The goal of PCB design is to create a fully functional circuit board, which I completely agree with. However, my first goal-post during PCB layout is to get the board 100% routed. A layout begins first by setting up the design parameters and then placing the components to satisfy their electrical, mechanical, and manufacturing requirements. The only thing left after that is to connect all of the nets with traces. That sounds like it should be pretty easy, doesn’t it? Well sometimes it is, but most of the time it’s much more complex than people realize.
Many details have to be considered and worked through to successfully route a printed circuit board. For instance, power and ground must be configured so that you have a good clean signal return path for your sensitive nets, and traces that carry sensitive signals must be a certain width and kept away from noisy circuitry that might impact their operation. And that’s just some of what a designer needs to take into consideration while routing the board. Let’s take a look at some of these PCB routing best practices here in more detail in this third part of our four-part series on circuit board design.
Good PCB Routing Practices Start Before Any Traces are Laid Out
In part one of this series, we looked at the best PCB design practices of developing the schematic for the printed circuit board design. Using the schematic as the foundation for the design, we saw in the second part of the series some of the layout best practices for placing the components on the design. Now in this third part, we are going to see how all of the preparation that we’ve done so far will help in the routing of the net connections between those placed components.
Before the routing can be done, however, it’s a good idea to review the design and make sure that you are ready for this next step:
- Has the database been properly configured with an approved layer stackup?
- Have all of the design rules been installed along with any unique routing constraints?
- Are all the components placed on the board?
- Is the component placement optimized for the best connectivity patterns?
Assuming that all of these items have been completed, you are ready to start routing traces.
The Nine PCB Routing Best Practices for Circuit Board Design
There are a lot of different types and style of traces that will need to be routed on a circuit board, and these nine best practices will cover the majority that you will be working with:
- Routing tools: The first step in routing your circuit board is to make sure that you understand how to use the design tools that you are working with. It isn’t unusual to find designers wasting their time doing manual routing tasks because they weren’t aware of the automated features in their tools that could do it better.
- Escape routing: It is imperative to get all of the pins of a high-pin count part, like a 600 pin BGA processor chip, routed to a via so that they can be connected on an inner layer. Via-in-pad and micro-vias are often used for escape routing, and bypass caps and other small discrete parts that connect to these same pins are often routed at the same time.
- Power supplies: Power circuitry should be routed using short and wide traces to help lower the inductance of the trace as well as to control electromagnetic interference (EMI), and heat. It is a good practice to use 45-degree corners when routing these traces, or round the corners. It is also better to keep your routing on one layer and avoid using vias, which requires an optimum placement of the power supply components.
- Signal-paths: In high-speed circuitry, signal paths are very important. It is important to use short direct traces to connect these nets. As with power supplies, this is very dependent upon the placement of the parts, and the schematic should be used as a guide for both placement and routing.
- High-speed transmission lines: Due to the need to contain the energy of transmission lines, these traces should be routed on an internal layer immediately adjacent to or between two reference plane layers. This is known as a microstrip or stripline layer configuration and is critical for providing the clearest and most direct signal return path. It is also important when routing transmission lines that you do not cross over a split plane, as this can ruin their signal return paths creating a lot of unwanted noise on the board. There are different forms of high-speed transmission line routing, and your PCB design CAD tools typically have features built into them for routing controlled impedance lines, differential pairs, or sensitive signals that require additional spacing.
- Bus routing: Digital circuitry will usually have groups of nets to be routed that are referred to as buses. These buses are often data and address lines that go between the processor and memory components, and therefore may have high-speed design constraints attached to them. Buses should be routed together so that their lengths match for signal timing, and some CAD systems offer specialized auto-interactive routing capabilities for these nets together.
- Analog routing: Analog routing should be kept isolated from digital routing as much as possible. It should have its own separate reference plane for signal returns to guard against analog signal noise from contaminating the digital circuits.
- Power and ground: The traces for routing power and ground should be wide, especially for higher current circuits. Remember to use thermal reliefs for thru-hole pins and connections to discrete parts to prevent thermal imbalances when soldering. It is always better to use a solid metal plane when possible, and ground planes should be kept clear of slots, cutouts, and splits to serve as clear signal return paths. If a split plane is unavoidable, make sure that the split isn’t in an area where high-speed routing is going through.
- Clean-up: This is more than just making the routing look tidy. There are some situations, such as dangling traces, that can affect the signal integrity of the board because they behave like antennas radiating energy. Make sure to use the tools you have to find and correct any problems left over from the routing.
These nine best practices will help you with most of the trace routing that will be required of you in your PCB designs. Remember that while trace routing has to conform to the constraints set up for them for both circuit performance and manufacturability, it is also an opportunity for creative expression on the part of the designer. In other words, trace routing can be a lot of fun, so don’t be shy about rolling up your sleeves and plunging in.
Your Local PCB Contract Manufacturer – The Best Source of Routing Information
You also have another source of information to help you with your trace routing, and that is from the PCB contract manufacturer that will be building your circuit board. PCB CMs are very well accustomed to working with all kinds of circuit board technologies, and they can help you with questions about layer stackups, assembly requirements, and much more.
At VSE we have been building circuit boards for over 30 years, and our goal is to make sure that your project is a success. Our procurement team will review your design’s bill of materials to make sure that there aren’t any price or availability problems with your components, and our engineering team will verify your design’s readiness for manufacturing. Our design staff is available to answer your questions about schematic capture and PCB layout or to offer their assistance in any other way.