A long time ago I was one of the photographers for my middle school’s yearbook. We used film that had to be processed in a dark room, and I still vividly remember the horrible smells of the developing chemicals. Thankfully there were other experiences from that time that are more pleasant to remember, including how we laid out all of the pictures and text of the yearbook. In a way, it helped prepare me for the layout process of designing circuit boards later on in my career.
Although it may look like the components on a circuit board are arranged haphazardly, there really is order and intent in their layout. They have to be located to not only provide the best performance of the circuits but also so that the board can be manufactured as easily as possible without any errors. Here in this second of four parts on circuit board design, we will look at some of the PCB layout best practices to help engineers create a superior design.
From Schematic Capture to Circuit Board Layout
In the first part of this series, we talked about some of the best design practices for creating a usable schematic. These included using up-to-date symbols from the part libraries and establishing a good logic flow throughout the schematic. We also talked about the importance of making the schematic symbols and text readable and legible, as well as including all of the pertinent part numbers, revisions, dates, and company legal and contact information.
With the schematic complete and in good shape, the next step is to make sure that it is ready to be coupled to the PCB Layout:
- Each symbol must be associated with the correct footprint library models for PCB layout. All of those capacitor symbols for instance must be assigned to the correct footprint package, whether that will be a 0402, 1206, 1812, or a thru-hole axial lead component. In addition, these PCB footprints must all be available for installation in the layout database.
- The board outline and layer stackup data must also be set up in the PCB layout database. Depending on the PCB layout tools that are being used, this may include the specific materials and widths of the layer stackup as well as board outline dimensions and specifications.
- Next, the layout design rules must be set up in the PCB layout database. These rules will govern how close components can be to each other during placement, as well as specifying the trace width and spacing rules for routing the connections together. Depending on the layout tools being used, these rules can get very detailed and be set up for the lengths and routing topologies of high-speed traces.
- Lastly, mechanical information must be added to the PCB layout database. Many tools will import this data from a mechanical design system, but for those that don’t, the layout designers will have to recreate this information. The data will include the location of specific components that have to interface with the outside world such as connectors, switches, and other mechanical features such as braces and stiffeners.
Once these tasks are completed, the board is ready to have its components placed on it.
The Seven Steps of PCB Layout Best Practices for Good Component Placement
For the best results, the layout designer will typically use the schematic as a guide for the placement of the components on the board. This will allow for the most optimum net connectivity between the parts, which will be placed according to this pattern:
- Fixed location parts: The first parts to be placed are those components that have specific locations designated by the mechanical input. These are often connectors, switches, LEDs, or other parts that either mate with other intra-system connections, or require human access.
- Processors and memory: Next, the processors and their associated memory components should be placed. These parts need to be located close to their connectors for best routing while giving enough room for the remainder of their associated parts to be placed around them. They also need to be close to the power components that will be supplying them, and for thermal reasons should not be placed on the edge of the board.
- Power supplies: These parts need to be between the connectors that are bringing power onto the board, and those high-current components that they are supplying power to (such as the processor and memory chips). At the same time, the different power supply circuits should have some distance between them to avoid creating thermal hot spots on the board. Additionally, these components need to have some separation from sensitive digital circuitry to avoid contaminating them with power switching noise.
- Bypass and decoupling capacitors: Processor and memory components usually have a large number of power and ground connections on them that will all need bypass capacitors close by for clean power. The placement of all of these caps can get very intricate around high pin-count parts, and a lot of care needs to be taken to make sure that you don’t compromise your trace routing channels.
- High-speed logic flow: With the main processor, memory, and power components placed, the next parts to be placed are those components that are part of the high-speed logic flows. It is important that these parts are placed close to each other for their signal paths to have the best signal integrity.
- General spacing requirements: While the different components of the board should be kept separate from each other for signal and power integrity, they also need to be close enough for short routing paths. At the same time though, care needs to be taken in their placement to allow for adequate routing channels between the parts. Some of the routings will also have specific spacing requirements for impedance controlled traces, differential pair routing, and other sensitive signals. All of this must be planned for while placing the components, while at the same time adhering to design for manufacturability (DFM) spacing requirements.
- Non-sensitive parts: Once the main components of the board are placed, the remainder of the parts can be placed. Even with the non-critical parts, however, it still is a good idea to place them so that their signal paths are short while at the same time giving them enough room for routing channels and DFM requirements.
Yes, there are a lot of placement requirements that have to be followed when laying out a circuit board. Designers typically spend most of their time moving components that are already placed, and it isn’t unusual at all to get down to step five from the list above only to find out that you need to re-do step three. For this reason, it is critical to get completely familiar with all of the component placement utilities available in your PCB design tools. These can help by allowing you to move multiple parts all at once, copy and paste unique placement patterns, or automatically aligning rows of components. And you also have another useful service available to you when placing parts on your PCB layout.
Working with Your Contract Manufacturer for Best Placement for Manufacturability
The PCB contract manufacturer that will be building your board can give you a lot of help with the component placement of your design. At VSE we have been working with PCB layout engineers like you for a long time now, and we understand the various design challenges that you are facing. We routinely provide feedback on DFM issues as well as recommendations on how best to increase circuit performance to our clients for their circuit boards. Our goal is to partner with you so that your design can be manufactured quickly, efficiently and without error so that it will operate the way that you intended it to.
