Design Reuse Within a PCB and Beyond

As PCB Designers, we are often tasked with filling out a board with geometry that repeats itself. Better physical resemblance usually equals better performance. For instance, Ethernet ports will have a transformer right after the connector. They will generally work the same if they look the same. And life will be much easier if you were not reinventing every instance of a circuit. Let’s walk through the process.

Some form of the Copy and Paste commands are always included with our tools. The tools themselves are referred to as PCB editors. The parts we place in the early stages of the design often incorporate parallel paths. Perhaps, all of those identical blocks of schematic diagrams can be randomly placed. Perhaps, not. When I scroll down to the next page and it’s the same as the last page, I keep on scrolling. The schematic may have more to tell.

There are a few intersecting considerations here.

• Number of Copies
• Circuit Complexity
• Timing/impedance Rules
• Real Estate - or lack of.
• Future Proofing

Two of a kind with a simple design and ample room is a case where you can do both layouts on a grid and position the groups as required. As the number of copies or the complexity increases, a more formal approach will be necessary. Step and repeat whenever an analog circuit is in play. Multiple Input, Multiple Output (MIMO) is a common architecture that will become even more common with the 5G rollout.

Image credit: Author - MIMO with two RX/TX chains plus one RX.

Go Big

Some advice if you’re moving up to bigger boards: At the very start of a design, the only components that are set within the board outline are those who have a designated location. The most significant device of all is the first one you encounter on a typical schematic. The “brain” may be a simple controller or a System In Package. The central nervous system flows from there. Beyond that, I find it most useful to gather the other pieces into functional groups for a rough placement outside the perimeter. Bear in mind that any of those groups have the potential for reuse.

The unconnected lines and especially the voltage pins will drive a lot of the functional placement sorting. Basically, the components that would suffer the most from being away from the center would get to be near the main device. Bypass caps generally go on the opposite side and everything else crowds around the boss chip.

The Memories

Memory devices typically get the first priority along the processor edges where those pins are located. The evergreen requirement for more memory often relies on using the same device multiple times. The group of components that accompany the repeated memory chips should be as consistently implemented as possible. The routing, while not identical, should be of similar geometry for the controlled-length and controlled-impedance connections.

Note that not that all of the stakeholders have a stake in being close to the main logic device. GPS and NFC devices are likely candidates for the far reaches of the board. Sensors of most types like their open space as well. These are not so much repeated circuits as they are types of circuits with similar functions. The common theme, in this case, is that the devices have trouble coping with noisy neighbors.

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