A Comprehensive Guide to Multiboard PCB Design

Many complex electronics systems are built as multi-board arrays of PCBs. There are many advantages to this type of design, the biggest one is the modular approach it offers. The PCBs can be plugged as daughter boards into the main board which make it more compatible for prototyping such as Arduino and Raspberry Pi. If you want to develop your own multi-circuit boards system, there are some basic steps you can take to ensure your design will have the connectivity you need.

• Gathering your information and configuring your data is essential in multi-board design.
• Schematic capture and PCB layout is still the same, but now involves working with all the system boards in the same environment.
• Multi-board system design gives you the ability to check and simulate the entire system instead of just a single design.

Here in this article, we'll cover some of the basic aspects of defining connectivity in your design while also ensuring signal integrity in the design.

Starting of Multi-board PCB Design

Multi-board design starts with a mechanical outline of each board in your system and a plan. Understanding how a single printed circuit board is going to fit into the system. Connection style may involve simple standardized connectors, such as mezzanine connectors or pin headers, or integrated edge connectors. After this, a placement and routing strategy needs to be developed to connect the components together keeping EMC/EMI and impedance matching parameters in mind. Multi-board design is intended to reduce development time by organizing multiple designs together within a single environment. To ensure that you benefit from this new design paradigm, you need to make sure that your processes and workflows are updated and ready.

Planning the Multi-board PCB System Design

Creating a multi-board PCB system is a system-level project, requiring a clear plan for board connections. Multi board PCB arrangement involves specifying connections between all PCBs in the system. Here's a streamlined approach:

1- Board Arrangement: Determine the orientation of each board, considering any moving components or mechanical constraints. Early mechanical modeling is essential here to shape the overall design.

2- Connector Selection: Choose connectors like board-to-board connectors, edge connectors, flex ribbons, or cables that align with your board arrangement and fit within the enclosure. You need to choose connectors to support the board arrangement while fitting in your enclosure.

3- Board Functions: A multi board system comprises several circuits with each performing certain functions. Also, these boards will only hold the required components that support this function. Therefore, you might need to reconsider your circuit board arrangement as well as the connector options.

4- Pinout Setup: Assign pins on each connector to support specific signals and maintain signal integrity. Define these pinouts in your connector schematics.

5- Schematics Entry: Organize schematics by board, ensuring each set only includes components specific to that board for easy management.

6- PCB Layout Design: With schematics ready, design the physical layout, placing components and connectors according to the planned board arrangement and functional requirements.

Setting the PCB Layout in Multi-board Design:

I am using EasyEDA, because of its free open source interface and huge online library support. The multiboard designs can be made as separate files under a project containing different schematics and PCB layout files. See our comprehensive guide on design layout. Here are some design insights:

Design 1:

Design 2:

Final Output:

Routing Tracks in Multi-Circuit Boards

For multi-board systems, routing should begin once initial design rules are set, impedance profiles are calculated, and the routing mode is configured. High-speed interfaces may not be necessary on every board, but can be routed between boards using edge connectors, cables, flex ribbons, or board-to-board connectors. Slower single-ended signals, like GPIOs or bus protocols, can also connect across boards via cables. To maintain signal integrity, ensure consistent grounding across all boards to avoid potential issues.

Defining Ground layer in Multi-board Design

In multi-board layouts, grounding must be carefully defined to support effective signal routing. To maintain a consistent ground potential throughout, follow these steps:

1) Use ground planes on each board to maintain characteristic impedance, reduce EMI/crosstalk, and provide robust decoupling for the power distribution network (PDN).

2) When connecting boards, establish a ground link across connectors to ensure continuous grounding across each board, enhancing shielding and signal integrity.

3) For ribbon or twisted pair cables, consider interleaving ground lines between signal paths to provide clear reference and stronger shielding.

These practices improve signal integrity by supporting consistent impedance, defined return paths, and minimized crosstalk in multi-board routing. However in some designs, this type of ground connection can’t be provided. This is often the case when the system is physically spread out over multiple cabinets, rather than having all the boards connected in the same enclosure.

Using Differential Protocols in Multi-board Design

When routing over long distances, a better approach is to use differential protocols for routing. In larger systems, particularly DC systems, ground can carry high current. Which can be a safety hazard and due to high current demand cable dissipates high heat in the ground connections which may lead to breakdown in PCB. In these types of arrangements the ground planes in each circuit board should be isolated and not connected to each other. Instead, the chassis and an earth connection should be used for shielding, not the PCB ground plane.

When routing signals between boards in a multi-board system, using differential pairs is advantageous as they can tolerate ground offsets between boards. Differential protocols are commonly used in these systems because they eliminate the need for a continuous ground reference while routing between boards. Once the differential signal returns to a board and the difference is read, data recovery is possible without concern for any ground offset introduced during routing.

Prevent Logic Errors in Multi-board Systems

One of the most important aspects of multi-board systems design is defining logical connections between each board in your system. Some of the critical errors that can arise in the interconnections are like, rotated orientation (I/O transpose, not matching of ports and pins), shorts and opens(missing nets/uncomplete nets) and transposed rows(header connector misplacement).

A logical connection represents the connections defined in schematics across multiple PCBs, where each connection between boards is assigned a net definition. If logical connections are incorrectly defined, it can lead to mismatched connections between the boards, potentially causing short circuits or misalignments across connectors, cables, and wires. For successful multi-board system design, the design engineer must have multiple sets of design data.

Conclusion

Designing a multi-board PCB system can significantly streamline complex electronic systems, especially by adopting a modular approach that facilitates prototyping and development. With some paid online software it is easy to design complex multi boards, but for the basic prototyping and up to 2 stack multi-board EasyEDA is a good option to go with.

An Engineer with expertise in PCB design can successfully design a multi-board by keeping tips like planning from defining board orientation, connectors, and functions in mind. Always ensure clear pinout definitions and maintain signal integrity across all connections.