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Layout Guidelines for High-Speed Boards

Updated: Jan 18

Do we need to write them or not?


Sometimes, we engineers try to be innovative. This is usually an advantage, but sometimes it can also be a disadvantage.

This time, I'm referring to the Layout Design Guidelines document, which gathers, or should gather, all the instructions, restrictions, and requirements for the specific board we are designing.

It's true that most PCB design tools now can define Constraints, but there are a few good reasons why it's still a good idea to write a document:

  • I write it as a checklist for myself. Then, I can also enter these restrictions through the Constraint Manager of the drawing tool without missing a single Net.

  • This Checklist allows us to go over all the requirements with the PCB editor together and coordinate expectations.

  • Sometimes, the drawing tool is not from the same manufacturer as the editing tool, and then, naturally, the Constraints will not be entered into the editing tool.


Engineers demand it...

In almost every High-Speed course I have given, I present a Layout Guidelines document as an example, and every time the engineers ask for a copy. For NDA reasons, providing a document from one project or another is impossible, but the following list from the Dgtronix workshop will faithfully serve the Board Designers, as follows:


Layout Guidelines for High-Speed Boards


1. General Block Diagram:

It is imperative to present a general block diagram to the PCB editor to clarify what the card contains on which he is working.


2. Power Tree of the system:

There are often many DC/DCs and LDO components in multi-Power Rails systems. It is advisable to present a diagram of the power tree indicating the current consumption from each supply. The PCB Layout editor can accordingly route power polygons and power planes for high-current consumers or power lines for low-current consumers.


3. Mechanical design:

Although the editor receives a dxf file, the Layout Guidelines document should provide information about the dimensions of the PCB, its thickness, the mechanical holes, and the mechanical constraints in it.


4. Stack Up:

A precise definition of the Controlled Impedance requirements of each PCB layer for both Single-Ended and Differential Signals.


5. Length matching:

A table that summarizes the list of lines and Buses for which lengths must be matched (length matching depends on the Set Up/Hold requirements of the component, the difference between the shortest and longest line in the Bus, and the propagation speed of the signal in the PCB).


6. Power planes, layers, and power lines:

PCBs nowadays employ multiple Power Rails, but each Rail has a different "weight" based on its current consumption. To ensure that the power planes will meet the required current consumption, it is advisable to summarize them in a table:




Table 1 - Power Rails


7. GND planes of its type

Similar to power supplies, the GND return path should be summarized in a table similar to the power plane table:



Table 2 - Power Rails


8. Differential Pair Signals:

As is known, differential lines require "special care and handling." The following table summarizes the requirements from the PCB Layout editor for the appropriate treatment of the different pairs:



Table 3 - Differential Pair Signals


9. Handling of "special" lines:

The routing nature, controlled impedance requirements, consideration of XTALK, skin effect, and other parameters will vary from one signal type to the other.

A list of "special" lines, such as Analog, RF, High Speed, Clocks, etc., should be summarized and defined for routing restrictions, route keepout distance, "rounding" of the lines, Guard Traces, etc., according to each line.


10. Silk Screen:

Finally, it is advisable to prepare a list of requirements for anything related to silk screen printing, such as company logo, Part Number, PCB version for the PCB, ESD antistatic logo, sticker borders, user guide instructions, etc. To locate the components more quickly in the debugging and re-work stages, performing the "Renaming" of all the components' "Reference- Designators" at the end of the routing and before generating the final manufacturing files is recommended.


Summary:


Centralizing all relevant requirements for a card in a single document is desirable. This document primarily organizes the designer, and if it is well written, it will also prevent many errors and problems, both in production and in the card's compliance with its requirements specification, as well as regarding Signal Integrity performance.

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