Multi Circuit Board Design Rules to Speed Up Design Time

October 7, 2019 Cadence PCB Solutions

A collection of odd sized nuts and bolts used for mounting hardware

 

One thing that I really appreciated about my dad was that if you needed a one-inch #6 pan-head machine screw, he would likely have one in his shop that he could give you. I guess that it wasn’t so much that he had all of this mounting hardware in different sizes available, but that his shop was so organized that he could go easily find the needed part. When it comes to my garage workbench, however, I am not nearly so organized. I consider it a good day if I can even find the light switch in all of that mess.

The point is that organization is the key to efficiency. I would be much more productive doing chores around the house if I had my shop area organized so that I could easily find and use the tools and parts that are out there. Similarly, working efficiently with our PCB design systems can be affected by a lack of organization in the same way. Take design rules for example. A lot of designers tend to reinvent the wheel with their design rules on each new design instead of relying on having an organized system in place. This can really slow things down when time is of the essence, and there are ways to be more efficient with multi circuit board design rules.

Multi Circuit Board Design Rules: Consistency and Library Management

It is far too often that PCB design rules have been created from scratch for each new project that is being worked on. This can be because the design department didn’t have a process in place to reuse the rules, or because the PCB designers simply choose not to do it. Either way it is indicative of a design workflow that needs to be updated for efficiency and consistency. Without improvement, this workflow could result in a lot of extra time and effort being spent, as well as the inherited problems due to inconsistency.

In the case of design rules for multiple circuit boards, each design could end up with differing rules. Usually the expectation is that the designers should know the rules well enough that they will always input the same values on each design. Even if their memories are correct however, there is always the chance for input errors. 

Without consistency, multiple circuit boards that are part of an overall system could end up being checked to different rules. In the case of different boards being used on the same fabrication panel, this can be an even bigger problem. If one layer structure is different between the different designs, it could cause fabrication delays, bad builds, and complete redesigns.

The best thing that you can do to streamline the creation of design rules for multiple circuit boards is to save and manage those rules for future use. Schematic symbols, simulation models, design constraints, PCB footprints, and STEP models are all contained in some sort of structure that we call a library. Libraries can be as simple as a central file directory for data storage, like throwing things in a bucket, or they can be complex systems of multiple directory locations and links

No matter how you have your CAD libraries set up, there isn’t any reason why design rule sets can’t be saved in the same way. To successfully manage a “library” of design rules, there are three basic practices that you should incorporate into your system:

 

  • Location: Establish a location that is centrally located on a server that all users have access to. Too often design rules and other types of data that are saved for later use get stored on someone’s personal computer. This can restrict access for others and there is a greater chance of the saved files being altered or deleted.

  • Naming Convention: Name the saved files in such a way that others can easily identify what it is. Dates, descriptions, and job numbers are all good candidates for file names. “Joes_really_cool_design_rules.txt” although colorful is not particularly helpful.

  • Restrict Write Privileges: It is best to come up with a way that design team members can save their files to this location, but can’t edit those files once they are in there. It can be a real frustration to find that the design rules for a six layer board have changed since the last time you used them. Design team members can always copy those rules into a new file, make their changes, and then save them under a new name.

 

Once your design data storage system is in place, start using it to save those files.

 

Screenshot of multi circuit board design rules on Allegro’s Constraint Manager

Utilities like Allegro’s Constraint Manager simplifies the capture and editing of design rules

 

Multi Circuit Board Constraints, Testing, and System-level Integrity

Even with one circuit board to work through, signal integrity and power integrity are crucial factors of design one needs to consider very carefully. With a simpler power supply, less interfering components, and less distance to travel for a signal, the considerations become much less expansive than what might be considered for multi-board systems today. 

System-Level Signal Integrity and EMI

The greatest way to ensure proper multi-board signal integrity (SI) is to have a reliable and accurate SI analysis and simulation tool to work with your design every step of the process. Having both high and low speed signals interacting can certainly cause problems for your SI, on top of already the greater potential for crosstalk with all of those active components. In light of that, multi-board designs will almost always be at greater risk for EMI due to the interaction between high speed signals. 

In light of the tool to help you, or perhaps augmenting its capabilities with your knowledge, keeping analog and digital signals separate, having smart trace routing (meaning no right-angle traces in your circuits), and keeping signal and return current closely coupled can significantly mitigate EMI concerns. 

Design Partitioning and Multi-board Cost Analysis

Physically grouping components based on purpose and function can enable greater security in a number of factors. This process, known as PCB partitioning, allows for multi-board designs to have subsystems viewable as a set of components. 

Proper PCB partitioning enables easier times and greater cost efficiency with things like:

  1. Modularity, or incorporating standardized units into a larger, multiple product design.

  2. Board spacing and actually fitting the circuit in your enclosure.

  3. Creating cheaper, smaller boards to connect to main boards instead of fitting in expensive, multi-layer board stackups.

  4. And EMI, by separating analog and digital circuits in your multi-board design. 

PCB Constraint Managers and Multi Circuit Board Assistance

Whereas many constraint managers will come equipped with the impedance, stackup, net and plane options you need to get your job done, a good constraint manager that can also account for component listings and parameters will make multi-board designs much easier to manage, especially when you’re working through thousands of individual components on each board. 

Managing advanced net classes, setting thermal requirements and being able to quickly adapt thermal specifications in constraints, as well as handling signal qualifications are all responsible portions of your multi-board constraint manager. 

 

Screenshot of high speed routing in multi circuit board design rules

Complex routing like this relies on the correct design rules and constraints to be in place

 

Multi Circuit Board System Architecture and Design Tools

Managing connectors in multi-board systems are of vital importance for the overall integrity of the system. While you may have everything planned out properly, determining how the connectors handle being in the enclosure environment, whether you have a vertical stack or horizontal layer of circuit boards, and accommodating for potential corrosion (Environmental, galvanic, electrolytic, and more) can all be of vital importance in determining the proper connectors for your design. 

Taking into consideration the connectors requires careful management throughout the layout stages, and if there hasn’t been enough to consider already, you might pay even more careful attention to your design rules and constraints manager. This tool saved my day more times than not when after a long spell of re-routing traces, I was left seeing the world in various-colored lines. I didn’t have the mental energy left to run a final check on, say, via impedance values. But, thanks to the DRC, I was able to see the violation tangibly and fix the problem quickly instead of fretting. 

When design rules first started being used in PCB design tools they were not the easiest things to work with. You often had to go through multiple menus and fill out several different forms. Often those systems didn’t have a reliable way to save the information either, or if they did it would be in an unformatted text file. Times have changed however.

Today’s PCB design systems typically feature advanced design rules and constraints that are accessed through spreadsheet type utilities. Additionally these rules and constraints are often connected to other functionality in the design system. For instance, the impedance calculator may input its values directly into the rules and constraints instead of making you manually input all that data. 

The next step is to make sure that you are armed with the best PCB design tools available for setting up and working with design rules and constraints. Allegro PCB Designer has the spreadsheet type rules and constraint system that we’ve been talking about that can make your life as a designer much easier. With it you can not only create and modify your design rules, but you can save them for future use as well.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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