Personal computing devices, IoT, and many other embedded systems now require a diverse amount of electronics built onto one circuit board. These systems usually combine digital and analog circuitry running at high frequencies, creating many problems for circuit board designers if they aren’t prepared. Controlling signal and power integrity in a design is essential to avoid EMI problems that can ruin the device’s performance and cause it to fail its validation tests. Here are some PCB design rules for high-frequency layouts that can help.
Preparing Your Design for High-Frequency PCB Layout
The first step in any PCB design of beginning with an organized schematic becomes even more critical when working with high-speed and high-frequency designs. The layout team will need to place components according to their signal path, and those connections must be detailed in the schematic with a logical flow. The schematic should also have its sensitive nets notated and grouped in net classes. These net classes will then pass into the PCB layout database and be available for design rule and constraint assignments. The last step is to synchronize the schematic with the layout database to ensure a smooth bi-directional flow of data from one to the other.
Over on the layout side, there are four areas of the design that should be set up before beginning work:
- Layout parameters: These parameters include layer colors, grid settings, fill patterns, default drawing widths, font sizes, and many others. Although you can adjust these at any time during the layout, it is always best to have a set pattern first so that you don’t have to go back and change some incorrect text fonts later. These settings are usually stored in a startup file that can be read in with the database for convenience.
- Board layer stackup: Most signal and power integrity problems stem from a poorly designed layer stackup. By starting with the right stackup, many impeding design issues can be avoided, including the overall routability of the design. Working together with a contract manufacturer is critical at this stage to define the best stackup for your high-frequency circuits.
- Controlled impedance: Along with the stackup, you need to calculate the controlled impedance lines to make the routing widths and spaces available for inclusion in the design rules and constraints.
- PCB padstacks & footprints: High-frequency designs may have specific pad size requirements, affecting how the PCB footprints are constructed. It is essential to settle on these before the components are placed to avoid redesigns later to accommodate changed pad sizes and footprints.
With these preparations made, it is now time to set up the rules and constraints of a high-frequency design.
PCB Design Rules for High-Frequency Layouts
PCB CAD systems typically have many rules that can be set up to govern the circuit board’s layout. These rules can be extensive, and some tools monitor silkscreen objects, solder mask coverage, and solder paste usage. These rules can be of great help, but only if the time is taken to set them up and use them. This article will focus primarily on the design rules that control component placement, trace routing, and power and ground planes.
High-speed and high-frequency circuitry needs to be as tight as possible to reduce the potential for crosstalk. At the same time, standard design for manufacturability (DFM) rules should be observed. Set up component-to-component minimum spacing rules as required by the manufacturer. Some parts will have different requirements than others depending on the assembly processes being used. Setting up specific clearance rules for a group of components in cases like these can be helped by combining these parts into one component class.
Design rules will provide the ability to set up a default width and spacing for routing traces, but it will still be necessary to set up some or all of the following to cover more specific needs:
- Individual trace widths and spacings
- Trace widths and spacings for groups of nets
- Specific trace widths and spacings for controlled impedance lines
- Trace width and spacing rules for differential pair routing
- Trace lengths and topology patterns for high-speed transmission lines
Some of these rules will help control crosstalk between traces and be careful during routing to avoid running lines close together for long distances. The crosstalk rules also apply to traces running together on adjacent signal layers, creating broadside coupling between layers. Also, make sure that the routing has a clear signal return path available on an adjacent reference plane layer, and don’t route high-frequency lines across any splits in the reference plane.
Power and Ground Planes
The design rules in your PCB CAD system will also provide control over the creation of your power and ground planes:
- Clearances between the planes, traces, or other design objects
- The connection style for pins and vias (solid connection or thermal relief pads)
- Minimal areas of metal to ensure the plane is robust enough for its current
In general, remember to keep different areas of digital, analog, and power supply grounds isolated but connected. Additionally, when power and ground planes are on adjacent layers of the board, make the power plane slightly smaller to avoid any coupling between the two on the edge of the board. And remember that ground planes are serving as reference planes and must be kept clear enough for high-speed signal return paths.
Many other design rules and practices will support in designing a better high-frequency circuit board, and a contract manufacturer can help.
Additional Help with PCB Design Rules
PCB contract manufacturers build many different circuit boards and are familiar with the various types of design technologies in them. With their experience, the PCB CM can help with the following:
- Part selection: The component engineers at the PCB CM help choose the best parts for the board’s functionality. They make alternative form, fit, and function component recommendations to improve the board’s performance and get better prices and availability.
- Circuitry enhancements: The CM’s engineers will make recommendations for design performance improvements or add testability and other necessary functionality.
- Design for manufacturability: The assembly team will also recommend changes to help improve the design’s manufacturability. These recommendations save time and expense by avoiding costly manufacturing errors.