High-Speed Layout Guidelines
How a clean and concise layout aids high-speed layout.
Material selection criteria for high-speed boards.
Steps to follow for land pattern creation to minimize work duplication and the chance of error in translating manufacturer’s data.
The complexity of printed circuit boards continues to escalate to meet the growing need for new automation in industry, wireless devices for consumers, and evolving technologies in the medical and aerospace fields. Keeping pace with this demand for smaller and faster circuit boards is the cornerstone of growth for PCB designers; engineering demands will ask layout designers to compromise less and innovate more. Design for high-speed PCBs is growing with the continued investment in IoT infrastructure for businesses and the general public.
A high-speed PCB layout can test a beginning designer. Layouts for high-speed design must place components to satisfy design for manufacturing and design for test requirements as well as route traces using industry-accepted design rules for width and spacing. Here are some stringent requirements and best practices associated with high-speed layout guidelines that designers should become familiar with.
Why High-Speed Design?
A Clear Schematic Enables High-Speed Designs
Though the schematic may be relatively disregarded for high-speed layout guidelines, easy-to-follow circuit diagrams provide a crucial first step in any layout. In addition to pushing the logic of the circuit into the layout tools, the schematic has always been a graphical representation of what the physical layout of the circuit should be. Creating a messy, haphazard schematic only adds to the difficulty of the layout because the intent of the circuit isn’t communicated. When it comes to high-speed design, clarity of intent is invaluable.
While multiple sheets can spread out the high-speed circuitry if necessary, it is more valuable to represent the circuitry flow in a logical manner that is easily understood when laying out the physical design. This holds especially for signal paths, which group components and nets that together represent a complete high-speed circuit. For a signal path to be laid out correctly in the PCB, the designer needs to see and understand what the path is on the schematic.
Some other details can be added to the schematic for additional clarity:
- Critical component placement locations and board sides
- Differential pair routing information
- High-speed routing information such as constraints for trace lengths, matched lengths, topologies, and controlled impedance lines
Don’t be afraid to add as much information as possible to the schematic to promote design intent in the board layout. Not only is this necessary if using layout is a joint task, but it will help individual designers stay organized as well.
Understanding your circuit board necessities is pivotal for high speed design
Board Materials and the Layer Stackup
One of the greatest resources for layout designers is the PCB manufacturer. When starting a new design, engage with fabrication and assembly managers as soon as possible: they will be able to help make the best decisions for board materials and stackups for high-speed designs.
Working with impedance calculators is also critical when planning the board layer stackup. Many advanced calculators will allow users to enter board materials and thicknesses to calculate trace widths for stripline and microstrip routing. Additionally, designers can browse catalogs of board materials to find the best match for the design based on settings like the dielectric constant (Dk), dissipation factor (Df), loss tangent, or more exotic use cases such as flex/rigid-flex. High-speed materials specifically will need to prioritize low Dk and Df values to minimize losses and out-of-phase signals, which can contribute to poor performance and excessive heat dissipation.
It is not only the materials themselves but also their construction that contributes heavily to high-speed performance. Most standard PCB materials contain a weave of glass fibers that provide rigidity and mechanical support to the board during lamination when the resinous component melts and sets. The electrical characteristics between the resin component and the glass fiber differ enough that length-matched high-speed traces can accrue timing delays throughout their travel if unaccounted for. While a layout could theoretically attempt to route in a manner to minimize the distance where the dielectric backdrop of length-matched lines vary, this can only be verified post-fabrication and is best avoided. Instead, weaves for substrate materials can be tightened, minimizing the potential for timing differences while relaxing constraints.
High-Speed Layout Guidelines for Component Placement
Component placement on a high-speed design starts with following the standard PCB layout practices and design rules. This means placing parts per design for manufacturing (DFM) and design for test (DFT) guidelines. Where it begins to get more complicated is in arranging components according to their high-speed circuit paths. A standard design affords considerable freedom to scatter components as needed across the board to balance out the placement or clear extra room for routing, but high-speed requirements require a designer to prioritize circuit paths. High-speed circuitry will often require very close placement between specific parts to minimize the additional distance that signals need to travel.
Follow the circuitry path laid out in the schematic to make sure that critical nets have the most direct connection between pins. This can often turn into a real balancing act to preserve the standard placement rules for DFM and DFT, and skilled designers will be asked to create a placement that satisfies all the requirements.
Additionally, components need to be placed so that signal traces do not cross-split planes. With high speed, this is even more important than standard layouts (already a huge EMI contributor) to ensure that each signal has a clear return path. If encountering larger than normal thermal issues in a design, improved placement to enhance passive cooling or even acting cooling methods may be required.
With smart high speed layout, you won’t run into trouble in your board design
Routing High Speed PCB Circuitry
A lot of the trace routing performed in a high-speed design will follow general best practices. One possible difference is that of length requirements between traces: some will have minimum length requirements while others might have maximums. Fortunately, CAD systems will have active measurement tools and features to assist in routing these lines. The aforementioned calculated trace widths for impedance-controlled routing (the ones first examined when setting up the board layer stackup) will help establish high-speed routing rules and attributes.
High-speed layout guidelines dictate the most direct trace path isn’t always going to be the ideal routing solution. For instance, the topology may call for a daisy-chain route, which will increase the total length of the net. From there, component placement may be adjusted to better set up the high-speed trace routing required. Similarly, high-speed transmission lines must be routed with the entire path of the signal in mind; it’s not only the trace from the driver pin of the IC to the resistor that needs attention. This is because a signal path continues through all components from driver to receiver.
Whatever the particular need for high-speed design, Cadence offers a host of PCB design and analysis tools that support the boards of today as well as the next generation of products. Coupled with the powerful yet easy-to-use OrCAD PCB Designer, design teams can hone in on expediting turnaround times while meeting demanding electrical performance requirements.
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