As the need of the hour is to stay connected anywhere, anytime and to anything, the data-rates and bandwidth required to cater to this connectivity euphoria is astonishingly huge and it keeps increasing. To meet these demands, datacenters, telecommunication, storage servers and similar system designs have evolved tremendously over the past few years. This has led to a rapid evolution of the existing high-speed interconnect standards like PCIe, USB, Ethernet etc. and introduction of novel standards like CXL. These developments eventually lead to revolutionary changes in high-speed board designing trends so that such insanely fast signals can be transmitted without compromising too much on the signal integrity aspects.

From Rubylith to Dot and tape to modern EDA tools, the evolution of PCB designing has been frenetic to keep pace with this insatiable data rate and bandwidth demands. Modern day PCB layout tools regularly introduce new features and automation to ease the effort needed to layout a high-speed board, thereby reducing the overall project schedule and quickening time to market.

Although many aspects can be automated in modern EDA tools, the human touch and the knowledge gained through experience cannot be substituted. Even to effectively use the right automation techniques, it is imperative to be aware of the technical and theoretical aspects while deciding where exactly to apply a particular technique. Apart from technical knowhow, the imagination and creativity of the “person in front of the tool” is highly critical to successfully completing a high-speed PCB layout. For instance, the ability to imagine and visualize the signal flow while reviewing the layout is crucial to identifying flaws in design. We will look at a few related scenarios in the sections that follow.

Creativity and Imagination: How do they matter?

A dense, high-speed layout can be equated to a complex “find your way out” puzzle where the aim is to guide the signal from transmitter to receiver through the shortest possible route having minimum discontinuities and reflections. A creative CAD engineer should be able to lay the pathway, namely PCB traces, for the signal to travel from its source to destination without impacting the signal integrity.

While placing the components, following the signal flow is of utmost importance. Having a mental picture of the signal flow makes it easier to place the components in a manner which minimizes signal loss. Picturizing analog, digital and power supply sections and reference planes ensures these sections will be well defined and care will be taken such that the sensitive analog section will have sufficient clearance from digital section and power supplies.

Laying down the high-speed traces is obviously the most critical part of a high-speed PCB layout. While planning the high-speed signal traces, being imaginative is key to identifying the shortest path for the signal considering the given stack-up to ensure minimal exposure of the signal to external noises while moving from the driver side to the receiver side. In addition, keeping in mind the discontinuities introduced by residual vias stub, sharp bends, and component pads, will ensure that remedial techniques like Blind vias, back drilling, arc routing, voiding reference plane under the larger component pads are not missed out.

Ground return path on the reference plane is one of the most important aspects of any PCB layout. At higher frequencies, current traverses through the least inductive path, which would be right below the signal trace. In case of stripline, identifying primary and secondary reference planes from stack-up is critical as return path will be through primary reference. If a signal crosses a split in reference plane, it can lead to impedance discontinuities and impact the signal integrity. Moreover, the return current would be forced to take a longer path as it would have to travel around the split. If along with the signal path, the CAD Engineer can imagine a path which the return current traverses while ensuring continuity, SI and EMI issues in board could be reduced to a large extent.

While phase turning, especially dynamic segmented phase tuning for very high-speed differentials, visualizing the signal traversing through the pair will help in identifying exactly where a mismatch is taking place and phase tuning the pair close to the region of mismatch. This is very significant as the data-rate of the signal increases. Having a signal match to within one clock period delay will ensure better cancellation of common mode noise.

Defining power planes is another factor which needs a high degree of imagination. If the designer is able to visualize the power generation and entry points along with the current flow path, it would be easier to avoid bottle necks in the plane and ensure a smooth current distribution. Having an idea of current rating of the rails and the amount of current a particular track or plane can carry makes it much easier to define proper power planes.

Silkscreen alignment and orientation, though electrically non-critical, are very important during testing and bring-up. A wrong silk marking can delay the debugging process considerably. During layout, the ability to visualize the perspective of test engineer makes it easier to correctly align and orient the silkscreen. Being imaginative also helps in making sure that the silkscreen is defined in positions whichwill be easily visible after components are assembled in the board.

Fiber weave effect can cause signal degradation at higher frequencies due to the minute gaps between the woven fibers which leads to discontinuities. If the designer has an image of high-speed signal flowing through the fiber weave, then care can be taken to overcome this effect. Having high speed traces routed in zig zag patterns helps minimize the fiber weave effect. Also, the whole artwork can be shifted by 10 degrees or so during fabrication to minimize losses due to a fiber weave effect. A simple Fab note indicating the angle of rotation would be sufficient to intimate the PCB manufacturer about this.

Conclusion

In short, along with technical knowledge, having a vivid creativity and imagination does helps in designing a first-time right high speed PCB Layout with minimal signal integrity issues. Aptly, one of the Gerber file formats is referred to as “art” files as it is the result of a creative artwork that is “High Speed PCB Design”.

 

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