Flex Bend Radius And Dynamic Flex Life

A flexible circuit that is bent once during assembly and left in place behaves very differently from one that flexes every time a lid opens. The first is a static application and almost any construction will survive it; the second is a dynamic application and the mechanics of fatigue decide the life. The difference is not a matter of degree but of design rules.

This article explains what strain the copper experiences when a flex circuit is bent, how the neutral axis governs it, and the design measures that turn a short lived dynamic flex into a durable one.

The controlling quantity is the bend radius relative to the thickness of the stack, and every other rule follows from it.

Strain And The Neutral Axis

When a circuit is bent, the layers on the outside of the bend are stretched and the layers on the inside are compressed. Somewhere between them is a surface that is neither, known as the neutral axis. Copper placed at that surface experiences no bending strain at all; copper placed away from it experiences strain proportional to its distance from it.

The strain in a conductor is therefore governed by two things: the radius of the bend and the position of the conductor relative to the neutral axis. For a single sided flex, the neutral axis lies in the base film, and the copper sits above it, which is the main reason a single sided construction survives more cycles than a double sided one of the same thickness.

The Radius Rule

The practical rule for a dynamic application is that the bend radius should be at least ten times the total thickness of the circuit, and for a demanding application several times that. For a static bend a ratio of five is often acceptable. The ratio, not the absolute radius, is what matters, because it determines the strain.

A circuit a quarter of a millimetre thick therefore needs a radius of at least two and a half millimetres to bend dynamically, and more if the copper is far from the neutral axis or if the copper is thick. Reducing the thickness by choosing a thinner base film is often the cheapest way to satisfy the rule. The influence of the conductor is discussed with copper thickness in flex circuits.

What Fatigue Does To Copper

Each bend cycle applies a strain cycle to the copper, and after enough cycles a crack begins at a grain boundary or at a surface defect and grows until the conductor breaks. The process is the same as in any copper fatigue failure, and the number of cycles to failure falls rapidly as the strain rises, which is why the difference between a radius that works and one that does not can be a factor of ten in life.

Electrodeposited copper foils have a columnar grain structure that cracks more readily than the equiaxed structure of rolled annealed foil, which is why the copper foil types matter in a dynamic design. Annealed copper also work hardens less during bending, so it accumulates damage more slowly.

Geometry At The Bend

Traces should cross the bend line perpendicular to it, so that each trace passes through the bend once. A trace that runs along the bend line for any distance accumulates strain over that whole length and fails early. Where the routing forces a trace to run parallel to the bend, it should be moved to another layer.

Plated through holes and vias should be kept away from the bend region entirely. The plating is brittle, and a barrel that passes through a bend concentrates strain where the copper meets the base film, which is exactly where a crack initiates. Where a via is unavoidable, it should be placed in a region that will not bend.

Surface condition matters more than it might appear. A scratch, an etch pit or an inclusion on the copper surface acts as a stress raiser, and a crack begins there long before it would in a smooth conductor. Handling damage to a flex circuit before it is assembled into the product is therefore directly relevant to its fatigue life, and the assembly instructions should say so.

Coverlay, Copper Thickness And Hatching

The coverlay changes the position of the neutral axis, and in a single sided construction it typically moves the axis toward the copper, which reduces the strain that the copper experiences. That is one reason a well designed flex with a coverlay outperforms one with a printed covercoat.

Copper thickness works in the opposite direction: a thicker conductor is stiffer and experiences more strain for the same radius, and the thicker the copper, the worse the fatigue life. A dynamic design should use the thinnest copper that will carry the current, and where a large area of copper is needed for shielding or for a return path, it should be hatched rather than solid.

Designing The Bend Region

The bend region should be a defined area of the layout with its own rules: no vias, no stiffeners, no component pads, and traces perpendicular to the axis. Where the design allows, the copper should be reduced in thickness in that region, and the coverlay should be continuous across it.

It also helps to give the bend region a shape that distributes the bend rather than concentrating it. A gradual transition at each end, rather than a sharp change from a stiff region to a flexible one, avoids the stress concentration that appears where the stiffness changes. The stiffener and coverlay design determines where that transition falls.

Testing And Qualification

The life of a dynamic flex is established by test rather than by calculation. A test fixture bends the assembly through its intended range at the intended rate while monitoring the resistance of the conductors, and the test continues until a specified number of cycles or until a failure. The failure criterion should be a defined change in resistance rather than an open circuit, because a partial crack appears before the conductor breaks.

The test conditions should match the application as closely as possible, including the temperature, because the fatigue behaviour of copper and of the adhesive changes with temperature. A flex that passes a room temperature test may fail in a product that operates warm.

Practical Rules

Four rules cover most dynamic flex designs. Keep the radius at least ten times the stack thickness, and more where the duty is severe. Use rolled annealed copper and keep it thin. Keep traces perpendicular to the bend, with no vias in the bend region. And test the finished assembly through the intended range rather than relying on the calculation.

Where the product allows, it is also worth reconsidering whether the flex needs to move at all. A design in which the circuit bends once during assembly and then is fixed in place removes the fatigue problem entirely and permits a much simpler construction.

FAQ

Does a larger radius always mean a longer life? Within the elastic range, yes, because the strain falls. Beyond the point where the design is no longer practical, the limit is the space available rather than the mechanics.

Can a double sided flex be used dynamically? It can, but the conductors are further from the neutral axis, so the strain is higher and the life shorter for the same radius. Where it is necessary, the radius should be increased and the copper thinned.

How many cycles can be expected? The number depends on the strain, the copper and the construction, and it is best obtained from a test on the actual design. Published figures are a starting point for comparison rather than a guarantee.