Flex Circuit Bend Radius And Bend Life

A flexible circuit is designed to bend, and the amount it can bend without damage is a design property rather than a property of the material alone. The bend radius, the number of bends the product will see, the direction of the bend relative to the conductor pattern, and the construction of the flex all combine to set the life of the part. A flex that is bent once during assembly can tolerate a radius that would destroy it after a thousand cycles.

This article explains what happens inside a flex when it bends, how the minimum radius is set, how the construction affects the life, and how the design is verified.

What Happens When A Flex Bends

The copper on the outside of the bend is stretched and the copper on the inside is compressed, and the strain is proportional to the distance of the conductor from the neutral axis and inversely proportional to the radius. A single sided flex with the copper on one side has its neutral axis inside the base film, so the conductor is always on the tension side when the part is bent in the natural direction, which is why the bend direction is specified in the drawing rather than left to the assembly operator.

The failure is fatigue. A bend that is within the elastic range produces no damage, while a bend that exceeds it leaves the copper with a permanent set and a reduced fatigue life, and repeated cycling at that radius eventually cracks the conductor at the edge of the bend region. A sharp crease, which is a bend with a radius near zero, can fail on the first cycle, which is why handling damage to a flex is easy to cause and difficult to see.

The static bend and the dynamic bend are different requirements and they should be documented separately. A flex that is folded once inside a housing and never moved needs only enough radius to avoid cracking the copper at the moment of folding, and the fold can be tighter than any dynamic specification would allow. A flex in a hinge, a sliding mechanism, or a moving head is a fatigue application, and its radius has to be set from the cycle count, the bend angle, and the number of conductors in the bend region rather than from a general rule of thumb.

Setting The Minimum Radius

The practical rule is expressed as a multiple of the total thickness of the flex. A single sided flex is often specified as a minimum radius of about ten times its thickness for a dynamic application and a much smaller multiple, sometimes five, for a bend that is formed once and never moved. A double sided flex with plated holes needs a larger radius, and a multilayer flex with a stiffener needs a larger radius again, because the stiffener and the plated barrels are both sensitive to the strain.

The radius is measured to the inside surface of the bend, not to the centreline of the conductors, and the two are often confused. The copper is at a distance from the inside surface that depends on the construction, so a radius that is adequate for the film may still overstrain the conductor if the copper sits far from the neutral axis. Where the design is tight, the conductor can be moved towards the neutral axis by using a construction with copper on both sides of the base film, which balances the strain rather than concentrating it.

Construction Choices That Affect Life

Rolled annealed copper is more ductile than electrodeposited copper and it tolerates far more bending before it cracks, which is why it is specified for a dynamic flex. The base film thickness sets the distance between the conductor and the neutral axis as well as the total thickness, and a thinner film therefore improves the bend life even though it reduces the mechanical stiffness of the part.

The coverlay matters as well, and the way a protective coating is applied to a finished assembly follows the same logic, which is described under board level protection. A coverlay that is bonded over the conductors adds thickness on one side and shifts the neutral axis, and a stiff coverlay material reduces the flexibility of the bend region. Where the bend is expected to see many cycles, the coverlay is often removed from the bend area, or a flexible coverlay material is used, and the conductors in the bend region are routed as a broad pattern rather than as a set of narrow traces.

Layout Rules For A Bend Region

The conductors should cross the bend perpendicular to the bend line, so that each trace is strained uniformly along its width. A trace that crosses at an angle is strained unevenly and fails earlier, and a trace that runs along the bend line is strained over its whole length and fails at the first irregularity. Where the routing cannot be perpendicular, the traces should be spread across the bend area rather than concentrated in one place.

Plated holes and vias should be kept out of the bend region altogether, because the plated barrel is a rigid feature in a flexible area and cracks at the barrel wall. Where a via must be close to a bend, it is placed on the side of the flex that is compressed rather than stretched, and the bend is located so that the transition from the flex to the stiffener is outside the bend radius. A stiffener itself should never end inside a bend, because the abrupt change in stiffness concentrates the strain at its edge.

Verification And Testing

The bend life is verified by a test that reproduces the motion the product will see. A flex specimen is cycled around a mandrel of the specified radius, or through the specified angle, while the resistance of its conductors is monitored continuously so that a crack is detected as a rise in resistance before it becomes an open. The number of cycles to failure is compared with the requirement, and the failure location is examined to confirm that it is in the bend rather than in a via or a termination.

The acceptance criteria for the finished part are visual and dimensional, and the mechanical envelope that the flex occupies is part of the same documentation as the outline of a rigid board, which is described under board outline and mounting design. The bend region is inspected for creases, for cracks in the coverlay, and for delamination, and the radius that the assembly instruction requires is stated in the drawing. The layout choices that follow from a flexible construction are described under layout decisions that affect production. Where the part is bent during assembly, a fixture that holds the correct radius is cheaper than the failures that an operator forming the bend by hand will produce. The stiffener design that supports the terminations is described under PCB design and fabrication.

FAQ

What is the usual minimum bend radius? Around ten times the total thickness for a dynamic bend, and roughly half of that for a bend formed once. A double sided or multilayer flex needs a larger radius than a single sided one.

Why is rolled annealed copper preferred? Because it is far more ductile than electrodeposited copper and tolerates many more bending cycles before it cracks, which is the property that sets the life of a dynamic flex.

Why must vias be kept out of the bend? Because a plated barrel is rigid. Bending it concentrates strain at the barrel wall and cracks the plating, and the failure is usually invisible until the circuit opens.