Top 6 Considerations for High-Speed PCB Design

When a PCB design is a high-speed design, the first thing most of us talk about is clock frequency, that is some cases the right thing to do. But if that’s all you look at, you are likely to have failures, because there is another problem. When logic switches from either 0 to 1 or 1 to 0, we have rising and fall times. When switching happens, we will get a thing called reflections on the transmission lines. If we have high density boards, we also have a crosstalk problem, meaning interaction between two traces running side by side. So, how do we decide when a design is high speed when switch from one logic state to the other? That’s going to be what drives nearly all of us, even when the clock is not high speed.

What Is High Speed PCB Design?

High speed design is any designs happening quickly, but it’s not just about the clock frequency used on the printed circuit boards (PCBs). A design is a high-speed design when it includes devices with fast edges – devices that switch state so quickly that the transition is complete before the signal can travel along the route and reach the target pin. In this situation, the signal can be reflected back to the source pin, degrading or destroying the original signal data. A signal with a fast edge can also radiate from the route and couple into adjacent routes, or radiate further and become electromagnetic interference (EMI), resulting in the product failing to meet mandatory emission standards.

When a signal has fast edges, it changes the way that the energy travels through the routing. In a circuit where edge rates change slowly, you can think of the energy flowing through the routing like water through a pipe. Yes, some energy is lost due to friction as the water is pushed through the pipe, but basically most of it arrives at the other end. For a DC or low switching frequency circuit you can work out the resistance of the route and make sure that the amount of energy lost along the way does not affect the circuit performance.

It’s not so simple in a high-speed design because as well as the energy flowing as electrons through the routing copper, in a fast-switching signal, some of that energy also travels as electromagnetic energy around the routing copper. Now you are no longer designing copper pathways for electrons; you’re designing a series of transmission lines embedded in a printed circuit board.

 

PCB Layout Design Is Key

Efficient manufacturing of high speed printed circuit boards is critical to the end result. But before high-speed board manufacturing, PCB layout design is key. Signal frequency or speed must be considered when designing high speed boards. Most high-speed boards have different signal types and speeds, which, unless solved in layout design stage, can bring signal integrity problems. Advanced planning and adherence to important factors will go a long way toward providing designs that result in desired functionality and successful fabrication of PCB boards. These practices of designing for manufacturability (DFM) and taking extra considerations for high-speed circuit demands need to addressed in the early stage of PCB layout design rather than being an afterthought.

If the circuit board assembly has no manufacturing problems, but has performance issues when fabricated PCBs are put into testing or production use, it must be caused by poor layout design. At that point, redesign or rework become costly and time-consuming as circuit failure or performance issues are evaluated and layouts of prototypes are reconfigured.

 

Considerations for High Speed PCB Design

For high speed printed circuit 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 fixe or controlled impedance. Achieving this objective is best achieved by involving your electronics contract manufacturer (ECM) to assist you in selecting materials and testing. Special considerations that should be considered for high speed design, listed below:

Top 6 Considerations for High Speed PCB Design
What to Consider?	What to Do?
Trace Termination	Use resistance for series or parallel termination on traces to provide a constant impedance value.
Dielectric Impedance	Choose dielectric material with a stable dielectric constant at high frequencies and low loss.
Copper Roughness	Choose low profile copper.
Single-Ended Traces	Use straight point-to-point routes on a single layer. Do not use vias or right angles.
Differential Pairs	Make sure that differential traces are identical in length, width and copper weight. Use meanders, if appropriate.
Impedance Verification	Specify that your ECM performs Time Domain Reflectrometry (TDR) testing to check impedance matching.

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

 

Step-by-Step to Achieve Successful High Speed PCB Design

• Schematic documentation – detailed schematics form the basis for good layout design. Questions often arise at the fabrication point related to physical layout of the board. A clear schematic can save time in resolving questions by providing the intended flow of the circuit. Include as much information as possible on the design schematic including any specifics available such as trace lengths, required component placements, board housing information, etc.
• Extra set of eyes – often the circuit designer and engineer doing the layout will not be the same individual. In such cases it will add value for the PCB layout technician to consult with the initial designer to ensure the layout meets the specific criteria of the design. Collaboration at this stage of the design can save time and money in producing PCBs that function as designed and meet the intended results and performance levels.
• Component location – there are some major components that must be well-defined for board functionality, such as critical power locations and input/output points. Other component placement can also have an adverse effect on the overall performance of the PCB, including placement of individual circuits such that their physical proximity does not impact the functionality of another.
• Power supply – power supply bypassing to minimize “noise” is especially important for high speed circuitry. Incorporating common methods such as rails-to-ground or rail-to-rail can provide effective results.
• Parasitics – high-speed circuits can often be negatively impacted by unintended stray capacitance referred to as parasitics. Understanding the potential source of such inductance and avoiding this potential instability or resulting oscillations in the circuit is an important consideration for high-speed PCB design.
• Routing/shielding – signals on boards can interfere with one another, and with high speed circuitry the negative effects become even more crucial. It is imperative that PCB designers plan accordingly for how the board will avoid such conflicts between signal paths through several techniques:

1. Keep long parallel runs to a minimum on the same board
2. Reduce the occurrence of long traces on adjacent board layers
3. Maintain as much distance between signal traces as possible
4. If individual signal traces do require higher levels of isolation, route them on separate layers when possible

• Materials – fabrication materials utilized in board construction play an important role in reduction of noise and crosstalk of signals. Different materials have varying dielectric constants, and a lower constant at certain frequency results in signals that propagate at higher speeds. Click to check
• Trace construction – use of microstrip or stripline trace layouts can also impact signal performance. Values that come into play include the trace width, thickness, and the dielectric constant of materials in use. Each of these values impacts the resulting resistance, inductance, and capacitance of the trace, impacting the associated signal path’s performance as well as potential noise separation attributes.

 

High Speed PCB Manufacturer

To assist you, it is advisable that you partner with your high speed printed circuit board (PCB) manufacturer early to ensure your design incorporates manufacturing considerations, such as material properties, that will impact the performance of your boards. MADPCB is a reliable PCB manufacturing, assembly and design company, supports high-quality, quick-turn PCB prototyping and high volume production, is committed to working with you to utilize the best materials and processes for your high speed PCB board design.

If you are ready to have your high speed design manufactured, just to send Gerber files, fabrication drawing, quantities and BOM files to us for a quick quote. If you want more information on high speed PCB design or PCB fabrication considerations for you development, contact us.