Factors to keep in mind when designing across multiple boards.
Performance considerations for different board-to-board connections.
The concerns of EMI for 3D PCB design.
3D PCB design for single- or multi-board assemblies helps reduce revision times by checking thermal and mechanical constraints
There’s only so much that can be done with a single printed circuit board (PCB). Advances in miniaturization and the steady growth in transistors that can be squeezed on a single chip have begun to butt up against physical limits. This limitation extends to the system level: with electrical system design more complex than ever, multi-board PCB design is becoming more of a necessity.
Supporting a multi-board PCB system design comes with its challenges, particularly how assembly fills 3D space since the dimensions are no longer bound to an in-plane outline and z-axis extrusion. Partitioning, intra-board connectivity, and EMI considerations all play a role in planning a 3D PCB design.
2D vs. 3D Board Design Challenges
Partitioning Assemblies With 3D PCB Design
Physically, partitioning is about grouping components based on functionality and nearby circuits (according to the schematic). Each functional subsystem can be viewed as components and their supporting circuitry. For example, a motherboard can be further subdivided into several functional units such as the processor clock logic, bus controller, bus interface, memory, video/audio processing modules, and peripherals (I/O).
In the context of multi-board PCB design, partitioning may be followed by placing component groups across different boards. Selective placement of components can yield many benefits:
EMC (electromagnetic compatibility) - Mitigate EMI concerns through best practices such as separating analog and digital circuits, isolating high-speed and rise time signals from surrounding traces, and utilizing ground pour with stitching vias.
Cost - For functional circuits that require more expensive multi-layer board architectures, it can be cheaper to use a smaller board that can be connected to the main board.
Modularity - Designing multiple products can save a business time and money by incorporating modular standardized units into a design, adding functionality to a baseboard as needed (e.g., think shields in Arduino chipsets).
Enclosure requirements - Fitting all your circuitry onto a single board doesn’t always make it practical for the physical dimensions and shape of the device’s enclosure.
3D PCB design involves craftiness and creativity when working to solve the particular problems of forcing multiple components with different voltage and current requirements together into a functional design.
Determining board partitioning is a good step in managing your system design.
Electrical System Design and Intra-Board Connectivity
Determining connectors for electrical systems is more than just determining what works best for your available production budget. Connectors are multi-faceted and can be make-or-break in some design cases when you’re working through particular power demands. Intra-board connectors serve as the cornerstone of multi-board PCB design. Here’s a quick look at the different types of intra-board connections:
Board-to-board - Male/female and pin/socket headers are the most common type of board-to-board connector out there. They tend to be low-cost and are not ideal for high-speed circuits. However, you can use higher pin counts and multiple pins to handle larger current draws. A good rule of thumb is to be mindful of the manufacturer’s rated current handling capacity per pin.
Card edge connector - Traces leading off the edge of one board can be inserted into a matching socket on another board such that the two boards are perpendicular to one another. Card edge connectors are often used as expansion slots on motherboards, backplanes, or riser cards; with the PCI-e (Peripheral Component Interconnect Express) slots used to add more RAM to your computer as a prime example. Corrosion-resistant gold contacts that directly contact trace on the board make them great for high-speed digital signal circuits.
Board-to-harness - There are many instances where it may be necessary to connect cables and wires to a board. The FFCs (flexible film cables), FPCs (flexible printed cables), and ribbon connectors characteristic of a server room are prime examples.
Direct-soldered - Castellated vias allow you to create PCB modules that can easily be soldered together. These are especially popular for attaching small wireless modules to larger boards. Just be sure to follow high soldering standards such as the IPC-A-610 or J-STD-001.
Flex - For additional cost and manufacturing complexity, flexible printed circuits can combine the benefits of both component assemblies and wire harnesses. Their ductile nature means they can more effectively fill 3D space in small and constrained enclosures.
Whether design requires a vertical stack of PCBs or sliding boards into racks or backplanes, it’s important to maintain a connection between boards that is neither susceptible to nor an influence on the signal quality of neighboring lines.
EMI Reduction in High-Speed Circuits
EMC/EMI concern is one of the major driving forces behind multi-board PCB design. All it takes to create EMI is energy and an antenna. The demand for higher-performing electronics means high-speed signal circuits are only going to become prevalent as demand for network speeds and bandwidth continues to increase.
With so many components at play, it’s no wonder EMI is a concern in multi-board systems.
Multi-board designs provide more room to accommodate EMI/EMC best practices: keeping analog and digital signals separate, avoiding right-angle traces on cramped boards, and the cost-effective use of multilayer boards on an as-needed basis. At the same time, multi-board designs also introduce new concerns, requiring an extension of analysis beyond single boards to the connections between boards and the entire system.
Putting It All Together With 3D PCB Design Considerations
Multi-board design is like an expensive 3D puzzle. Each board that makes up your system must fit into a physical enclosure or case. There’s nothing worse than drafting up the “perfect” CAD drawing and procuring all the materials, parts, and connectors, only to find out on assembly day that the 3D clearances are incorrect. Worse still: not leaving enough room for proper ventilation subjects your product to heat-related performance and aging issues.
Fortunately, software exists to help designers keep track of all these puzzle pieces. Designers can now take a holistic approach to multi-board PCB design, performing signal integrity analysis across all boards, connectors, cables, sockets, and other structures.
Cadence’s PCB Design and Analysis tools like the 3D Step Viewer support complex sub-assemblies and confirm enclosure clearance even on single-board products. Coupled with the powerful OrCAD PCB Designer, layout teams can reduce turnaround times for the most challenging 3D PCB designs.
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