Flex Circuit Stiffener Design

A flexible circuit is thin, and thinness is what allows it to bend but also what makes it difficult to attach a connector or to mount a component. A stiffener is a piece of rigid material bonded to a defined area of the flex, which restores the local rigidity without giving up the flexibility of the rest of the circuit.

This article covers why a stiffener is used, what it is made of, and where its boundary should be placed relative to the bend.

Why A Flex Circuit Needs A Stiffener

A connector that is soldered to a thin flex can be peeled off by the force of mating, because the flex deforms under the load and the stress concentrates at the joint. The stiffener spreads that load over an area and holds the joint flat, which allows the connector to be inserted without damaging the solder.

The same reasoning applies to a surface mount component that has to be placed on the flex, particularly a ball grid array where the coplanarity of the pads matters. A stiffener behind the component area keeps the surface flat through the reflow and afterwards. It also allows the area to be processed with the same tooling as a rigid board, which simplifies the assembly.

Materials Used

The common materials are a polyimide sheet, a glass reinforced epoxy sheet, and a thin metal such as stainless steel or aluminium. The polyimide is thin and light and is used where the requirement is to keep the area flat for a connector. The epoxy sheet is thicker and stiffer and is used where the component needs a more rigid base. The metal stiffener also conducts heat and is used where the component has to be cooled as well as supported.

The selection is driven by the thickness that the assembly can accept and by the stiffness required. A thick stiffener achieves the rigidity but adds to the height of the assembly, and on a product where the flex passes through a narrow gap that height may be the limiting factor. The material also has to tolerate the assembly temperature, because it is bonded before the reflow.

Where The Stiffener Goes

The stiffener is placed behind the area that needs support, and it must not extend into the bend. The boundary between the stiffened area and the flexible area is where the stress concentrates when the circuit is bent, so it should be placed away from the bend region and, where possible, aligned with the direction of the bend rather than across it.

The shape of the stiffener matters as well. A rectangular stiffener with a sharp corner creates a stress concentration at the corner, and the copper under it can crack there. A stiffener with a rounded or a tapered edge distributes the transition and reduces the risk. The edge should also be positioned so that it does not sit under a conductor that runs from the stiffened area into the flexible area, because that conductor will be the first to fail.

The Bend Region Boundary

The bend region should be free of stiffener, of adhesive and of any additional layer, because those materials resist bending and transfer the stress to the conductor beside them. The adhesive that bonds the stiffener flows slightly during lamination, so the designed boundary is not the actual boundary, and the allowance for the flow has to be built into the placement.

The distance from the stiffener edge to the start of the bend should be stated in the design rules rather than left to the layout, and it should be based on the bend radius of the application. A tight bend close to a stiffener edge is the combination that produces cracks, and the fix in a mechanical design is usually to move the bend or to reduce the stiffener rather than to change the flex.

Adhesive And Bonding

The stiffener is bonded with a layer of adhesive that is either a separate film or a coating on the stiffener. A thermoplastic adhesive is bonded under heat and pressure, and a thermoset adhesive requires a cure. The bond has to survive the assembly temperature and the mechanical loads of the application.

The adhesive thickness affects the result. A thick adhesive layer accommodates a variation in the surface and reduces the stress at the bond line, while a thin layer gives a stiffer assembly. The adhesive must also not flow into the bend region during bonding, which is why the boundary allowance exists. Where the stiffener is also a thermal path, the adhesive has to be selected for its thermal conductivity as well as its mechanical properties.

Assembly Consequences

The stiffener changes the thickness of the flex locally, which affects the stencil printing, the placement and the reflow. The area is no longer flat if the stiffener is on the same side as the components, and the paste deposit is different over an edge. A component placed across a stiffener edge is at risk of a poor joint, so the keep out around the edge is part of the design rules.

The stiffener also changes the way the panel is handled. A flex with many stiffeners is stiffer and less prone to sagging, which can help the assembly, while one with a few large stiffeners may not lie flat. The balance is found during the process development, and the mechanical requirements of the flex as a whole are described under board outline and mounting design. The layer arrangement of the flexible construction is described in layer stack up, and the manufacturing questions in the guidelines for manufacture.

Additional Considerations for This Build

Practical attention to component mounting pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating component mounting explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, component mounting is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

FAQ

Can a stiffener be added after the flex is made? It is usually bonded during the flex fabrication, because the bonding requires heat and pressure that are applied while the layers are laminated. A later addition is possible but the process is less controlled.

Why does the stiffener edge have to stay out of the bend? Because the transition from rigid to flexible concentrates the stress, and a conductor crossing that boundary is the first to crack under repeated bending.

Does a metal stiffener help with heat? It does, and it is used where the component on the flex needs a thermal path. The adhesive then has to be selected for thermal conductivity as well as for mechanical strength.