High speed PCB design can be a complex topic with a range of in-depth design guidelines. Design requirements span all areas of ECAD design, including component placement, stackup design and routing, materials selection, and even selection of passives. Things also get complicated when you consider EMI/EMC requirements and the need to balance high speed functionality with compliance and reliability.
To help designers get started with a new project and to ensure their boards will be fully functional on Rev. 1, we’ve compiled some of the essential high speed PCB design guidelines covering the majority of new products. Textbooks have been written about these design guidelines, but we’ve distilled the important insights needed to ensure your board is functional, compliant, and reliable on the first revision.
Getting Started With High Speed PCB Design
When you’re starting a high speed PCB layout, you already know the important interfaces and major components in your design. Before transitioning to the PCB layout, there are three important steps to take as you plan your routing strategy and build your PCB stackup.
#1 Learn Your Signaling Standards
Before you route a single trace in the design, take some time to read an overview of the design requirements in your signaling standards. Make sure you understand the bus topology, length matching limits (if any), single-ended and differential impedance targets, noise margins/loss budgets, any off-die termination, and any other important requirements. Signaling standards information is available online and should be consulted if you’re working with any unfamiliar interfaces.
#2 Focus on the Stackup
PCB stackup design is the foundation of high speed PCB design. An incorrect stackup can cause a design to be a strong radiator of EMI, create power integrity problems, or make routing very difficult. If you focus on the stackup first, you can overcome many of the common challenges in high speed design.
Determine the required layer count: There are a few ways to accomplish this. For high net count designs, the typical approach uses the number of rows in the largest BGA component on the board to determine the number of signal layers. For most coarse-pitch BGAs with a single trace routed between balls, the number of signal layers will be the number of rows divided by 2. Add in required ground and power layers, and you have a complete stackup.
Select the right materials for your design: The main concern here is loss tangent. Most design “gurus” will tell you that you must use low-Dk materials, but the reality is that the loss tangent is the more important quantity. If your board is very large, such as a rackmount backplane with very long interconnects, then you will need the lowest possible losses, as insertion loss will dominate signal behavior on the interconnect. For very short lines, worry about precise impedance matching with low dispersion, as return loss will dominate.
Basic FR4 laminates may not be sufficient for your design.
Determine your impedance: By taking time to understand your signaling standards and selecting materials first, you’ll know your target impedance and can size trace widths/spacing to ensure you meet these targets. Most ECAD applications will have an impedance calculator you can use to determine the trace width required to hit a specific impedance target.
Adjacent power rails and ground planes: Placing these two layers adjacent enables natural decoupling of the power and ground nets to provide stable power. Using adjacent power and ground adds capacitance into the PDN and helps ensure lower impedance out to higher frequencies. Make sure you can do this on all power rails that will provide power to high-speed digital components.
Once you’ve designed your proposed stackup, send it to your fabricator to make sure it is feasible with their materials. Don’t wait until the design is finished to find out that the stackup needs to change. Otherwise, you might trigger a significant redesign, particularly on impedance-controlled nets.
DFM for High-Speed Digital PCBs
#3 Have a Routing and Layout Strategy
The location and orientation of your components will dictate how easy it is to route the board. A good idea in most designs is to centrally locate the largest BGA component and try to orient it to minimize net crossings. Try to implement the following strategies:
- Opt for shorter traces where possible, and try to minimize via transitions.
- Only route impedance controlled high-speed signals over a uniform ground plane; never place cuts in a ground plane in an attempt to provide isolation between groups of nets.
- If you’re producing a prototype, place test points on the board for important interconnects so you can evaluate high speed signal integrity in your layout.
- Route parallel buses on the same layer so that all signals have the same propagation delay and length matching tolerances.
- Eliminate very long via stubs with backdrilling. A less expensive option is to route those signals across the entire board thickness so that these transitions have small or no stubs.
Your CM Can Help With High Speed PCB Design
High speed PCB design guidelines can be difficult to balance alongside DFM/DFA requirements. Partnering with an experienced CM early is the best way to ensure your design is manufacturable at scale, test requirements are specified, and all important design rules are followed.
|Tempo's Custom PCB Manufacturing Service
Tempo Automation is an industry leader in PCB fabrication and assembly for nearly any application, ranging from small consumer products to complex high-speed embedded systems. We’ll help ensure your design follows the best high speed PCB design guidelines to ensure correct functionality, signal/power integrity, and EMC compliance.
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 Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
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 CAD files or how to incorporate your design into a CAD format, contact us.