What is high-speed PCB design?
Blogger: Zachariah Peterson
Today, most PCBs can be considered to be at risk for some type of signal integrity issue, often associated with high-speed digital designs. High-speed PCB design and layout focuses on creating circuit board designs that are less susceptible to signal integrity, power integrity, and EMI/EMC issues. While no design can completely avoid these problems, by following these high-speed circuit board design guidelines, they can be reduced to the point where they are not noticeable and do not cause performance issues in the final product.
After you create your schematic and prepare to transition to PCB layout, you need to take advantage of specific features in your PCB design tools to place and route it correctly. In your PCB design software, you will have the opportunity to prepare power and ground plane layouts in stackups, calculate impedance curves for traces, and view PCB material options for stackups. Most aspects of high-speed design revolve around PCB stackup design and routing to ensure signal and power integrity, and a suitable ECAD software can help you succeed in these areas.
High-speed digital design basics
So, what is high-speed circuit board design? High-speed designs specifically refer to systems that use high-speed digital signals to transfer data between components. The line between high-speed digital designs and simple boards using slower digital protocols is blurry. A general metric used to denote a particular system as "high speed" is the edge rate (or rise time) of the digital signals used in the system. Most digital designs use both high-speed (fast edge rate) and low-speed (slow edge rate) digital protocols. In today's era of embedded computing and IoT, most high-speed circuit boards have an RF front-end for wireless communications and networking.
Although all designs start with a schematic, the main part of high-speed PCB design focuses on interconnect design, PCB stack-up design, and routing. If you are successful in the first two areas, you are likely to be successful in the third area as well. Read the following sections to learn how to get started with high-speed design and the important role of PCB design software.
Plan your high-speed PCB stackup and impedance
The PCB stackup you create for your high-speed board will determine the impedance and how easy it is to route. All PCB stackups include a set of layers dedicated to high-speed signals, power, and ground planes. There are a few things to consider when distributing layers in a stackup:
Board size and net count: How big your board is and how many nets you need to route in your PCB layout. A physically larger board may have enough space to allow you to route traces throughout the entire PCB layout without having to use multiple signal layers.
Routing Density: With higher net counts and board size constrained to a smaller area, there may not be much room for routing around the surface. So when the traces are closer together, you will need more internal signal layers. Using smaller board sizes may force higher routing density.
Number of Interfaces: Sometimes it is a good strategy to route only one or two interfaces per layer, depending on bus width (series vs. parallel) and board size. Keeping all signals on the same layer of a high-speed digital interface ensures that all signals see consistent impedance and skew.
Low-speed and RF signals: Will there be any low-speed digital or RF signals in your digital design? If so, these may take up space on the surface layer that could be used for high-speed buses or components, and additional internal layers may be required.
Power Integrity: One of the cornerstones of power integrity is the use of large power planes and ground planes to meet every voltage level required by large integrated circuits. These should be placed on adjacent layers to help ensure there is high planar capacitance to support a stable supply to the decoupling capacitors.
PCB material options, number of layers and thickness
Before designing your PCB stackup, consider the number of layers needed to accommodate all digital signals in your design. There are several ways to determine this, but they rely on a little math and past experience with high-speed board design. In addition to the points listed above to consider the number of layers, large high-speed ICs with BGA/LGA packages can also determine the required board size. When doing BGA fan-out, each signal layer can usually fit 2 rows, and when building the stackup make sure to include power and ground plane layers in the layer count.
BGA fanout on FPGA with large polygons for powering in high-speed designs
FR4-grade materials can generally be used for high-speed digital designs as long as the traces between components are not too long. If the cabling does become too long, there will be too much loss in the high-speed channel, and the components at the receiving end of the channel may not be able to recover the signal. The main material property to consider when selecting materials is the loss tangent of the PCB laminate. Channel geometry will also determine losses, but generally choosing an FR4 laminate with a lower loss tangent is a good place to start in smaller boards.
If your wiring is too long, you may need a more specialized material as a substrate for the high-speed signal. PTFE-based laminates, polarized glass laminates, or other specialty material systems are good choices for supporting larger, high-speed digital circuit boards that have very long traces and require low insertion loss. 370HR is a set of entry-level high TG laminates suitable for small high-speed PCBs. For larger boards, materials like Megtron or Duroid laminate are also good choices. Before proceeding, check with your manufacturer to ensure your material selection and recommended stackup are manufacturable.
Impedance can only be determined after you create the proposed stackup and complete verification with your fabricator. Manufacturers can propose modifications to the PCB stack-up, such as alternative PCB material options or layer thicknesses. Once you have the gaps on the stack up and finalized layer thicknesses, you can start calculating the impedance values.
Impedance is usually calculated using formulas or a calculator with a field solver tool. The impedance required in the design will determine the size of the transmission line and its distance from nearby power or ground planes. Transmission line width can be determined using some of the following tools:
・IPC-2141 and Waddell formulas: These formulas provide a starting point for impedance estimation, and they produce accurate results at lower frequencies. Learn more about using the trace impedance formula.
· 2D/3D Field Solver Utility: The Field Solver is used to solve Maxwell's equations within the transmission line geometry you define for your high speed plate. Learn more about the industry's best field solver built into the PCB stackup calculator.
Using the Layer Stack Manager that comes with a field solver will give you the most accurate results, taking into account copper roughness, etching, asymmetric line arrangements, and differential pairs. Once you have calculated the impedance curve for your trace, you need to set it as a design rule in your routing tool to ensure that the trace has the required impedance.
Impedance calculations for transmission line designs in high-speed circuit boards. The Layer Stack Manager in Altium Designer contains an impedance calculator for calculating copper roughness.
Most high-speed signaling protocols, such as PCIe or Ethernet, use differential pair wiring, so you need to design for a specific differential impedance by calculating trace width and spacing. The Field Solver tool is the best tool for calculating differential impedance of any geometry (microstrip, stripline or coplanar). Another important result of the field solver tool is propagation delay, which will be used to perform length adjustments during high-speed routing.
Layout planning for high-speed PCBs
There are no specific rules or standards for where components should be placed in high-speed PCB layouts. Generally speaking, it's a good idea to place the largest CPU IC near the center of the board, since it usually needs to connect to all the other components on the board in some way. Smaller ICs that connect directly to the central processor can be placed around the central IC so that routing between components can be kept short and direct. Peripherals can then be placed around the board to provide the required functionality.
High-speed layout works best when the main controller IC is placed near the center of the board and other high-speed peripherals are placed around it. This is one of the reasons why motherboards place large processors in the center of the board. MiniPC projects in Altium Designer place their PCIe, DDR4, USB 3.0 and Ethernet peripherals around a central FPGA SoC so routing is easier.
After placing the components, you can use PCB design tools to help you start designing the layout. This is a sensitive part of high-speed digital circuit design because incorrect routing can destroy signal integrity. Signal integrity is easier to achieve if the previous steps are completed correctly. You should set up impedance curves in your PCB design rules so that any traces in your design are placed with the correct width, clearance, and spacing, maintaining controlled impedance during routing.
Cabling, signal integrity, and power integrity
Signal integrity begins with designing a circuit board for a specific impedance value and maintaining that impedance value during layout and routing. Some other strategies for ensuring signal integrity include:
・ Designed to shorten paths between components to ensure high-speed signals
・Minimize routing through vias, ideally using only two vias to access inner layers
・ Eliminate stubs on ultra-high-speed lines (such as 10G+ Ethernet) through backdrilling
・Note whether any termination resistors are required to prevent signal reflections; check the datasheet to check if on-chip termination is present
・Consult your manufacturer to learn which materials and processes can help avoid fiber weave effects
・Use rough crosstalk calculations or simulations to determine appropriate spacing between nets in the board layout
・ Keep a list of buses and nets that need to match in length so that adjustment structures can be applied to remove the offset
These points can be set as design rules for routing tools, which will help ensure you adhere to best practices for high-speed designs.
High-speed PCB routing
The design rules you set up in your high-speed design project will ensure that you meet impedance, spacing, and length goals when designing routing. Additionally, important rules in differential pair routing can be enforced in your routing, specifically minimizing length mismatches to prevent offset between traces and forcing spacing to ensure differential impedance goals are met. The best routing tools will allow you to edit trace geometry constraints into design rules, ensuring performance.
Length adjustments are used between traces across parallel buses and differential pairs to ensure delay matching and eliminate skew between the signals seen at the receiver.
The most important aspect of high-speed PCB routing is placing a ground plane near the traces. The stackup should be constructed to have a ground plane in the layer adjacent to the impedance control signal in order to maintain consistent impedance and define a clear return path in the PCB layout. Traces should not be placed over gaps or cracks in the ground plane to avoid impedance discontinuities that create EMI problems. Ground plane layout is not limited to ensuring signal integrity, it also plays a role in power integrity and ensuring stable power delivery.
Power integrity is a broad topic that is highly relevant to high-speed PCB design. Ensuring stable power delivery to high-speed components is critical in PCB design because power integrity issues can masquerade as signal integrity issues. Power integrity focuses on delivering low-noise power to components. The layout of the PCB stackup and PDN is a major factor in determining the level of power integrity in digital designs. If done successfully, power is delivered to fast digital components with low noise and very weak transient oscillations on the supply rails. Designing high-speed PCBs with good power integrity ensures low-emission, low-noise power delivery and eliminates some of the SI issues seen in high-speed interconnects.
Advanced tools for high-speed design and layout
A good high-speed PCB design software will consolidate all these capabilities into a single application, rather than forcing you to use separate workflows to overcome different design challenges. High-speed PCB layout designers must perform a lot of work on the front end to ensure signal integrity, power integrity, and electromagnetic compatibility, but the right high-speed layout tools can help you implement the results as design rules to ensure that the design performs as expected.
More advanced PCB design software will interface with simulation applications to help you perform industry-standard analysis. Some simulation programs are designed specifically for evaluating signal integrity and power integrity in new designs, as well as checking EMI in PCB layouts. Simulations are very useful in high-speed designs because they can help users pinpoint specific SI/PI/EMI issues before the design enters manufacturing. Some examples include return path tracing, locating impedance discontinuities in traces, and ideal placement with decoupling capacitors to prevent EMI.
Complete your physical layout using high-speed design software
When you need to build advanced, high-speed digital systems while ensuring signal and power integrity are maintained, use the best-in-class high-speed design and layout toolset based on a rules-driven design engine. Whether you need a densely laid out single-board computer or a complex mixed-signal PCB, excellent PCB layout tools will help you maintain flexibility when creating high-speed PCB layouts.
Achieve high-speed design and layout with Altium Designer® PCB design tools trusted by circuit designers, layout engineers, and SI/PI engineers. When designs are complete and ready for production, the Altium 365™ platform enables you to easily collaborate and share your projects.
#highspeed #PCB #design
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