Flexible Circuit Dynamic Bend Design Rules

A flexible circuit that is bent once during assembly is a different design problem from one that bends ten thousand times in service. Repeated bending accumulates fatigue damage in the conductors, and the design rules that allow a single form will fail quickly under cycling. This article covers the parameters that determine dynamic bend life and how it is verified.

Static Bend Compared With Dynamic Bend

A static bend is a forming operation. The material is taken past its yield point once, and the design has to ensure that the conductors are not cracked by the single event. A dynamic bend is a fatigue problem: each cycle applies a small strain, and the damage accumulates until a conductor cracks. Where the application bends only when a door is opened or a module is serviced, the cycle count is low and a static design may be adequate. Where the circuit follows a moving part, the count can be in the millions and the material choice becomes the dominant factor. Rolled copper is the material that makes such a life possible.

The difference is large. A design that survives a single bend at a tight radius may last only a few hundred cycles at the same radius, because the strain per cycle is far above the fatigue limit of the copper. Dynamic applications therefore need a larger radius, a different copper type and a different construction.

Copper Type and Grain Structure

The copper used in a dynamic flex is not the electrodeposited foil used on rigid boards. Rolled and annealed copper has an elongated grain structure that resists fatigue crack growth, and it is the standard choice for any application that cycles. Electrodeposited foil has a columnar grain structure that cracks more readily under bending.

The difference is measurable in a bend test, and the number of cycles to failure can differ by an order of magnitude. The choice should therefore follow the application rather than the cost, and the drawing should state the copper type explicitly, because a substitution during fabrication is invisible on the finished part.

Conductor Orientation and Bend Radius

Conductors should cross the bend line at right angles. A conductor that runs along the bend line is strained along its length and across its width, and it cracks with far fewer cycles. Where the routing cannot avoid a parallel run, the conductor should be as wide as possible and the radius should be increased to compensate.

The bend radius is normally expressed as a multiple of the circuit thickness, and the multiplier for a dynamic application is much larger than for a static one. The radius should be set by a bend test on the actual construction, because the multiplier depends on the number of layers, the coverlay thickness and the copper type. The rule of thumb is a starting point, not a specification.

Neutral Axis and Layer Construction

Bending puts the outer surface in tension and the inner surface in compression, with a neutral axis between them where the strain is zero. Conductors placed on the neutral axis see the least strain, so a symmetric construction with conductors between two coverlay layers lasts longer than one with conductors on the outside of the stack.

For a two layer flex, the conductors should be arranged symmetrically about the neutral axis and the layers should be equal in thickness. An asymmetric stack bends around the stiffer side, which puts one conductor layer into higher strain than the design intended. Where the layer count cannot be made symmetric, the design should account for the shift in the neutral axis.

Coverlay and Adhesive Selection

The coverlay protects the conductors and influences the stiffness of the bend area. A coverlay that is thicker than necessary raises the strain in the conductors, and one that is bonded with an adhesive that flows into the bend creates a stiff region. The bend area should be defined so that the coverlay, the adhesive and the stiffeners all stop before it.

Adhesive selection matters for the same reason. An adhesive with a lower modulus accommodates the movement, while a rigid one transfers the strain to the conductor. Where the application cycles at a low frequency and a small amplitude, a stiffer construction may be acceptable, but the assumption should be tested rather than assumed.

Terminations and Strain Relief

A conductor rarely fails in the middle of the bend region. It fails where the geometry changes: at the edge of a stiffener, at a pad, at a via or where the conductor width changes. Those locations concentrate the strain, and each of them should be kept outside the bend area or reinforced.

Strain relief is the means of protecting them. A coverlay extension, an adhesive bead or a shaped relief that spreads the movement over a longer length reduces the strain at the critical point. The design should place the terminations outside the bend region where possible, and where that is impossible, the geometry should be tapered rather than stepped.

Bend Area Layout and Mask Openings

The bend area should be kept free of everything that is not a conductor. Pads, vias, test points, stiffeners and silkscreen all change the local stiffness and create a step that concentrates strain. Where a feature cannot be moved, the conductor geometry around it should be tapered so that the strain is spread over a longer length rather than concentrated at the transition.

Coverlay openings define where the circuit can bend, and the same features that are electrically useful become mechanical problems when they sit inside the opening. The layout should reserve a clear band for the bend, and the dimension should be derived from the bend radius rather than from what is left over. The related conventions for features on the circuit surface are described in PCB fabrication notes.

Assembly Handling and Routing in the Enclosure

A flexible circuit that cycles in service is also handled during assembly, and the handling can introduce damage that only appears later. Bending a circuit by hand to fit an enclosure creates a local radius that may be much smaller than the specified value, and the damage is a crack that grows under later cycling. The assembly should use a fixture or a forming tool that sets the radius, and the operation should be defined in the work instruction rather than left to the operator.

The route the circuit takes inside the enclosure matters as much as the forming. A circuit that rests against a sharp edge, that is pinched by a cover or that is pulled tight between two mounting points will fail at the point of contact. The design should include features that hold the circuit in place with a defined slack, and the assembly should be checked for the intended route rather than for the electrical result alone. The mechanical quality questions that apply are the same ones described in judging PCB quality.

Verification by Bend Testing

The only reliable verification is a bend test that reproduces the application. The sample should be from production tooling, formed to the intended radius and cycled at the intended frequency while the resistance of the conductors is monitored. A rise in resistance indicates a crack forming, and the number of cycles at which it occurs is the design life.

The test should be run at the temperature extremes if the product will experience them, because the fatigue behaviour of the copper and the adhesive changes with temperature. The gopcb engineering team requires a dynamic bend test for every flex design that will move in service, and the result is recorded with the construction so that a later material or thickness change is not made without requalification.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Can a dynamic flex use electrodeposited copper? It can be built, but the bend life will be much shorter than with rolled and annealed copper. For a cycling application the rolled material is the practical choice.

What bend radius should be specified? The value from a bend test on the actual construction. A general multiplier is useful for estimating but not for specifying a life requirement.

Why does the conductor fail at the via? Because the geometry change concentrates the strain. Vias and pads should be kept out of the bend region or reinforced with a local strain relief.