High Speed and RF PCB Routing Best Practices

October 11, 2019 Cadence PCB Solutions

3D image of a high speed circuit board

 

The need for the next best thing is a never ending cycle, and nowhere is this better exemplified than in the world of electronic design. You probably would have never thought twenty years ago that you would speak a command for the lights in your living room to come on, but smart home technology has made voice activated lamps a daily part of our lives. 

For technology like this to exist however has taken the continuing advancement of electronic design to achieve that “next best thing.” The smart hub in your home for example has some fascinating features in it requiring a printed circuit board with design technology that have taken many years to refine and perfect.

To handle the advanced computing needs of these devices requires high speed circuitry in dense PCB designs. On top of that is the RF portion of these devices which increases the complexity of their small yet powerful circuit boards. To successfully design these boards, PCB designers need to balance the requirements of a board that has both high speed and RF PCB routing on it. Here is some information on how you can do this.

What Are the Key Points of High Speed and RF PCB Routing?

When you start working on a printed circuit board with high speed and RF circuitry in it, there will be a lot of specific high speed design rules that you will have to observe. You also need to remember however that no matter what, you are still designing a circuit board. This means that in addition to the new design parameters that you will be working with, you will still have to follow all of the regular rules of PCB design. These will include:

 

  • Design For Manufacturing (DFM) rules. If your design can’t be built because of basic DFM rules violations, you’ve got a much bigger problem on your hand. This includes designing your board for both assembly and test (DFA & DFT).

  • Keep in mind the basic PCB design rules for digital, analog, power, and mixed signal technologies. Just because you need more room for high speed traces or RF shielding doesn’t mean that you can ignore your regular PCB design spacing rules and squeeze things together.

  • Thermal issues with components that run hot. High speed circuitry tends to generate more heat, and you need to keep thermal issues in mind now more than ever. Make sure that you board is getting cooled the way that it should be.

 

Now that you PCB design is set up the way you want it, you are ready for the additional rules and constraints for high speed and RF PCB routing.

 

 3D PCB routing key graphic

Let’s look at some key points of high speed and RF PCB routing techniques

 

 

PCB Design Practices that will Help with High Speed and RF PCB Routing

For smart home, mobile, or other IoT devices, size is often a big factor in your designs. With smaller board sizes for high speed circuitry, you need to plan accordingly to make sure that you have the space that you need. Certain high speed routing may require specific trace widths to accommodate the specified ohm values, as well as extra spacing to minimize any crosstalk.

When laying out your high speed and RF circuitry, make sure your placement follows the signal paths laid out in the schematic. High speed circuitry is dependent on short direct traces between pins within a signal path, and you don’t want these traces wandering around the board before they connect. On the other hand, large data and memory busses will need to be equalized in length which often means that some of those lines will have to be lengthened. In this case you need to allocate enough room for tuning measured trace lengths to the correct values.

Another consideration is the correct layout of your power and ground networks. Some components need to be placed within dedicated power or ground plane areas to isolate their noise from sensitive high speed routing. You also need to place bypass capacitors next to each supply pin of an IC and keep them as close as possible. Another critical concern is to make sure that high speed transmission lines are not routed across power and ground plane splits. These traces need a contiguous plane for a return path, and a split will create a canyon that it can’t cross. This can cause EMI problems that will ruin the design’s signal integrity performance.

RF routing will introduce a whole new set of requirements for how you create your traces. Some of the trace topologies will be very different than what you are used to in order to create the correct size and shape. You will also be adding additional vias for shielding and increasing widths and spaces, all of which will require more room. Another aspect of both high speed and RF designs is in their layer stackup. It is important to precisely configure the layers and materials in the stackup to support high speed and RF microstrip and stripline routing.

The success of your high speed and RF PCB routing is dependant on many factors. These include adherence to high speed layout guidelines in order to correctly route high speed and RF PCB trace lengths. How you plan ahead for your RF PCB routing is also important as well as simply designing the basic circuitry of your board such as in how to connect a relay in a circuit. Let’s take a closer look at some of these details.

 

OrCAD PCB Designer 3D high speed and RF PCB routing

Some 3D high speed routing with tuned traces

 

Further Details on Routing High Speed and RF Printed Circuit Board Designs

For your high speed or RF circuit board design to work as intended, the routing must be done according to specific parameters that we’ve listed below.

A Closer Look at PCB High Speed Layout Guidelines

For success in routing high speed and RF circuitry on your PCB design, you need to follow some basic guidelines that will help you to avoid the more common problems. One of the first is to make sure that your schematic is set up with enough information to help the designers in layout with their routing. Another is to know precisely what materials and stackup configuration your high speed or RF design needs so that critical design data such as trace widths and spacing don’t change in mid-layout. Component placement also needs to be precise in order to produce short and direct signal paths, while high speed trace routing needs to follow topology patterns designed to create the best signal integrity performance.

 

If you’d like to learn more about high speed layout guidelines, you can read about it here.

Layout Tips for High Speed and RF PCB Trace Length

High speed and RF trace routing is extremely important for the best signal integrity performance of the circuit board. You need to keep the impedance of a trace within a narrow margin by controlling its trace width, as well as maintaining enough separation to guard against crosstalk. Power and ground return paths must be kept clear of splits or voids that can interrupt signal returns, and differential pairs and measured lines need to be within their tolerances. The best friend of the PCB designer in the fight to maintain good signal integrity during design is the design rules and constraints of their PCB design tools.

 

For more information on high speed and RF PCB trace lengths, take a look here.

 

Picture of an RF PCB trace length

An example of a tuned trace to control its length

 

How to Effectively Plan and Complete RF PCB Routing

Planning ahead is one of the best things that you can do when designing a printed circuit board with high speed or RF circuitry on it. One of your first objectives will be in deciding what the correct board layer stackup configuration will be as well as what board materials you will be using. This will allow you to set up the correct impedance controlled trace widths for your design for the best performance of your high speed transmission lines. There are also many other tips that will help you in your design such as providing an abundance of protection for your RF circuits with ground planes, shielding, and filtering.

 

To find out more about how to effectively plan and complete RF PCB routing, read here.

Finishing the Rest of the PCB Design and How to Connect a Relay in a Circuit

One area of high speed and RF design that can be a real problem is when not enough attention is spent on the non-critical areas of the design. When this happens, those non-critical areas can become a real problem in how the rest of the board operates. Here is where falling back on basic PCB design skills can be a great help, even if it something as simple as connecting a relay in your circuit.

 

To see how to connect a relay in a circuit, read more about it here.

To Design the Next Best PCB, You Need the Best PCB Design Tools

To design a printed circuit board with the best high speed and RF PCB routing on can be a challenge for any designer. With a standard board, you’ll have plenty of room and opportunity for component placement optimization, power delivery network (PDN) challenges, as well as mitigating crosstalk and noise. 

Thankfully, you do have an ally in your task in your PCB design tools. Design rule checks (DRCs) can be your best friend when it comes to highly demanding signals across your circuit boards. With the right PCB design system you can create a schematic with all the information needed for high speed and RF designs, even get all the information needed for your stackup from the beginning. 

The top systems also have simulation and analysis tools to model your circuits before you lay them out, and analyze them afterwards to give your design the best performance. And finally you need the most powerful layout tools available. These tools should have multiple routing features as well as a full and complete system of design rules and constraints to keep you at the top of the PCB routing technology curve.

 

Screenshot of OrCAD PCB Designer Constraint Manager

Being able to manage high speed and RF PCB routing constraints is easy in this manager

 

Fortunately for all of us engaged in high speed and RF PCB routing projects, the PCB design tools that we’ve described are already available. The PCB design system that has all of the features and capabilities that we’ve talked about here is from the Cadence line of high performance EDA tools. OrCAD PCB Designer has the features and functionality you need to precisely route your high speed and RF designs. With OrCAD you have access to a variety of different routing features as well as their state of the art design rules and constraint system.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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