Several years ago, a report came out of St. Louis of a strange apartment on the market. It was in the community of Central West End. With a small floor plan of only 200 square feet, the entire bathroom was placed right in the middle of the kitchen. Well, that's interesting. It gives new meaning to the studio apartment. Well, with closer examination, there are several convenient features available here. You got a lovely footstool to get to those pesky upper cabinets and could save even more space by removing the sink entirely and using the tub to wash those late-night dishes. But seriously, who would do something like this? Yes, it's a terrible floor plan.
Here’s my point: In this story lies some fantastic lessons for us as PCB designers. In real estate, it's pretty uncommon to find the bathroom in the kitchen; but metaphorically speaking, it's done all the time in a PCB design (ouch). What do I mean by that? Well, the floor plans of many PCBs have a lot to be desired. The subject of circuits and component placement is usually an afterthought, usually when the completed board returns, and it just doesn't work like it did on paper. Then you think, “Maybe I shouldn't put the tub next to the stove; it’s not a good idea.”
The steps of arranging the rooms into functional PCBs are often ignored or rushed through because everyone wants to get into what they consider the "exciting" part—the routing—not realizing the routing started with the placing of the very first component. Those functional areas and their locations do impact product quality. Placing the rooms or sections of your design willy-nilly is the equivalent of the tiny apartment in St. Louis with the toilet/footstool next to the kitchen sink. The advice I always give, especially to new designers is, "Just because you can do something doesn't mean you should.”
With more complex designs, the name of the game is to make it smaller and higher in performance. I refer to it as the incredible shrinking FR-4 real estate, dwindling significantly, and causing significant issues for us. We're always going through different paradigm shifts in the industry. To get everything placed and arranged correctly, we may not always "break" the rules, but we often bend them to the point they look more like a pretzel. Although PCBs are getting smaller, we don't have a license to violate the basic design principles. Just the opposite should be the case.
As the initial step of placing your components on the PCB, you should take a step back and look at the bigger and more important picture of your PCB floor plan. Several considerations drive your decisions, which I look at in more detail.
Consider Critical Placement
The first components that usually go on the PCB and are locked down are called critical components. These are the items that must have a precise location on the PCB. Driving those locations could be mechanical, testing, EMI, user interface, or design development access. Here you are taking your first steps to solving the puzzle of the PCB. Look at them as anchors for what follows. Since the first rule of good signal integrity is to keep the connections as short as possible, that drives locating the first “rooms.” That results in components like connectors being on the edge of the PCB for easy access to other essential components.
Types of Circuits and Function
First, understand the function and the purpose of your circuit. Please don't fall into the ruse that often happens; some believe the responsibility of a designer is just to put everything on the PCB and connect the dots. Instead, dig deeper and know not just what, but rather why you are doing something.
We all know that the two sides of electronics are digital and analog. A digital circuit is one where the signal must be one of two discrete levels. Each level is interpreted as one of two states (on/off, 0/1, true/false). An analog circuit, on the other hand, is one that represents continuous signals in electrical form. These two are the “odd couple” in the electronic world. (Those younger folks in the audience might need to Google that reference.) On one side, you have Felix Unger, the neat freak, and on the other, Oscar Madison, the sloppy sportswriter. Both are divorced and end up sharing a New York City apartment. It makes for a funny sitcom but not so funny on a PCB design.
A well-designed digital/analog circuit will have isolated power and grounds. The current flows through a digital circuit vs. analog, and the varying demands on the power rail cause instabilities and spikes. The fact is that integrated circuits are susceptible to power fluctuations. Thus, the reason for capacitors on each power pin (bypass capacitors). I’ll give you this one for free: The standard practice is to have separate supplies and return paths for each circuit. In isolation, to use the modern vernacular, socially distance different sections of your design; the digital, analog, and power sections must have their own isolated working areas. I can always tell the designers that didn't take this subject seriously. Those are the boards that come back from assembly with issues of noise and EMI. It's now too late to consider that maybe Maxwell was right with his equation regarding electrical charges.
Isolation is so severe that unique components bridge the different areas (rooms). Look at this as the doorways in your home; you only move (hopefully) from room to room through the door. These areas have special rules in those areas. There lies a problem with our tiny apartment in St. Louis—no consideration of the functions of the two areas. What are the rules of interaction between them, which is what if both areas got used simultaneously? It would be a bit, let's say, awkward. The same is true of our PCB.
Within the major areas of digital, analog, and power, there are minor areas, like the closets in the master bedroom, such as keeping various components like decoupling caps close to power pins and clock drivers/synchronizers close to the clock oscillator. Oscillators must be close to their integrated input pins. Also, always isolate the outputs from the inputs to reduce feedback problems.
Finally, the need for isolation goes much deeper than the types or the circuit functions; you must consider the frequency of your design. Intermingling different circuit operating speeds causes more problems in a design. Identify the high, medium, and low-frequency circuits, then isolate them from each other. According to IPC-2221, the correct location for those high-frequency circuits is close to the connectors or source. You want short signal routes that don't go across the board. What happens with an electrical charge on a low-frequency vs. high-frequency circuit? How could they interrelate and cause problems with signal integrity, time delays, reflections, electromagnetic interference, and crosstalk? That's a discussion for another day.
In real estate or PCB design, it doesn't make sense to place your bathroom in your kitchen. It's not a good practice in your PCB design, either. Know the different major and minor areas when you look at the floor plan of your board and know the bridges (doorways) between them, keeping everything inside the room and isolated from the other. You'll have a much quieter design with fewer problems. The best part is you won't need to use a toilet for a footstool.
John Watson, CID, is a customer success manager at Altium.
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