Estimated reading time: 6 minutes

Elementary, Mr. Watson: The Printed Circuit Board Design

One of my greatest joys as a grandfather of five (believe it or not) is spending time with them. Entertaining young children in activities that don’t involve a screen is a significant task, but we enjoy working on jigsaw puzzles. Very often in our home there is a partially completed puzzle on our dining room table with all the individual pieces spread out everywhere. The appeal of puzzles is the challenge of fitting together all the unique pieces so as to create a completed, beautiful picture.

Much in the same way, I have been a part of the PCB designers' community for many years. I have had the opportunity to encounter some of the most phenomenal people you could ever meet. I've learned that PCB designers are a special breed of folks who look at what they do, not just as a career, but also as a passion. We love the process; for those of us who undertake a PCB design, just like my activity with my grandkids, we solve the puzzle. Essentially that's what a PCB design is—one big puzzle. Our pieces are the schematic parts wired together, followed by getting all those components on the board. It’s the constant challenge of routing all the connections, with the multitude of what looks to the untrained eye as just crisscrossing colored lines.  

In the January issue of PCB007 Magazine, I-Connect007 introduced the theme of 2021 as the year of continuous improvement. They presented the concept as a simple process improvement formula: X = X_c – 1.

It is a fantastic issue, and I highly recommend it. If we are to experience constant improvement, it must involve the examination of the processes used. Because most problems and issues that come up during a PCB design are, first, a direct result of a breakdown of the PCB process, let's look at the printed circuit board design and its process. 

The Process
When we look at a PCB design process on the macro level and take relatively simple items through the numerous steps to a finished design, the required sciences include various advanced mathematics, materials, chemistry, problem-solving, mechanical engineering, etc. On top of that, “Mr. Murphy” is always waiting to make sure that anything can go wrong. It's no wonder that problems or issues can sneak into our designs—problems that, many times, don't show up until it's often too late. However, if we first understand what the PCB design process is, how to control it, and by putting in place the correct milestone checks at various points. I guarantee you that every PCB design will be right the first time.    

The PCB design process is just that, a process. The business dictionary defines process as:

"A sequence of interdependent and linked procedures which, at every stage, consume one or more resources (employee time, energy, machines, money) to convert inputs (data, material, parts) into outputs. These outputs then serve as inputs for the next stage until you reach a known goal or result."

That definition is probably very simplistic for most of us. But when we break it down into its parts, or jigsaw puzzle pieces, the process is just made up of inputs, resources, outputs, and final results. Every stage is relying on the one before it to make sure we reach the final product. That forms the interconnect of the PCB design. 

One further thought: There is a concept when we speak of various items or documents in the PCB design called the parent/child relationship. That is where you use an input of data, material, or parts (parents) as the foundation or resource for another Item or document (child). An example of this concept is when a schematic document (parent) is the source for the bill of material (the child). Keep in mind that an item or document can be both a parent and a child, depending on where they are in the PCB design process. 

Beginning of the Process
Many believe that the beginning of the PCB project is the schematic. I respectfully disagree. The foundation of every PCB design starts and is built on your component library. Everything begins and ends with the quality of that component sitting somewhere in your library. Most of the problems I have seen in PCB designs have related right back to library problems. In the February issue of Design007 Magazine, I discuss components and libraries in greater detail. But, for now, the components in your library are the parent of everything to follow. If you have either weak or, even worse, wrong components, you will also have a weak and wrong PCB design. Inside of each component, you should have everything needed for your design. What I mean is a component consisting of Information, models, and other various items. Each of those items is used somewhere in the PCB design process. If they are missing, it introduces risk and probably errors of some kind. 

Figure 1 shows the construction of a typical PCB design items/documents and their relationship. The four general stages of a PCB design are the library, schematic, PCB design, and outputs. You can see that the schematic and the PCB has inputs and outputs, making them both a parent and a child document. It's essential to understand this relationship because whenever changes are required, the rule is that you must change the parent item, not just the child.

Figure 1: The four general stages of a PCB design. 

For example, let's say you needed to change an item on the BOM—a child document—if you did something as drastic as adding another component to just the BOM. The first problem is your schematic and BOM are no longer synced up. Also, that would cause problems with your PCB. You would want to make the change on the schematic (parent) and possibly the library. This simple rule keeps the integrity of your design, where everything is synced up. I would add that it also saves you major headaches in the future when, let's say, other changes are needed and the person doing it does not know the history of the changes to just the child document. That could unknowingly undo previous changes and cause significant issues. 

Red Flags
The three red flags in Figure 1 are milestones and gatekeeping events in the design. It is essential at those points to stop everything you are doing and conduct required various checks. These are gatekeeping items, meaning that you do not move forward without first verifying the design's status. I know that in the rush of deadlines, many times known issues become a preferable tin can you kick down the road. I have always said that good PCB designs are often sacrificed on the altar of expendiency. This might be controversial, but I believe the driving force behind a PCB design is not an arbitrary schedule but rather the design process and the milestone checks you correctly conduct. Suppose you push known problems down the process, thinking you will come back to it later; too often, those issues are lost in the rush of everything. You are opening yourself to possible issues. 

Here are a few tips for performing electrical rule check (ERC) and design rule check (DRC) that can keep those red flags from popping up in your design process: 

1. During ERC, all new components should be checked and compared to the datasheet, IPC standards.
2. Full ERC should be run on the schematic.
3. Full DRC should be run on the PCB design.

When we understand the design process and steps required, a PCB design is not complicated; it’s like solving the jigsaw puzzle—one piece at a time. 

John Watson, CID, is a customer success manager at Altium.