Designers crave rules, but few rules apply in every situation.
A recent discussion with fellow designers led to the realization that designers truly appreciate rules. They crave formulas that always bring them to the design board with an “electrically correct” posture. They want to use flow charts to answer their questions and checklists that can help them critically weigh the pros and cons of each situation and further support best design practices. In other words, they want to follow the best combination of rules, formulas, methods and ideas that cover all the issues related to the design of a particular PCB. These include not only HS, SI and EMI considerations, but also proactive DfM, DfA and DfT protocols that smooth the design for easy processing through manufacturing.
The board design world has been rapidly changing in recent years with signal speeds increasing, EMI becoming more of a problem than ever, component size shrinking dramatically and lead-free issues creating changes in procedures. All of these changes affect design dramatically. Many designers may have come to the realization that they really do need to investigate these issues and learn more about them.
Most designers have a strong attention to detail and want to understand the ramifications of what they are doing, but have worked in environments that limit their growth. Some have worked for companies that did not encourage training or the exchange of ideas and information, while others have just not had the opportunity to attend meetings and conferences that would give them more knowledge.
Some designers may rely on engineers to help them find the right way to do things, but many engineers do not have the same background knowledge about today’s issues that they do about logical topics. These issues, as they apply to today’s PC boards, were not (and are not) taught in engineering schools. The engineer may have basic knowledge about physics, but the current edge rates and interactions of parts and signals require revisiting the information with a new, questioning eye.
A few designers may still rely on the old, traditional way of doing things, but one thing is certain–we cannot continue to do things the way we have always done them. Just because something worked before, does not mean it will work in today’s environment.
Unfortunately, there are no rules that will apply in every situation. Even though the laws of physics will never change, the circumstances can, and all the factors MUST be considered because they affect each other. In some instances, one rule will dominate, and in other instances, another will. Any design rule that might be written would be so complex as to be unusable because of all the exceptions contained within it. There is a speaker on the conference circuit that is famous for asking questions in his classes, to which the correct answer is almost always “it depends,” teaching the students there are always variables that must be considered.
I would like to propose several rules that might be universal. First, in an effort to help people understand how the electronics affect each other, quite a few authors and speakers have written “Rules of Thumb” in their articles and books. These “rules” are born from analysis, formulas and long years of evaluation. They involve various design parameters including placement, stack-up, routing, spacing and more. The rules of thumb suggest generalities and should definitely be used to start the design process. But please allow me to vehemently caution… While rules of thumb are an excellent tool to use for estimates, they can hardly be the final decision makers that designers want.
Secondly, designers must take the initiative to discover their weaknesses and learn about any topics that can help them improve their work. The laws of physics, for example, are incredibly important to know and to understand for today’s designs that include electric and magnetic fields, transmission lines, return current, Ldi/dt, inductive loops and all other forms of signal integrity and EMI control. Other areas that have changed over that past few years and that need more study may include PCB materials, HDI, RoHS and DfM.
For instance, there are user forums where designers ask questions, join discussions and participate in the exchange of information. There is an abundance of magazines and books, with articles written on a variety of levels, providing an outlet for authors to address current questions. Webinars and the Virtual PCB shows are quickly becoming a mainstream way to interact with speakers and suppliers, too, and if designers are able to attend, there are conferences that enable designers to network and attend classes.
Lastly, designers should encourage the engineers whom they work with to read the same information and attend the same classes, so that they can all discuss the topics and create the best possible boards. PCD&F
Susy Webb is a senior PCB designer at Fairfield Industries in Houston, TX, and is a regular speaker at the PCB Shows, This email address is being protected from spambots. You need JavaScript enabled to view it..
