Most Common Logic Errors in Your Multi-Board PCB Assemblies

Many electronic systems and products don’t just use a single PCB. They can use multiple boards, a board and external modules, or a cable connection to an external device. In a multi-board system, it’s possible for logic errors to occur between the two boards, but you might not ever catch these errors without a thorough design review or until the board is prototyped. Fortunately, there are some simple solutions and design choices you can implement that can help you prevent these logical connection errors between boards.

What Are Logical Connection Errors Between Boards?

Logical errors occur when incorrect net connections exist between two PCBs, where the two PCBs are connected with a cable, board-to-board connector, edge connector, or some other method. This mistake always originates in the design data for the two boards, and then the mistake gets incorporated into the two PCB layouts.

This can happen when the two PCBs don’t have a direct logical connection between them in your design software. For example, if you have two projects, and you’re designing them in isolation, it’s easy to mistake a net connection between connectors on two different boards. The result is an unintentional short, incorrect data connections, incorrect power connections, or an open circuit.

Below are four common problems that can occur in multi-board interconnects, as well as the solution that can be applied to fix or prevent each problem.

Wrong Net Assignment

Problem: The most common problem occurs with net assignments. Sometimes, the net assignment on specific pins creates a function problem or causes a board to shut down. If this occurs on specific pins in a connector pinout, then it’s possible the net assignments on isolated pins are incorrect across the two connectors.

Solution: Aside from very thorough design reviews and checking net assignments on connector pins, there is a simple design process strategy that can help prevent this problem.

Consider designing schematics for both boards in the same project. This way, you will have connectivity checks between two components inside your schematic capture software. Then when you’ve verified the design and you’re ready to proceed to the PCB layout, make a copy of the project and narrow the circuits down to the individual boards.

Rotated Board-to-Board Orientation

Problem: If the boards in your system mate together in the incorrect orientation, you are probably using rotationless board-to-board connectors, and one of the connectors was placed incorrectly during assembly. In this case, the pinouts will also be rotated and the electrical interconnections will not match.

Solution: The solution here is to make the connector orientation obvious in the PCB layout data so that there will not be any assembly mistake. If you used rotation-proof connectors, then check your footprint; the pin-1 marking might be missing from the silkscreen layer, or the orientation may have been ambiguous. Another option to ensure orientation is obvious is to draw the correct outline of the connector in the assembly layer and in the silkscreen layer.

Other options to prevent this include:

• Use a rotation-proof connector set and pinout
• Use connectors with hybrid rotation-proof THT+SMD land patterns
• Use slotted/keyed connectors that prohibit rotation

PCIe connectors use a slotted keyed pin arrangement so that the inserted card cannot be rotated.

Rotated Pinout

Problem: The connector body orientation is correct, and your boards/cables mate correctly, but the pinout on one connector is rotated. In other words, you can only create the correct connections by totally rotating the cable or board by 180 degrees.

Solution: Go back to the footprint for one of the connectors; the pin numbering may have been applied in reverse. If this is the case, it’s simple enough to pick up all the pins and rotate their coordinates around the footprint center to produce the correct pinout.

Transposed Pin Rows on Ribbon Cables

Problem: In some connector pairs connected with ribbon cables, the rows of connector pins might appear to be reversed or transposed. This could be an annoying consequence on the construction of ribbon cables, where the pin 1 connection appears transposed in the cable assembly. It could also be a mistaken transpose by the designer.

Solution: Use shrouded pin headers with a keyed ribbon cable. These connectors have a specific pin-1 connection that will always be reflected onto the other board. As long as you always use the same pin 1 assignment, you can rest assured that your ribbon cable will make the correct connections. Take note of the key location on standard ribbon cable connectors, as shown below.

Pin-1 has the same orientation with respect to the notch on ribbon cable assemblies. [Source: User dim on stackexchange.com]

The Right PCB CAD Tools Can Help

Not all PCB design software applications can provide logical connection support for multi-board PCB systems. But with the right software, you’ll have net connectivity checking built into the schematics and the PCB layout. This extends the solution under the “Wrong Net Assignment” section above to the PCB and even into 3D mating.

No matter the complexity you build into your multi-board design, you can create high-quality multi-board systems with the complete set of CAD features in OrCAD from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

Subscribe to our newsletter for the latest updates. If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.