Flex Cable or Custom Harness: Which Should You Use?

Let’s set the scene: you have multiple boards that you need to connect together, but you don’t have a lot of space or the budget for a rigid-flex PCB assembly. Instead, you have to decide between an FPC connector, custom flex ribbon cable, or off-the-shelf connectors wired into a harness. Which one should you choose?

If you’re unsure which route is best to pursue, then take a look at the list of pros and cons below. The use of flex cables or even a rigid-flex build often comes up when we start talking about connectors, but custom wire harnesses provide a lot of advantages in exchange for space savings.

Flex Cables vs. Custom Wire Harnesses

FPC Connectors and Flex Ribbons

Flex ribbons aren’t just used in rigid-flex PCBs, they are available as off-the-shelf components that interface with a standardized set of connectors (known as FPC connectors). These connector and cable systems are low cost and they allow you to get all the benefits of a flex interconnect, but without the cost of a rigid-flex build. They can also offer high pin density due to signals being routed on multiple layers.

Flex cables are often brought up as a cure-all for space savings, but this is not always true. There are two reasons for this:

1. Flex cables can only bend along one direction, so you are limited in how you can route them
2. You can consolidate signal nets onto flex cables, but typically you cannot do this with power nets

If your interconnect will require multiple turns or twists to route between boards, or if you need to provide high power, then it may be better to use a custom wire harness.

The other option is a custom flex cable. If you can’t find something that hits your form factor, you can always build a custom flex cable that works just like a wire harness. You can do it with PCB design software that has rigid-flex support, you would just be designing a flex interconnect with multiple endpoints.

Wire Harnesses

The easiest way to build a wire harness is with wire-to-board connector and cable systems. Some of these systems are modular and they support a variety of wire sizes, so they can be configured for signal and power in a single connector. They can also be used in much tighter spaces than a typical flex connector because they can bend in any direction, so they can wrap through small conduits and around obstacles.

Here, the drawback is that the pin density tends to be lower and the density will be limited by the wire gauge. When power is needed, you will need smaller gauge wire, which also drives down the pin density. Still, there are many off-the-shelf options for the wire-to-board connector systems needed to build custom harnesses.

When off-the-shelf wiring and cabling systems will not hit your requirements, you might consider using custom connectors for your wire harness. Going the route of a custom connector can be costly due to the design and tooling requirements, so this is more of an option for higher volume assemblies.

Which is Best?

In summary, selection of a flex ribbon with an FPC connector versus a custom harness is about balancing trade offs, just like many other design decisions. The table below should help summarize the advantages and disadvantages of each approach.

CAD tools have come a long way from the days of cabling and wiring diagrams. Today you can access wiring harness design capabilities inside your PCB design software as part of multi-board PCB design, and you can implement the standard logical checking between each end of your custom harness.

If you’ve considered the tradeoffs and you’ve landed on the use of FPC connectors with flex ribbons for your multi-board system, you can still design flex ribbons in PCB design software. An integrated toolset eliminates the need for an external wiring harness design application. Today you can do it all in a single program.

Whenever you need to design wire harnesses and flex cabling for connecting multi-board systems, make sure you use the complete set of PCB design features in Altium Designer®. To implement collaboration in today’s cross-disciplinary environment, innovative companies are using the Altium 365™ platform to easily share design data and put projects into manufacturing.

We have only scratched the surface of what’s possible with Altium Designer on Altium 365. Start your free trial of Altium Designer + Altium 365 today.