Our industry changes everyday, and we need to make sure we keep up. Design constraints today may not be an issue tomorrow, so make sure you’re following fact and not fiction. Here are 5 myths of PCB design.
There are a number of guidelines to PCB design, to help you create circuit boards that are low-power, bug-free, and generally functional. Some of these guidelines can be bent occasionally, depending on what your design needs, like component spacing in a dense design. Others are hard and fast rules that must always be followed, like electrical rules. And some are just outright myths: things you were taught in school, or by a mentor, that have been perpetuated for years. Blindly adhering to these myths can spell trouble for your design in any number of ways, so it’s important to be aware of them, understand their origins and what they actually mean for your project. Here are five of the biggest PCB design myths you may have heard of.
Top PCB Design Myths
1. A Complete Connection Is Just from Pin to Pin. Many PCB designers simply route their signal traces from pin to pin, and don’t worry about any timing issues except matching the traces to the delay. However, designing this way neglects one important fact: Current doesn’t just flow one way. It flows in a loop. And that doesn’t mean simply flowing back the way it came, either. The return current takes the path of least resistance, or, at high speeds, the path of least inductance, charging the local parasitic capacitance as it makes its way back through. Because of this, the ground plane can’t be used to dump unwanted signals.
2. Trace Widths Can’t Be Smaller Than 3 Mils. In the past, it was true that using trace widths smaller than three thousandths of an inch caused problems for anything more than a very short span. However, new developments in semi-additive manufacturing now allow trace widths to be much smaller without causing problems.
3. “Ground” Is a Region of Zero Potential. At regular DC voltages, ground has zero resistance and zero impedance at all frequencies. However, when it comes to high speed PCB designs, the term “ground” is no longer a viable concept. Any metal has some amount of resistance. It may be as near to zero as (seemingly) makes no difference, but as we covered in the first point, that current forms a loop, which then causes inductance. Once you get up to those higher speeds, these seemingly insignificant figures are no longer insignificant, and finding a place with completely zero potential becomes virtually impossible.
4. “Getting It to Work” Is Your Chief Concern. Of course, you want your PCB to do what it’s supposed to do. And, obviously, that’s the first issue you tackle as you’re designing: basic functionality. But once you achieve that, your job isn’t done. Often, “getting it to work” can result in a lengthy debugging phase. You want your design to be sleek and efficient. You want it to be cost-effective. You want it not just to work, but to work well, and without bugs. All these issues are things you still need to resolve, long after you’ve gotten it to work.
5. All PCB Design Tools Are Basically the Same. All you really need is a platform to work in, right? Get the cheapest one, or, better yet, one you can download for free, and you’ll be able to design just fine. Hopefully you know better than that. But maybe you have a personal favorite design tool, and refuse to use anything else, no matter how many problems it causes you. The process of choosing a PCB design tool can’t be rushed or taken lightly. First, you need to catalogue your design needs. There are a variety of factors to look at when it comes to choosing the correct tool, such as the size and quality of its content library, availability of support and troubleshooting, supply chain and MCAD integration, and much more. Decide what it is you need from your PCB design tool, then look for a tool that can meet those needs without disrupting or overturning your current procedures and methodologies. Do your research and find out what you need, what you can afford, and what kind of ROI you’ll be looking at, before you make any final decisions.
As a designer and an engineer, it’s important not to take everything you hear at face value. Some of these myths may have been ingrained into your mind for years. But take a look at them again in a new light. Do they really make sense to your design? Have new technological advancements made these rules obsolete? What happens if you take a different path? That’s what engineering is all about: taking a different path and finding a new solution to a problem. With time and effort, you can help to debunk even more of these PCB design myths.
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