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Keyed Flex Cables Help Prevent Assembly Errors

flex printed circuit cable

Ambiguously interchangeable parts are one of the factors that can cause incorrect assembly in a multi-board product. With connectors and harnesses, this usually is solved by using keyed mating connectors, where the connectors are forced into the correct orientations by the connector housing. But many times, a cable must be custom-designed as a flexible printed circuit (FPC), and that printed circuit with its mating attachments may need to be a unique flex PCB.

Unfortunately, if you're trying to optimize product costs, this use of many unique flex cables goes against cost reduction and part number reduction, which adds costs and assembly complexity into the product BOM. Although making everything unique goes against part number reduction and cost reduction, there are ways to consolidate cables to reduce BOM cost while also preventing assembly errors. This could mean keying your connectors, as well as keying your flex cable shapes so that installation errors will be nearly impossible.

What Makes a Keyed Flex Cable?

Much like a keyed connector, a keyed flex cable is a type of cable that is very difficult or impossible to install or mate backwards. Because it cannot be mated or installed backwards, it is very difficult to use the cable incorrectly in an assembly. Just like a keyed connector housing forces a specific orientation, a keyed flex cable shape forces a specific cable orientation. When a set of boards gets assembled into an enclosure, keyed flex cabling can only be installed in a single way, and it is very difficult to install the cable backwards without creating an incorrect bend or damaging the cable/connectors.

Keyed flex cables should have a shape that is not rotational or can't be recreated by flipping or bending the cable. This could mean a keyed flex ribbon has an L shape, J shape, or similar shape that lacks symmetry. Static bends in a flex cable can also be a form of keying that eliminates symmetry in the cable and forces a specific orientation.

Keyed flex cable

The L-shape on one end of this FPC keys the cable so that it cannot be reversed in an assembly.

Keyed connectors can be used with flex ribbons to provide additional insurance that the correct part is being installed in the correct location. It also provides a further check that the connector has been placed with the right orientation. Keyed connectors should typically be used with a keyed cable shape as an additional measure of insurance against an assembly error.

One additional measure on a connector that can provide assurance of a correct mate is a connector latch, which is sometimes combined with keying. Some keyed connectors use latching as the key for the mating connector bodies. One instance where this is used is on pin headers, where the latching mechanism will be implemented on the shroud around the pins.

 Latching SMD board-to-board connector

Latching SMD board-to-board connector.

Keyed cables still have to be used with other documentation to ensure correct assembly. For a product being assembled into a custom enclosure, creating a mechanical assembly drawing for your internal or contract assembly team will be a requirement. A mechanical assembly drawing for a multi-board product needs to include:

  • 3D drawing of the assembled product from at least one perspective
  • Clearly denoted part numbers for every mechanical and electrical component
  • A clearly marked order of assembly or installation
  • Clearly marked or drawn bends, including bends in keyed flex cables

These factors can help prevent assembly errors and will help the assembly team ensure a streamlined process for installing components into an enclosure.

Get the Job Done With MCAD Collaboration

Keyed flex PCBs are as much mechanical elements as they are electrical elements. Because of this, the electrical designer needs their cable shape and bends qualified in the assembly housing. This is normally done by a mechanical engineer in solid modeling software.

Incorrectly oriented mating connectors, and incorrect shapes and lengths of flex ribbons are major causes of assembly errors in products with multiple circuit boards. At minimum, they are just a nuisance that have to be corrected with odd twists in the flex cable. In the extreme case, the incorrect mating forces with a rotated pinout. When the pinout is not rotationally symmetric, this creates a short that damages the system.

To help prevent this kind of problem, there is a simple process that electrical and mechanical engineers can follow:

  1. The MCAD designer should create the board shape and provide it to the electrical engineer
  2. Electrical designer can use an exported DXF to create the board shape
  3. PCB layout can then proceed as normal with component placement
  4. The PCB designer must export the PCB with placed components for mechanical verification

If the placement creates no interferences, then the PCB designer can proceed with routing and finalize the board.

Multi-disciplined design teams need the best set of tools for interchange between PCB layout and mechanical designs. No matter what you need to design, you can build it with the best set of PCB design features with MCAD support in Allegro PCB Designer from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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