How to Specify FFC Cable Length, Stiffeners and Exposed Contacts

FFC length should be measured between clearly identified physical reference points, not written as a standalone nominal number. A 100 mm cable can produce a different usable routing length when each end has 4–6 mm of exposed conductor and 6–10 mm of stiffener. Connector families from major suppliers commonly use pitches such as 0.50, 1.00, and 1.25 mm, while mating thickness may fall near 0.30 mm in many compact connector designs. A ±0.5 mm termination error can consume more than 10% of a 4 mm exposed-contact zone. Length, exposed contacts, and finished mating thickness should therefore be dimensioned separately.

The first dimension to settle is the cable length reference. “120 mm long” is incomplete unless the drawing says whether 120 mm is measured conductor-tip to conductor-tip, insulation-edge to insulation-edge, or between two other datums. On a cable with 5 mm of exposed conductor at both ends, choosing the wrong reference can shift the usable insulated section by 10 mm, or 8.3% of a nominal 120 mm assembly. That difference is large enough to change where the cable bends or whether it reaches the second connector without tension.

For a fixed PCB-to-PCB installation, length should come from the installed routing path rather than the straight distance between connector centers. Two connectors separated by 100 mm may need a cable longer than 100 mm when one board is 12 mm higher than the other, the cable passes around a housing rib, or insertion requires several millimeters of free movement. Adding length without checking the route is equally poor practice because a 5% surplus on a 200 mm cable adds 10 mm that must fold or bow somewhere inside the enclosure.

Measure the cable along the intended routing centerline, then separate installation allowance from the controlled finished length.

Once the route has been established, the drawing can assign a realistic tolerance. A long static cable may tolerate ±1.0 mm more easily than a short termination feature, while a compact assembly can require tighter control. A ±1 mm tolerance on a 200 mm overall length is only ±0.5%; the same ±1 mm applied to a 5 mm exposed-contact length is ±20%. Using one tolerance for every dimension therefore creates very different functional results.

That percentage difference matters most at the termination, where the exposed conductor must align with the connector contact system. Many FFC/FPC connectors use contact pitches around 0.50 or 1.00 mm, and the conductive fingers often extend only a few millimeters in the insertion direction. If the specified exposed length is 4.0 mm with ±0.3 mm tolerance, the possible range becomes 3.7–4.3 mm, a total spread of 15% relative to the nominal length. Connector drawings should be used to check whether that range still maintains contact overlap and insulation clearance.

The exposed dimension should always be tied to a visible edge. “4.0 ±0.3 mm from conductor tip to insulation edge” is much harder to misread than “4 mm exposed.” The same drawing should show conductor width, pitch, circuit count, and cable width because pitch alone does not define whether the fingers align. A 30-position cable at 0.50 mm pitch spans roughly 14.5 mm from the center of the first conductor to the center of the last before conductor widths and edge margins are considered.

The table also shows why stiffener thickness should not be treated as an isolated film dimension. A connector that accepts a nominal 0.30 mm mating section does not necessarily require a 0.30 mm stiffener. The base FFC already contains conductor, insulation, and adhesive layers. If the flexible portion measures 0.12 mm and the added reinforcement system contributes another 0.18 mm, the finished area reaches about 0.30 mm. A 0.03 mm thickness change at that interface equals 10% of the nominal 0.30 mm section.

That thickness influences insertion force and clamping geometry, so the finished stack should be compared with the connector manufacturer's mating-thickness range. Connector series released during the 2010s and 2020s often include several variants sharing the same 0.50 mm pitch but accepting different cable thicknesses, contact orientations, or locking structures. Using pitch as the only selection parameter can therefore pair an electrically aligned cable with a mechanically incompatible connector.

Stiffener length needs the same level of definition. An 8 mm stiffener behind a 4 mm exposed-contact section leaves a reinforced region extending beyond the conductive fingers, but the exact overlap depends on the supplier's construction method. If the stiffener position moves by ±0.5 mm, the edge location varies across a 1 mm total band. For an 8 mm reinforcement, that movement represents 12.5% of its nominal length and can move the stiff-to-flex transition closer to the required bend.

That transition should stay away from an aggressive fold. FFC can bend through a small enclosure, but repeatedly bending directly at the stiffener edge concentrates strain in a narrow zone. A design using a 5 mm bend radius has very different mechanical conditions from one folded almost flat at the connector exit. Where repeated motion is expected, bend radius, conductor construction, stroke, and cycle count should be specified separately rather than assuming a standard static FFC will tolerate 100,000 or 1,000,000 movements.

The stiffener supports insertion; it should not become the point around which the cable is repeatedly folded.

Contact orientation becomes the next item because two cables with identical length, pitch, and conductor count can still be incompatible. Some assemblies need exposed conductors on the same side at End A and End B; others need opposite sides because one connector faces upward and the other downward. A side view showing both ends removes the ambiguity. In a 2026 production drawing, relying only on supplier terms such as “Type A” or “Type B” is risky because naming conventions are not universal between cable vendors.

End A and End B should also be dimensioned independently when different connectors are used. One end may require 4 mm exposed conductor and an 8 mm stiffener, while the other uses 5 mm exposure and 10 mm reinforcement. Treating both ends as identical would create a 25% difference in exposed length at the second connector and a 20% difference in stiffener length. Separate end views also make inspection easier because each termination can be checked against its own dimensions.

A supplier drawing can then use a small set of controlled datums rather than dozens of loosely related dimensions. The conductor tip, cable centerline, insulation edge, and stiffener edge are practical references. Dimensions that affect connector mating should be taken from those references whenever possible. Chain-dimensioning several small features can accumulate tolerance: four consecutive ±0.2 mm dimensions can create a theoretical end-to-end range approaching ±0.8 mm if every feature reaches the same tolerance direction.

The connector part number should appear on the drawing or purchasing specification because it gives the cable manufacturer a second method of checking the interface. A supplier can compare the requested 0.50 mm pitch, 20 positions, 0.30 mm finished thickness, contact orientation, and exposed length with the connector documentation. This does not replace dimensions, but it catches mismatches before tooling or sample production. Even a 3% dimensional mismatch is easier to resolve before a first article than after several thousand cables have been produced.

Material notes should remain limited to properties that affect fit or performance. If PET stiffener material, adhesive type, conductor plating, temperature class, or flame requirement matters, state it. If the connector needs a particular contact finish, the cable contact area should be compatible with that interface. Plating thickness should not be guessed from appearance; when it matters, use the cable supplier's controlled process specification and the connector manufacturer's mating recommendations.

Inspection requirements can be equally compact. A first-article report can record overall length, width, pitch, exposed length at both ends, stiffener dimensions, finished mating thickness, and contact orientation. For a 20-piece first-article sample, measuring all 20 terminations provides 40 end measurements rather than checking only one cable. Production sampling levels can then be agreed with the supplier according to volume, historical capability, and the importance of the dimension.

For a practical release drawing, the information can be reduced to a short set of entries:

• overall length with explicit measurement endpoints;

• routing or bend restrictions where geometry is constrained;

• circuit count, conductor pitch, and total cable width;

• exposed conductor length for End A and End B;

• stiffener material, length, side, and position;

• finished thickness in the connector mating area;

• contact orientation at both ends;

• mating connector manufacturer and exact part number;

• dimensional tolerances applied by function rather than one blanket value;

• inspection dimensions for first-article and production checks.

A specification containing those fields gives manufacturing and incoming inspection the same geometry to work from. A 150 mm cable with ±1 mm overall tolerance, 4.0 ±0.3 mm exposed contacts, 8.0 ±0.5 mm stiffeners, and a controlled 0.30 mm mating section describes measurable interfaces rather than a general cable shape. That level of detail is usually sufficient for a supplier to review manufacturability, quote the part, produce samples, and verify the finished cable against the drawing without filling missing dimensions by assumption.