Multi-board vs Multilayer Designs and the Need for System Level Design

May 3, 2019 Cadence PCB Solutions

Picture of car engine as a metaphor for multi-board vs multilayer designs

 

The first time I opened the hood on my first car as a teenager to do some maintenance, I was instantly intimidated by the complexity of the engine and all of its different components. All I wanted to do was to simply change the oil, but I had no idea where to begin and needed my dad’s help in order to proceed. Years later I would routinely do more complex repairs such as replacing water pumps, refurbishing brakes, and tuning the engine without giving it a second thought.

It is human nature to feel overwhelmed when first starting out in something new, and as PCB layout designers we can see the same thing happen in our work. Do you remember the first time that you looked at all the unrouted nets on a design and had no idea where to start routing? Very quickly, you learned how to organize all those unrouted nets so that you could effectively route the board.

The next big challenge was probably the first time you looked at a complex multilayer design, and found that it was just as intimidating (or at least I did). We all learned how to master the art of laying out a multilayer design in time, now the next challenge is here: system level designs. Let’s take a look at system level multi-board vs multilayer designs and see how knowing how to work with system level designs is becoming more and more important for PCB designers.

Multi-Board Vs Multilayer Designs and What that Means for Layout

When first starting out in PCB layout, most designers work on simple one-sided or double-sided boards. Adding layers to these PCBs means increasing the complexity of the board design since the designer now has more layers to manage. The additional amount of layers does give increased latitude as far as routing options, but it also adds more rules to follow as well. Some of these rules include:

  • Trace Widths: These may change depending on which layer is being routed on, especially with controlled impedance routing requirements.
  • Spacing: This will change from external to internal layers depending on the needs of electronic or mechanical keepout zones and spacing requirements.
  • Routing Direction: Usually the routing needs to follow a general horizontal or vertical routing pattern per layer to keep the metal on the board balanced and to minimize crosstalk between signals on adjacent layers.
  • Routing Across Plane Splits: When routing high speed transmission lines, care must be made not to cross an area where an adjacent power plane on the next layer is split. This will negate the return path of the transmission line and seriously degrade its signal performance.

Multilayer designs are growing in necessity due to devices requiring higher speed, more complex circuits in smaller sizes. As you can see, there is a lot for a designer to learn when stepping into multilayer designs from the simple double-sided boards that they started from. This is a pretty normal adjustment, however, and designers will continue to grow in their knowledge of designing multilayer boards.

Just as when a layout designer had to deal with these new challenges when first laying out a multilayer design, there will also be some new challenges ahead for their first multi-board design. They will have to learn new features of their PCB design tools to make sure that all boards and their associated schematics are accounted for and connected.

They will have to learn how to leverage the power of multi-board design systems as they design each board to fit together with the other systems boards. The need for multi-board design is becoming increasingly important, especially in light of saving time and effort over the old way of doing system level design.

 

Picture of dense multilayer design in multi-board vs multilayer designs

PCB designers will need to ramp up their multilayer skills for multi-board design

The Need for Multi-Board Designs

In order for system level design to work, all the system boards need to work and fit together. Traditionally this has meant a system of design and prototype builds that takes time and resources. The workflow for this traditional system is generally like this:

  • Conceive the top level design and determine each individual board size and shape.
  • Design each PCB as an individual unit.
  • Build prototypes of all individual PCBs.
  • Test all prototype boards for form, fit, and function in a complete system mockup.
  • Go back to each board design and make corrections and enhancements as needed.
  • Repeat the prototype build and testing phase again.

As you can see, this workflow is dependent on building prototypes to test whether or not the PCB designs will actually fit and work together as planned. Unfortunately, this workflow is also time consuming and expensive, and time and money are luxuries that companies can no longer afford in order to meet the demands of today’s fast-paced market.

Yes, prototyping will always be needed for full final system verification, but the key is in being able to reduce the amount of intermediate board spins. Multi board designing makes it easier to decide the position and type of connectors to be used at an early stage, saving time and money. Furthermore, it reduces interactions with the mechanical team to decide positioning of various pcbs in consideration of EMI or EMC effects of boards on each other. By utilizing multi-board design, PCB designers will be able to reduce the amount of board spins for prototype testing, which will carve a lot of time and expense out of your design cycle.

 

Screenshot of OrCAD layout 3D in multi-board vs multilayer designs

For multi-board design, powerful PCB designs tools with 3D capabilities becomes obvious

 

How Multi-Board Design Will Advance System Level Design

Multi-board systems are being used in increasing amounts today in multiple different technologies. You can find instances of multi-board designs being used in everything from spaceflight to self driving cars, wherever the need for adding more processing power to a system exists.

With the increasing need for these multi-board systems, the need for PCB designers to be able to design multi-boards within their PCB design tools has become more important than ever before. PCB designers need to be able to take their current multilayer PCB design skills to the next notch for multi-board design.

To do this will require designers who are willing to step up their game. They will need powerful software tools that will give them the resources they need for creating multiple schematics and layouts that are interconnected. These tools will also need to give PCB designers the ability to work with these layouts in 3D in order to see and correct mechanical form and fit problems before going to a prototype build.

The Allegro PCB design system has the multi-board design capabilities that we’ve been talking about. Allegro PCB designer has the power you need to step into the challenge of designing multi-board PCBs for system level design.

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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