CC BY 3.0 Frank Zheng
There’s only so much you can do with a single printed circuit board (PCB). We’ve seen advances in miniaturization and the steady rise in the number of transistors you can squeeze on a single chip. Even so, factors such as EMI concerns, thermal limits, and the overall rise in circuit complexity have turned multi-board PCB design into an industry necessity.
A multi-board PCB system is any design that requires more than one PCB working together. From partitioning, to intra-board connectivity, to 3D design considerations, and tips for EMI (electromagnetic interference) reduction, let’s dive into common component placement strategies for multi-board PCB systems.
PCB partitioning (not to be confused with the digital partitioning of a hard drive for memory purposes) is about physically grouping components based on functionality. Each functional subsystem can be viewed as a set of components and their supporting circuitry.
For example, your typical motherboard can be further subdivided into a number of functional units such as your processor clock logic, bus controller, bus interface, memory, video/audio processing modules, and peripherals (in/out).
In the context of multi-board PCB design, partitioning may be followed by refactoring components onto different boards. There are many reasons one might do this, including:
EMC (electromagnetic compatibility): Mitigate EMI concerns through best practices such as separating analog and digital circuits (we’ll dive into more of these EMI reduction techniques later in this article).
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: When designing multiple products, it can save a business time and money by incorporating modular standardized units into a design, allowing you to add functionality to a baseboard as needed (e.g. think shields in Arduino chipsets).
3D Space: Just because you can fit all your circuitry onto a single standard 10” x 16” PCB (roughly the size of a pizza box), doesn’t mean that’s practical for the physical dimensions and shape of your device’s enclosure.
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: Your male/female, 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 traces 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.
Whether your design requires creating towers of PCBs stacked on top of one another, or sliding boards into racks or backplanes, it’s important to ensure you’re able to get a solid connection between the different boards that make up your product.
EMC/EMI concerns are 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 more prevalent in the years to come.
Multi-board designs give you more room to accommodate EMI/EMC best practices. Things such as 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 you to extend your analysis beyond single boards to the connections between boards and the entire system.
Putting It All Together With 3D 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, procuring all the boards, parts, and connectors, only to find out on assembly day that you didn’t get all your 3D clearances right. Worse still, not leaving enough room for proper ventilation, subjects your product to heat-related performance degradation. And we haven’t even scratched the surface of the physical realities of EMI.
Fortunately, we now live in a time where software exists to help the designer keep track of all these “puzzle pieces.” With the July 2018 release of Cadence® Sigrity™, Sigrity tools have been integrated with Cadence Allegro technology and a new 3D Workbench to bridge the gap between mechanical and electrical domains in PCB design. 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. Ready to streamline your next multi-board PCB project? Check out Cadence’s suite of PCB design and analysis tools today.
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