Flexible Circuit Materials And Construction

A flexible circuit is not a thin rigid board. It is a different construction with different materials, and the rules that govern it are written by mechanics rather than by impedance. Understanding the stackup is the first step towards designing one that survives being bent.

This article describes the layers of a flexible circuit, the materials used for each, and the design consequences that follow from the construction.

The technology is used for three reasons: to fit a circuit into a shape that a rigid board cannot take, to connect two rigid boards that move relative to each other, and to remove a connector and its failure modes from a product.

The Base And The Conductor

The base film is usually polyimide, chosen for its combination of mechanical strength, thermal stability and dielectric properties. It is available in thicknesses from about twelve to a hundred and twenty five micrometres, and the choice determines both the stiffness of the finished circuit and its ability to survive repeated bending. Polyester is cheaper and is used for static applications where the temperature range is modest.

The conductor is copper foil, either rolled annealed or electrodeposited. Rolled annealed foil has a grain structure that resists fatigue cracking better than the electrodeposited type, which is why it is the standard for dynamic applications. The copper foil types available for flex are a smaller set than for rigid boards, and the choice affects the achievable feature size as well as the flex life.

The Adhesive Question

Copper can be bonded to polyimide with an adhesive layer or directly, in what is called an adhesiveless construction. The adhesiveless type is thinner, has better dimensional stability and withstands higher temperatures, because the acrylic or epoxy adhesive is the weakest layer in the stack both thermally and mechanically.

Adhesive based laminates remain common because they are cheaper and easier to process, and for static applications they are adequate. For a circuit that will bend repeatedly, or that will be soldered at high temperature, the adhesive backing is usually the first layer to fail, and the adhesiveless construction is the better choice.

Coverlay And Its Function

The coverlay is the flexible equivalent of solder mask, applied as a film of polyimide with an adhesive layer, laminated over the finished circuit with openings for the pads. It is thicker than solder mask and it contributes substantially to the mechanical properties of the finished circuit, particularly to the position of the neutral axis.

A common alternative is screen printed covercoat, which is cheaper and can be applied in thinner layers. It provides less mechanical protection and it does not shift the neutral axis in the same way, so it is used for static applications. The choice and its effect on stiffness is discussed with the coverlay and stiffener design rules.

Stiffeners And Their Purpose

A stiffener is a piece of rigid material laminated to a local area of the flex circuit. It serves two purposes: to provide mechanical support where a connector is mounted, and to constrain the flexing to the regions where it is intended. Materials include polyimide of greater thickness, FR-4 and stainless steel.

The stiffener also affects assembly. A connector footprint that is not supported will flex under the insertion force and crack the solder joints over time, so the stiffener is a reliability feature rather than a convenience. Its thickness is chosen to give the required stiffness without creating a step that the assembly process cannot handle.

Single Sided, Double Sided And Multilayer

A single sided flex has one conductor layer and can be bent in one direction with a very small bend radius. A double sided flex has conductors on both faces, which requires plated through holes and makes the circuit stiffer. Multilayer flex circuits exist and are used where the routing density demands it, at a further cost in flexibility.

Where a design needs to bend, the layer count should be kept as low as possible, because each additional layer moves the conductor away from the neutral axis and increases the strain it experiences when the circuit is bent. That is the same principle that governs the choice of copper thickness in a flex design.

The number of layers is not the only stiffness variable. The total thickness of the stack, including the coverlay and the adhesive, determines how much force is needed to bend the circuit and how much strain the outermost copper experiences. A design that adds a stiffener or a thicker coverlay for assembly reasons changes the mechanical behaviour of the bend region as a side effect.

Design Consequences

Several rules follow from the construction. Conductor traces should run perpendicular to the bend line rather than parallel to it, so that each trace crosses the bend at a single point and the bend radius required by the design is uniform along its length. Plated through holes should be kept out of bend regions, because the plating is brittle. Copper should be thinned in a bend region if the design allows it.

Ground planes need care as well. A solid plane in a dynamic bend region will crack, so a cross hatched pattern is used instead. The hatch reduces the stiffness and allows the copper to deform without accumulating damage in one continuous sheet.

Panel utilisation drives cost in a way that is worth understanding. Flex circuits are made on small panels and the material is expensive, so a layout that leaves large unused areas between circuits is directly visible in the price. Nesting the circuits efficiently, and accepting a shape that panels better rather than a shape that is convenient to draw, reduces the cost per unit substantially.

Assembly And Handling

Flexible circuits are damaged more often in handling than in service. A circuit that is folded during assembly in a way that the design did not intend, or that is pulled through a housing without support, can be creased and the copper cracked. The assembly instructions should specify how the circuit is to be handled and where it is allowed to bend.

The connection to a rigid board is a common failure point. It can be made by a connector, by soldering, or by an anisotropic conductive film, and each has different requirements for the pad geometry and the stiffening behind it. Where the joint carries the mechanical load of the assembly, a strain relief feature should be designed into the flex itself.

It is worth specifying the flex requirement in mechanical terms. The number of cycles, the direction of bending, the radius and the temperature during the movement all belong in the drawing, because a circuit qualified for a thousand cycles at a large radius is a different product from one qualified for a hundred thousand cycles at a small radius, even though the artwork is identical.

FAQ

Can a flexible circuit carry power? Yes, and thicker copper is used where it does. The trade is that thicker copper reduces the flex life, so a power carrying flex is usually designed as a static or limited movement part.

Is a flex circuit more expensive than a rigid board? For a small simple circuit, usually, because the materials cost more per unit area and the panel utilisation is lower. The comparison should include the connectors and the assembly steps that the flex removes.

How is a flex circuit tested? Electrically like any circuit, and mechanically by bending the finished assembly through its specified range for the required number of cycles while monitoring for continuity. The mechanical test is the one that predicts field behaviour.