Special Considerations for High Speed Board Design

May 9, 2019 , in Blog

When you first learn to drive, the expressway can seem scary. After all, with so many lanes and so many cars, it can seem like you are in a high-speed race. Comparatively, driving in the city seems safer, primarily because it is slower. However, the fact is, each situation presents its own set of challenges, which must be addressed to safely navigate through. For example, when driving in town, you have to be cognizant of pedestrians, while on the highway, vehicles entering and exiting are special considerations.

High speed driving

Likewise, signal frequency or speed must be considered when designing PCBs. Most boards have different signal types and speeds, which, unless addressed, can present problems for your board’s signal integrity. For example, failing to match impedances for differential pair routing can introduce electromagnetic interference (EMI). In contrast, lower frequencies are less likely to present such noise issues. Therefore, let’s take a look at high speed board design, the associated special considerations, and what to do for the ones important to your PCB development.

What is High Speed Board Design?

For many PCBs, especially the more complex designs, there are typically many signal types flowing across the traces, through the vias and to and from planes. These may include power, ground, and slow or low frequency, fast or mid frequency and high speed or frequency RF, analog and digital signals. When high speed signals are present it is common to refer to the board design in terms of the highest signal speed type or as a high speed board design.

Many designers would classify boards that where signals above 50MHz are propagating as high speed; however, signal frequency starts to become a factor at around 10MHz. Boards that are typically rated as good for high speed design are boards built to respond well to frequencies in the range of 500MHz through 2GHz. This may be a little confusing so let’s try to give a more definitive description of high speed design.

Definition of High Speed Design:

Generally,

High speed board design refers to the selections made and processes undertaken during the design of a printed circuit board such that signals that are categorized as high speed propagate along and through traces with the least distortion.

This definition obviously requires a definition of signal categorization. From the eBook DFM for High Speed Digital PCBs, signals can be categorized in speed domains as shown below:

Speed domain criteria table

In the above table, Tflight is the signal propagation time along the trace path while Trise is the signal transition time from low to high. Boards are classified according to the fastest domain for which there are signals on the board. Therefore, a high speed board contains a signal (or signals) that fall within the High Speed domain. If your board design is slow, no action is necessary. If it’s fast, then you can convert it to slow by reducing the trace length or following other simple methods. For high speed board design, on the other hand, special considerations are necessary.

High Speed Board Design Considerations

For high speed domain boards, small parameter variations can cause large reflections or signal loss. Avoiding or minimizing this requires that you design target impedance with the objective of establishing and maintaining a fixed or controlled impedance. Achieving this objective is best accomplished by involving your contract manufacturer (CM) to assist you in selecting materials and testing. Special considerations that should be considered for high speed design include the following:

Special Considerations for High Speed Board Design
What to ConsiderWhat to Do
Trace terminationUse resistance for series or parallel termination on traces to provide a constant impedance value.
Dielectric impedanceChoose dielectric material with a stable dielectric constant at high frequencies and low loss.
Copper roughnessChoose low profile copper.
Single-ended tracesUse straight point-to-point routes on a single layer. Do not use vias or right angles.
Differential tracesMake sure that differential traces are identical in length, width and copper weight. Use meaners, if appropriate.
Impedance verificationSpecify that your CM perform Time Domain Reflectrometry (TDR) testing to check impedance matching.

In addition to the above, it is also advisable to utilize frequency domain simulations during design to help hone in on the best impedance value. High speed board design is a challenging endeavor that may require a good deal of calculation, simulation, and testing to achieve the best result.

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To assist you, it is advisable that you partner with your CM early to ensure your design incorporates manufacturing considerations, such as material properties, that will impact the performance of your boards. Tempo Automation, the industry leader in fast, high-quality PCB prototyping and low volume production, is committed to working with you to utilize the best materials and processes for your high speed board design.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on high speed board design or PCB manufacturing considerations for development, contact us.

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