As signal speeds get faster in circuit boards, designers must do more to ensure the best signal integrity in their layouts. One of those areas that require diligence in design is controlled impedance traces. High-speed transmission lines must have a carefully calculated trace structure to guide their path without differences in impedance that can distort the signal. Not only does this affect trace routing but also the board layer structure. Here is a brief look at impedance control in PCB design and how to successfully set up PCB impedance control.
Controlled Impedance Trace Structures
| Embedded Microstrip | A trace within the PCB with a plane on one side and laminate on the other. |
| Offset Stripline | A trace within the PCB with a plane on both sides. |
| Edge-coupled Coated Microstrip | Differential traces with a resin coating on one side and a plane on the other. |
| Edge-coupled Offset Stripline | Interior differential traces between two planes. |
| Broadside-coupled Stripline | Two traces surround a laminate layer with planes on the other sides. |
| Coplanar Waveguide | Single-ended traces between in-plane ground traces for better isolation and shielding. |
| Offset Coplanar Waveguide | Like the coplanar waveguide, but internally between two planes for maximum isolation and shielding. |
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What Is PCB Impedance Control?
Electrical impedance measures a combination of resistance, capacitance, and inductance in the traces of a high-frequency circuit and determines the opposition of the current flow’s amplitude and phase. Although this opposition is measured in ohms the same way that resistance is, impedance is implicitly in the domain of alternating current (i.e., a non-zero frequency). Resistance, on the other hand, is a DC characteristic. Impedance can vary in value at different points along the length of the trace, which can degrade the signal’s quality. This risk of degradation makes impedance control in high-frequency circuits essential for signal integrity.
The circuit board’s traces behave differently at higher frequencies than regular connections and will act more like high-speed transmission lines instead. If one of these lines has different impedance values along its transmission path, the mismatches could reflect, reducing signal strength and introducing significant distortion. The magnitude of these reflections will depend on the differences between the phases, where a quarter-wavelength difference results in the greatest possible difference.
It is essential to control the signal’s impedance values during trace routing to ensure the best signal integrity and the continued high-frequency functionality of the circuit board. However, many factors affect the impedance of a trace, including the circuit board’s materials and the trace parameters. We’ll look further into these details next.
Impedance Control Layout Considerations
Impedance-controlled routing begins with the schematic. Design engineers should specify their controlled impedance signals’ values and types so the layout team understands which nets are single-ended or differential pairs. This specification can use the design tool’s rules and constraints to set up different net classes for controlled impedance routing and notating it on the schematic.
The following parameters must be considered for routing to ensure that the same impedance values remain constant throughout the length of a controlled trace:
- Trace geometry – The trace’s width and thickness (copper weight).
- Signal spacing – The spacing between the signal trace and its return path is usually on the adjacent reference plane layer. Configuration of trace spacing follows the board layer stackup.
- Dielectric material – The core and prepreg materials electrically isolate the controlled impedance trace layers. Their thickness and dielectric constant will be part of the impedance calculations.
PCB layout designers must calculate these parameters with the circuit’s impedance requirements to determine the appropriate trace width for routing. To perform these calculations, designers can usually use the trace impedance calculators built into their design tools. There are also circuit simulators that will provide these calculations and different online calculators and charts.
When it comes time to start routing controlled impedance traces in the layout, here are some considerations to keep in mind:
- Trace widths: Make sure the controlled impedance traces are distinguished from other nets to help the PCB manufacturer spot them easily. During manufacturing, these trace widths often need subtle changes, which can be complicated if the traces aren’t easily distinguishable. Highlight these by setting up net classes in the design data or specifying a slightly different trace width value in the Gerber files.
- Differential pairs – These traces must be routed parallel with consistent spacing between them. Do not split the pair around obstacles like vias; arrange them via fanouts to keep lines symmetrical.
- Spacing to other objects – Controlled impedance lines need to be isolated from nearby tracks, and a good rule of thumb is to use three times the trace width for their clearances. If a controlled impedance line is five mils wide, allow 15 mils clearance to other traces and components. Using design rules and constraints to set up net classes for controlled impedance lines will help set and maintain the extra spacing necessary.
- Clear signal return paths – Ensure that the reference planes adjacent to a controlled impedance routing will provide a clear return path. A blocked return path is one of the most common sources of EMI problems in a circuit board.
Your Contract Manufacturer’s Controls for Impedance and Quality
With the number of high-speed designs today, manufacturers dedicate significant resources to PCB impedance control during stackup design and routing. Their experience can help with the material and configuration choices a project needs. Here is some of the information necessary to get started:
- Net names of controlled impedance signals
- Target impedance for single-ended and differential pairs
- Routing layers of the circuit board for the controlled impedance traces
- Desired raw board materials
At VSE, we use design data, manufacturing drawings, DFM tools, and experience to validate your design against the requested board materials. If we find any discrepancies, we will work to bring the target impedance values within their specified range.
