EMC, EMI, and what they mean to your printed circuit board design.
PCB design techniques that will help you to control the EMC of your design.
PCB design tool features that can help control EMI on your circuit board.
Circuit boards should be designed with care to manage EMC
Longer ago than I would care to admit, my parents gave me a “Playball” metal lunch box to pack my school lunch in. What made this particular lunch box unique was that it included a baseball game on the back panel that you played with little magnetic pieces. This first piqued my interest in magnetics, which is an interest that has stayed with me ever since. These days, though, my focus is more on the electromagnetic forces that can affect a printed circuit board rather than simple games on an old lunch box.
Electromagnetic interference (EMI) can cause serious problems for the intended operation and performance of a circuit board. Managing this interference requires that you design the circuitry on your board to be electromagnetically compatible (EMC) with other circuits on the board, or in nearby electronics. We’re going to take a closer look at what this means, along with some techniques for refining your design for the best EMC circuit performance. And yes, I still have that lunch box; I’m just waiting for its price to escalate on eBay for a big payout.
EMC and EMI on Your Circuit Board
Electromagnetic interference can be generated in different ways on a printed circuit board, and it can be radiated through the air or conducted by the board materials. The sources for EMI include how the board layer stackup is configured, how the parts are placed, and what techniques were used to route the nets. As you can see, good PCB layout techniques are critical in controlling these EMI emissions, which in turn prevents them from contaminating other circuitry or nearby equipment. This whole process of EMI prevention in order for circuits and nearby equipment to work together is known as electromagnetic compatibility.
The goal of all of this is to create a circuit board that can pass the standards and testing that are required of it. There are generic standards, product standards, and product family standards. What standards your board will be tested to will depend on the function and application that it was designed for. For instance, circuit boards that are designed for automotive use have multiple standards to meet such as CISPR 12 and CISPR 25 as well as ISO 11451 and ISO 11452. In the environment of a car, there can be hundreds of different automotive systems creating both narrowband and broadband emissions. A circuit board must be able to function without being victimized by the emissions of that environment as well as to not contaminate other nearby systems.
Next, we’ll look at some potential areas of design on a circuit board that can create EMI problems.
Potential Problems Due to Uncontrolled EMI in the PCB
As we said earlier, how a PCB is laid out is critical to how well it can be controlled for EMC. Let’s break this down into three different areas:
Board Layer Structure
Circuit boards are constructed with layers of conductive and non-conductive materials, and how these layers are configured can have an enormous effect on EMI. For instance, a six-layer board with its power and ground planes in the middle of the stackup, on layers 3 & 4, will not offer much in the way of shielding for the other routing layers. By moving these plane layers to 2 & 5, however, they will now provide very good shielding for the signals routed between them on layers 3 & 4.
Components should be placed with their most sensitive or “noisy” nets in mind so that they can be routed as short and direct as possible. This is especially true of power supplies where you want high current lines to have the shortest and widest traces. High-speed clock lines and other sensitive signals will also want the shortest and most direct path to minimize their ability to radiate EMI. In order to do this, the component placement must be optimized for the best signal path routing.
As we said, high-current and sensitive lines need to be routed as short as possible, but there are many other connectivity considerations as well:
Signal routing on different layers should alternate routing directions in order to prevent broadside coupling (crosstalk) between signals on adjacent layers.
High-speed transmission lines should be routed using micro-strip or stripline configurations with adjacent ground planes.
Sensitive lines should be isolated with more trace-to-trace spacing than regular routing.
Controlled impedance lines should be calculated and routed at precise widths.
Ground planes should be designed for clear signal return paths, especially for high-speed signals.
By starting with these, you will be well on your way to decreasing the potential of EMI in your PCB design. Let’s look a little further now at some specific design techniques that can help.
Using design constraints can help with EMC management by controlling the routing of power traces
PCB Design Techniques to Optimize the EMC Circuit Performance
One of the best techniques that you can use to help control EMI is to put the design rules and constraints system to work in your PCB design system. Many CAD tools, such as Allegro PCB Designer from Cadence, have a superior rules and constraints system that will allow you to specify specific line widths, spaces, layers, and directions for routing. This becomes very important when you are trying to control the trace width for nets such as power and ground. In the picture above, you can see an example of Allegro’s design constraints in specifying the width of the power trace that is being routed.
High-speed nets will also require unique trace patterns or “topologies” when you are routing. Here, the constraints can help by designating the connectivity between pins so that they are routed using the correct topologies. The constraints can also be set up to control the length of specific nets, or to match the length of other nets. This is important when routing lines whose lengths must be precise in order to make sure that the data can be captured on the rising and falling edge of the associated clock signal.
Design rules and constraints apply to much more than just routed traces; they can also help with many other aspects of the design. For instance, you can set up clearances for component placement to ensure that your parts are placed the correct distance from each other. This can be done for individual components or classes of components. You can also set up rules for which via to use with specific nets as well as many other rules to ensure that the board can be successfully manufactured.
Another important method of controlling EMI on your circuit board is in how you design your power and ground planes. Each signal that you route on your board must return to its origin, and that is usually accomplished through a ground plane. If the ground plane is full of obstacles, however, such as splits for other power or ground nets, cut-outs, or even a lot of holes, the return signal may end up wandering around trying to find its way back. This wandering can create a lot of EMI, especially when it is the high-speed signals that are blocked up. Fortunately, tools like Cadence Allegro have simulators and analyzers within them that will help you to find and correct problems in your design, such as blocked up signal return paths.
PCB design tools such as Cadence Allegro have many features to help with controlling your design
How Your PCB Design Tools Can Help
Here are some of the features in PCB design tools that can help you with creating a PCB design with controlled EMC:
Constraint management: You need a fully-featured rules and constraint management system that will allow you to control every aspect of your design from trace widths to routing topologies. The constraint management system in Allegro allows you to easily enter the rules for your nets using a spreadsheet format that works from either the schematic or the layout side of the design.
Simulators and analyzers: By finding circuit and design problems before you lay out a board, you can avoid a lot of the EMI problems that may surprise you later. Plus, many of the analysis features available within the layout system, such as Allegro’s Return Path Analysis tool, can help you to locate design problems before you get the board built.
Impedance calculators: To set up the correct layer structure you need to know what order the layers need to be in for the best signal performance. Here is where a built-in tool such as an impedance calculator can help you define your board requirements before starting the place and route.
Online links and data exchange: There is a lot of data and information available on how to build a successful board layer stackup for your design, but getting that data into your tools has always been the bottleneck. With the IPC-2581 two-way data exchange format, you now have the means to pull board layer stackup information, complete with board materials and thicknesses, directly into your design from the manufacturer.
All of these PCB design features can help you to better control the EMC of your circuit board while it is still in layout, and they are all available in Cadence Allegro PCB Designer. Cadence also has a lot of other useful information that can help you with controlling the EMC in your design, which you can see in this E-book.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.