much more apparent which design guidelines are most important and which are not important at all for a specific design.

In my next article i will discuss all the points one by one mentioned above.

Thanks for reading..

Ruby