Early in my career, I worked for a supervisor who would occasionally utter, “form needs to meet function.” While I respected his knowledge, I was new in the position and—to tell the truth—had no idea what he was talking about. I had no point of reference. But…”form needs to meet function” sounded cool enough so I liked it.
In our world of PCB design, “form needs to meet function” has an increasingly important meaning as we talk about multi-board design form factor. Very small devices work at very fast speeds within consumer and industrial applications. Rigid-flex designs often include multiple interconnected boards. While we must pay attention to signal and power integrity, each board must fit within an enclosure that meets the product specifications.
The challenges of working with multi-board PCB designs include managing interference across multiple boards and controlling signal timing from one PCB to the next. In addition, your PCB design must check for system level connectivity within the electrical and mechanical environments. Finally, the combined characteristics of each PCB and the overall enclosure must match the product design requirements.
Working with Multi-Board PCB Designs
Multi-board PCB design can take a few twists and turns. Depending on the product design, a single PCB design may have multiple uses. Or multiple devices may use the same PCB design. Interconnecting multiple PCB designs forms a complete and functional system. When moving from concept to design to production, you should manage each board as a single unit that has its own lifecycle. Also, you should manage the interdependencies between the boards. Many times, the placement of a component on one board impacts the layout of a second board in multi-board PCB design.
Managing individual PCBs within a multi-board design requires consistent signal names, design rules, and good design software. You also need to establish whether the design works from a single schematic that segments into smaller sub-circuits or whether the design uses a top-level schematic that contains multiple lower-level schematics. With the second choice, you can compile multiple PCBs from the sub-circuits. The schematic consists of several pages.
A third method—called logical system design—may serve as a better option. With logical system design, each child board associates with a module. Each module references the specific PCB project and the board within the project while connecting to other modules as part of a multi-board schematic. You can use design software to establish connectivity between the modules and to populate the modules with your design information. Logical system design also provides a method for tracing signals across multiple PCBs.
Logical system design requires attention to document management because of the need to transfer information from schematics to assembly documents. You can achieve a better document management structure by establishing a document hierarchy that includes the multi-board schematic, the multi-board assembly document, and the child PCB projects. Because any change to a child project impacts the entire system design, your document strategy must also include Engineering Change Orders (ECO). With this approach and the proper versioning tools, you can maintain changes between the original design and the current design.
When you apply logical system design within your design software, you can assign signal names, place connectors, plugs, and sockets, and verify the signal paths. Your design software should also control trace lengths, detect net-to-pin assignment errors and interconnection wiring errors. Each of these factors assists with the design as you move from schematic to layout and begin to consider issues such as routing, trace width, and component clearance.
Achieve the Right Form Factor through ECAD and MCAD
We do not have the luxury of using multiple enclosures for a multi-board design. Instead, product designs require the appropriate multi-board design form factor and dictate the physical placement of multiple boards within a single enclosure.
Cluttering your circuit board without proper checking can be disastrous
Solving this dilemma occurs through the ability to transition designs from the electronic computer-aided design (ECAD) environment to the mechanical computer-aided design (MCAD) environment. Working between the ECAD and MCAD environments allows us to support electrical connection needs while considering the placement side, position rotation, and origin of board connections and cable connections in relationship to other boards.
For example, because rigid-flex designs utilize dense component packing, you can observe electrical rules and maintain the mechanical design needs. ECAD tools also provide a method for verifying how PCBs electrically connect. MCAD tools add the functionality of collision checks for the components. In addition, we can use the capabilities found within the MCAD environment to rotate and align a PCB with other boards.
Obtain the Correct Multi-board Form Factor with 3D Design and Visualization
Working with multi-board designs requires new skill sets. Along with having the knowledge needed to work with ECAD and MCAD software, knowing how to efficiently use 3D design tools can smoothly and rapidly move your design into production. 3D design begins with importing 3D format STEP and body object files into the MCAD system. You can use 3D design tools to simulate the position of all the boards while remaining focused on electrical and mechanical functionality.
3D design tools allow you to:
Create 3D component models
Import standard-format 3D components
Model 3D component footprints
Import 3D models of cases or enclosures, and
Perform 3D collision checks for the PCB, components, and enclosure.
Collaborating on design and production ensures easier device finalization
For example, you can use the tools to zoom into a specific part of the board to examine if the height of components or connectors presents a problem. 3D design environments allow you to select objects, move objects along an axis or rotate an object around an axis. With the added capability of 3D visualization, you can seamlessly design and model mechanical and electronic details. As a result, you can easily and quickly achieve Design for Manufacturability (DFM).
Of course, along with these tools perfect for design visualization and achieving multi-board form factor, you’ll want a tool with all of the standards you’ve gotten used to with the power you need: high-speed layout, pin-count adjustment, integrative design rule checking and analysis functions. Combine your PCB design necessities with your manufacturing demands in Allegro’s PCB Editor.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.