High Speed PCB Layout Guidelines: Placement Tips and Strategies
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In real-estate, the buzz word is; “location, location, location”. Interestingly enough, the same can be said of high speed PCB layout. Although all aspects of the high speed PCB design process are important, component placement is especially important for ensuring easy routing, minimizing EMI, and possibly eliminating the need for some extra layers. Placement methods that work without a problem in a standard PCB design may not satisfy the critical signal flow requirements of a high speed design. For the design to work, it really is all about “location, location, location”.
Here are some tips and strategies to consider when creating your high speed PCB layout. First, we will look at some basic component placement considerations for high speed designs followed by the benefits of creating a floor-plan before placing components on the board. Lastly, but certainly not the least in importance, we will discuss termination resistors and where they should be placed.
Any PCB layout is like a difficult puzzle to be solved, with many competing objectives to be considered. Often times, you have some form factor constraints and layer count objectives you need to meet, and you need to ensure parts are placed in such a way to satisfy these constraints and many more.
Parts in a high speed PCB layout should generally be arranged in the following ways:
- Grouped by circuit block: First, group components that work together to perform specific tasks in the system together. As an example, all the components that play a part in power regulation should be grouped together.
- Grouped around large processors: These components tend to have high I/O count and will interface directly with your grouped circuit blocks. Try to arrange the first-level circuit blocks around your central processor, and then try to arrange the downstream blocks around those.
- Grouped by access to routing channels: If you have a set of components that need to access a common interface on another component, try to arrange these components so that their pins face each other. This won't always be possible, but if you're successful you won't need to route through internal layers or in long paths around other components.
In the image below, you'll see a large MCU in the right-most area of the layout, and there are other components grouped around it with their pins facing the MCU. Farther to the left, we can see that there are secondary components like connectors, LEDs, and some passives. Notice that these are roughly lined up to face one side of the MCU. This enables some routing direct from the MCU to those board regions in the left side of the board.
High speed PCB layout example
When planning out the of placement functional blocks of circuitry, keep in mind the power and ground plane needs as well. Using continuous power planes is usually preferred, but in the event that the needs of the design necessitate a split power plane for multiple voltages, exercise caution placing connected components across the split. High speed transmission lines should not cross splits in power planes as that will break up the return path for those signals. Also, avoid placing other components that are not part of a circuit between components of that circuit. This too will affect the return path for that circuit.
Let’s look a bit deeper at part placement for different component blocks, connectors, and other circuits.
Creating a floor-plan of your placement is an effective way to prepare a high speed PCB layout. By planning ahead, you can account for groups of components as was mentioned above, rather than being surprised as they are placed at the very end of the design.
Functional blocks of circuitry such as power conditioning, RF, digital, analog, etc., should be organized and placed as groups in order to minimize signal crossing. A pre-placement floor-plan will allow you to see what the signal flow between functional blocks is and how best to plan for it. For instance, group your power conditioning together as much as possible in order that their signals do not have to cross through sensitive areas of RF circuitry.
Also, plan your placement to keep connection lengths as short as possible. Additionally, components that are part of high speed signal paths (multiple nets that connect a series of components together), should be placed as close together as possible and not spread apart. You should also consider your routing channels when planning your placement to make sure that you have the necessary space.
You should avoid placing sensitive high speed devices close to the edge of the board. This is because the edge of the board can act like an open cavity that allows electromagnetic radiation to escape the edge of the board, meaning there can be more electromagnetic interference (EMI) that impacts other components in your system.
Any cables your board needs to data or power will need to reach a connector in your board, and these can radiate EMI. Therefore, it's usually a good idea to try and separate your connectors and high speed components, high speed digital functional blocks, and high frequency analog blocks. Most guidelines state that you should place connectors closer to the board edges and sensitive high speed devices closer to the board center to reduce EMI in your design.
High speed edge connectors for PCIe cards.
Thermal effects are another aspect of high speed design placement to consider. This is because high speed layout guidelines expect that devices may run at higher temperatures than standard components. To ensure that your hot components placement stay cool, plan your placement so that these components receive an unrestricted airflow. For example, don’t place taller components, like connectors, in the direction of airflow to a hot BGA.
Termination and Impedance Matching Networks
The final and most specific placement strategy is considering the placement of your termination resistors. High speed PCB designs may need some termination applied at the source end or receiver end of an interconnect, depending on the port impedances of the components and the impedance you need to match. These resistors are often treated as an afterthought drop-in once the main portions of the circuitry have already been placed. Since these resistors are part of the circuit as a whole, their placement is extremely important for it to function correctly.
No matter where you want to put your high speed components, you'll need to make room somewhere for any required termination resistors. So where should a designer place termination resistors, and what effects will this have on signal behavior? First, we need to consider whether we're adding termination resistors in series or parallel.
Parallel Termination (Pull-up, Shunt, or Thevenin)
Parallel termination is normally used to shunt the load end of a transmission line to ground when the load component has high input impedance. Sometimes, a pull-up resistor is used to adjust the signal level to accommodate the receiver. Shunt and pull-up resistors are sometimes used together, called Thevenin termination, which adjusts the signal level at the receiver and sets the load's input impedance to match the transmission line impedance. Check your component datasheets to see which termination method you should use in your designs.
This scheme puts one side of a termination resistor on the end of the circuit closest to the receiver while the other side is tied to power or ground plane. The greater the trace length from the load pin to the resistor, the more the circuit is prone to signal reflection resulting in signal degradation. This is why parallel resistors should be placed as close as possible to the load pin of the receiver, and they should have an immediate connection back to the power/ground plane through a via.
The purpose of series termination is to set the driver's output impedance equal to your interconnect impedance. In this termination scheme, a resistor is placed immediately on the output pin of the driver. Since the resistor is very close to the driver's output pin, the input impedance seen by the driver will be approximately equal to the transmission line input impedance.
High pin count components, such as BGA packages, will generally not require termination on every single driver pin. Certain interfaces may have on-die termination, so an external termination resistor won't be required. For those pins that do require series termination, there needs to be some available room around the outside of the component to apply termination. However, placing multiple series termination resistors for a large scale device will take up a lot of board space around the device. This will require planning ahead to make sure that adequate space is available without having to rip up and replace tracks in your high speed PCB layout.
Watch Out for Electrical Lengths
No matter which type of termination you need to apply, place the resistors close to the components they are terminating. Do not place them in the middle of a transmission line. If you do, the component can create a reflection at very high frequencies, causing signal distortion. The same problem occurs with high speed protocols like PCIe, where DC blocking capacitors placed in the middle of a PCIe lane create an impedance discontinuity that causes reflection and distortion.
This occurs because the trace section leading from the driver to the termination resistor, or from the parallel termination resistor to the load, is longer than the critical electrical length. For high speed signals, make sure you are only creating transmission line sections in your layout where you intended. This requires smart floorplanning to help ensure your high speed PCB layout will work as designed.
Floorplanning and other strategies that we’ve talked about will give you a great start when you're creating your high speed PCB layout. The best high speed PCB design tools can help you with the placement as well as many other aspects of the design. Professional PCB design software, like Altium Designer®, comes with the right tools for the job.
Would you like to find out more about the different high speed layout guidelines and capabilities that Altium offers its users? Talk to an expert at Altium.