Rigid Flex: Preparation, Placement and Process Control

A rigid flex board is rigid where components are mounted and flexible where the assembly has to fold. The region between the two is the transition zone, and it is where most rigid flex failures originate. The rules that govern it are not complicated, but they are unforgiving, because the mechanical stresses concentrate exactly where the structure changes.

What Happens at the Transition

In the rigid section the circuit is bonded to stiffening layers of prepreg and core. In the flexible section those layers are absent, and the copper is covered by a coverlay or a flexible mask instead. The transition is where the rigid material stops, and everything about the design follows from that boundary.

Because the rigid material carries load and the flexible material does not, the stiffness changes abruptly. A bend placed at or near that boundary concentrates strain in the flexible layers, and the copper there is the first thing to fail.

Layer Drop Off and Staggering

Rigid layers should not all end on the same line. Staggering the ends of the prepreg and core layers over a defined distance spreads the change in stiffness over a longer region, which reduces the peak strain at any one point.

The stagger distance is typically of the order of a few millimetres and depends on the number of layers and the thickness. The drawing should show each layer end explicitly; leaving them to be aligned by default produces a sharp step and a weak transition.

Adhesive Flow and Squeeze Out

Prepreg flows during lamination, and at the edge of a rigid section it squeezes out into the flexible area. If it flows too far, it extends the rigid region unpredictably and creates a thickened, stiff lump where the board is supposed to bend.

Controlling it means specifying the prepreg type, the layup and the lamination parameters so that the flow is predictable. The transition drawing should state the maximum allowed flow beyond the intended boundary, and the finished board should be checked by cross section rather than assumed correct.

Bend Radius and Its Position

The bend radius is measured on the inside of the fold, and it must be large enough that the outer copper layer does not exceed its strain limit. A common minimum is ten times the total flexible thickness for a static bend and considerably more for a dynamic one.

Position matters as much as size. The bend should sit entirely within the flexible region, at a defined distance from the transition, so that the strain is distributed through flexible material rather than concentrated at a boundary. That distance should be a dimension on the drawing. Our article on bendable circuit materials describes how the limit is derived from the materials.

Coverlay and Copper Orientation

Copper that will see bending should be rolled annealed rather than electrodeposited, because its grain structure tolerates flexing better. The direction of rolling should align with the bend axis where the design permits, which requires the fabricator to know the bend direction.

The coverlay protects the copper through the bend and controls where the neutral axis falls. A coverlay that is too thin leaves the copper exposed to surface strain, and one that is too thick stiffens the bend and pushes the stress elsewhere. Both properties are set by the construction, not by the assembly process.

Routing Through the Transition

Traces should cross the transition straight and perpendicular to the boundary. A trace that runs diagonally or parallel to the transition spends more length in the highest strain region, and a trace with a via or a plated hole at the boundary is a guaranteed failure point.

Vias belong in the rigid sections. Where a layer change is unavoidable near the transition, it should be placed as far into the rigid region as the layout allows. The layer construction choices that govern this are described in our article on multilayer flex processing.

Ground Planes and Shielding

A continuous ground plane in the flexible region improves both signal integrity and mechanical behaviour, because it distributes strain and reduces the incidence of a single trace taking the whole load. Where the plane has cross hatching rather than solid copper, the pattern should follow the bend axis so that the material can flex.

Where shielding is required, the same reasoning applies. A shield layer that terminates abruptly at the transition creates a stress riser, and the termination should be staggered like any other rigid layer. The via and layer transition rules are covered alongside our notes on blind and buried vias.

Verification

The finished transition should be verified by cross section on a sample, checking that the layer ends are staggered, the adhesive flow is within limits and the flexible region is free of rigid material. A bend test on a sample to the specified radius confirms that the construction survives the intended fold.

Both checks are cheap and both catch problems that are invisible from the outside. Where a product will be folded repeatedly during assembly, the test should be cycled rather than performed once, because the failure mode is fatigue rather than a single overload.

Verification and Records

On a design of this kind, layer transition is the item that decides how the rest of the board is arranged. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

The sequence of operations is part of the specification, because a different order produces a different result from the same steps. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

Points to Confirm at First Article

On a design of this kind, layer transition is the item that decides how the rest of the board is arranged. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

FAQ

Can a rigid flex board be folded at the transition? It should not be. The bend belongs in the flexible region, at a defined distance from the rigid material.

How many bend cycles can a flex region survive? It depends on the radius, the copper type and the layer construction. Dynamic applications need the radius and the material chosen for the cycle count.

Does the coverlay matter to the bend radius? Yes. It sets the thickness that enters the strain calculation and it protects the copper surface, so its thickness is part of the mechanical design.