A Flex Cable Design Example From Brief to Build

A camera module that must fold into a compact robotic inspection head has little tolerance for an interconnect that is merely convenient. The cable must survive movement, preserve high-speed image data, fit a controlled bend path and remain straightforward to assemble. This flex cable design example shows how those requirements can be translated into a manufacturable flex assembly rather than a drawing that only works on screen.

The example is illustrative, but the design logic applies to AI vision equipment, portable instruments, robotics, medical devices and other next-generation electronics where board position, motion and signal performance are closely connected.

The application brief

Consider a vision module with a sensor board mounted on a moving arm and a processor board fixed inside the enclosure. The two boards sit 85 mm apart when deployed. During use, the moving board rotates through 70 degrees several times per hour. Space is limited to a 12 mm wide cable route, and the camera carries MIPI CSI-2 differential pairs, I2C control, power and ground.

This immediately rules out treating the cable as a passive afterthought. A standard straight flex may be suitable where the route is direct and static, but this assembly needs a shaped outline and a defined dynamic bend region. The electrical and mechanical requirements must be established together.

A useful engineering brief records the operating voltage and current, interface speeds, cable length, bend geometry, expected flex cycles and environmental exposure. It should also identify connector family, board termination method, permitted cable thickness and the locations where fastening or stiffening is allowed. Missing any one of these details can force a late redesign.

Flex cable design example: defining the construction

For this application, a two-layer flex circuit is a practical starting point. It offers enough routing freedom to separate power and high-speed signals while keeping the assembly thin and compliant. A typical construction might use rolled-annealed copper for the flexing conductors, polyimide dielectric, a polyimide coverlay and local FR4 or polyimide stiffeners at the connector ends.

Rolled-annealed copper is generally preferred in a dynamic bend area because its grain structure tolerates repeated flexing better than electrodeposited copper. That does not mean every circuit requires it. For a one-time installation bend, other constructions may be acceptable and more economical. The duty cycle should decide the material choice.

The proposed cable has a 10 mm main body, widening to 14 mm at each connector tail where stiffeners support insertion and termination. The outline includes smooth, radiused transitions rather than sharp inside corners. Those curves reduce stress concentration and make the part more predictable during forming and assembly.

The moving section is assigned a 20 mm long bend zone. No pads, vias, component footprints or abrupt trace direction changes are placed in this area. The cable may flex, but it should flex where it was designed to flex.

Routing for movement, not just connectivity

Within the bend zone, conductors run perpendicular to the bend axis wherever possible. This allows them to share strain more evenly as the cable curves. Tracks are routed with gentle curves rather than hard 90-degree turns, and their spacing is kept consistent.

For a dynamic application, a single broad bend is preferable to several tight bends. The precise minimum bend radius depends on total thickness, copper weight, layer count and movement profile. As an initial engineering rule, dynamic designs need a more generous radius than static installations. Validation with the real enclosure remains essential, because a cable can meet a nominal bend-radius calculation yet fail if a housing edge creates a local pinch point.

The cable should also be long enough to move naturally. An overly short flex is placed under tension at the end of travel, while excessive slack can rub against sharp features or obstruct neighbouring mechanisms. In this example, the developed length is set after modelling the full motion path, not by measuring the straight-line distance between boards.

Protecting high-speed signals and power delivery

MIPI CSI-2 lanes carry fast differential signals, so pair geometry must remain controlled from the processor connector to the camera connector. Each pair is routed together, with matched lengths appropriate to the interface budget and stable spacing through the route. The stack-up is selected to provide a known impedance environment, usually with a nearby reference plane or ground structure.

A two-layer flex can support this if the routing density and impedance target permit it. Where signal margins are tighter, a multilayer construction with dedicated reference layers may be the better choice. That adds thickness and cost, so it should be justified by the electrical requirement rather than assumed by default.

In this example, ground conductors are placed adjacent to sensitive pairs and a ground return strategy is maintained across the cable. Power traces are sized for current, voltage drop and temperature rise, not simply made as wide as the layout allows. If the camera has short current peaks, the transient demand should also be considered alongside average consumption.

Shielding is not automatically required. A grounded copper area, shielding layer or external shield can improve electromagnetic performance, but each option affects flexibility, thickness and manufacturing complexity. The decision should follow the actual noise environment, compliance requirements and test evidence.

Stiffeners are local mechanical tools

Stiffeners do not make a flex circuit generally stronger. They create controlled rigidity where it is needed, usually beneath ZIF connector contacts, soldered tails or mounting features. In the camera example, stiffeners are confined to the two termination ends and stop clear of the dynamic bend zone.

Their thickness is chosen to meet the connector manufacturer’s requirements. A tail that is too thin may not latch securely; one that is too thick may not insert. This is a small detail with a direct effect on field reliability.

Designing for production from the first revision

The best flex design is not the one with the smallest possible geometry. It is the one that meets performance targets with repeatable yields. Trace width and spacing, coverlay openings, annular rings, tooling features and tolerances need to reflect the selected fabrication process.

For this example, connector pads are lengthened where the connector specification allows, giving a more forgiving contact region. Coverlay openings are reviewed to ensure they do not expose more copper than necessary or create unsupported edges near soldered areas. The shaped outline is supplied with clear dimensions, datum references and bend-zone notes rather than relying on an informal drawing annotation.

The assembly drawing should state which side faces the enclosure, where the cable may bend and where adhesive or clamps can be applied. Clamping directly over active traces or inside the moving zone can undermine an otherwise sound design. If pressure-sensitive adhesive is used, its thickness, temperature range and placement need consideration as part of the stack, not as a packaging decision made later.

Prototype inspection should check more than continuity. Engineers should inspect connector engagement, actual bend behaviour, clearance at both ends of travel and signal quality under movement. Cycle testing is particularly valuable for dynamic applications. It reveals whether a bend zone is genuinely controlled or whether stress is migrating to a transition, pad edge or enclosure feature.

When an off-the-shelf flex is enough

Not every project needs a bespoke design. A standard Straight Flexi can accelerate an early prototype where the board spacing, pinout and bend route are uncomplicated. A Shaped Flexi can be the efficient option when the mechanical route is known but does not justify a fully new circuit design.

Custom development becomes more valuable when connector positions are fixed by the product, signal integrity has little margin, the cable must move repeatedly or the enclosure imposes an irregular path. It can also reduce assembly effort by replacing hand-wired harnesses and separate support parts with one purpose-designed interconnect.

Cocom supports this progression from standard flex products to custom flex and PCB engineering, helping teams align the interconnect with the wider system rather than treating it as an isolated component.

The decision that prevents late failure

In this flex cable design example, the critical choice is not simply two layers versus four, or 10 mm versus 12 mm width. It is defining where the circuit moves, how signals reference ground, and how the part will be installed before layout begins. Those decisions establish the useful design space for every later detail.

Bring the mechanical model, connector data and electrical requirements into the same review at the outset. A flex cable then becomes a controlled part of the product architecture: precise where it must connect, compliant where it must move, and ready for production when the design is released.