Flex Stiffeners: Adding Rigidity Where It Is Needed

A stiffener is a piece of rigid material bonded to part of a flexible circuit to stop it flexing where it should not. It is a small addition that solves a large class of problems: a connector that cannot be soldered to a film, a component that cracks when the flex bends, a board that will not stay flat during assembly, and a bend that always happens in the wrong place.

What a Stiffener Does

A flexible circuit is thin, and that thinness is exactly what makes several operations difficult. A connector needs a flat and rigid surface to be soldered or crimped onto, and a film that flexes under the iron will not produce a reliable joint. A surface mount component needs a dimensionally stable substrate, or the solder joint will crack as the film moves. A circuit that slides into a housing needs a leading edge that will not fold. And a bend has to occur where the designer intended, not wherever the assembly operator happens to crease the material.

A stiffener addresses all of those by locally raising the stiffness. It is bonded to the flex, usually with the same adhesive used for the coverlay, and it can be applied to one side or both.

The thickness and the material decide how much rigidity is added. A thin stiffener stops a component site from bending; a thick one turns the end of a flex into a rigid tongue that can be inserted into a connector or a slot.

Materials

Four materials are in common use, and each suits a different requirement.

FR-4 is the cheapest and the most common. It is available in standard thicknesses, it machines and routes easily, and it provides good rigidity. It is not flexible, so it must not be placed where the circuit will bend, and its thermal expansion differs from polyimide, which matters on a long stiffener that sees temperature cycling.

Polyimide is more expensive and thinner for the same stiffness. It is often used where the stiffener has to be thin, where the assembly sees high temperature, or where the stiffener must not add much thickness to a stack that slides into a narrow housing.

Stainless steel provides very high stiffness in a thin section, and it is used for insertion tongues and for the ends of circuits that have to be pushed into a connector. It is conductive, so it must be isolated from the circuit, and it is usually bonded only in an area with no exposed copper.

Aluminium is used where the stiffener also has to conduct heat, since it spreads heat away from a component site as well as supporting it.

Where Stiffeners Belong

Stiffeners are used in a small number of predictable places.

Under connectors. The most common use. A zero insertion force or crimp connector needs a rigid backing so that the insertion force does not flex the film and so that the solder joints do not crack. The stiffener extends beyond the connector body by a margin on every side, because a stiffener that ends at the connector edge still allows the film to bend at the last solder joint.

Under surface mount components. A component on a flex that is flexed in service will crack its joints. A local stiffener limits the movement, and it is usually specified under any component larger than a small passive, particularly where the circuit is expected to be handled after assembly.

At insertion ends. A flex that plugs into a connector needs a rigid tongue, and stainless steel is often used because it gives the required stiffness in the thinnest section. Gold fingers on a flex are almost always on a stiffened area.

Under test points and pads that are probed. A probe applies a point load, and an unsupported film deforms under it, which damages the pad over repeated probing.

Where the mechanical design requires a flat surface. A stiffener is sometimes added purely so that the flex sits flat against a housing or a heatsink, or so that an adhesive can be applied to a defined plane.

Thickness and Bend Radius

Two numbers govern the design. The first is the stiffener thickness, which is chosen for the stiffness required at that location rather than for the circuit as a whole. A connector site might use 0.8 mm FR-4 while a component site uses 0.2 mm polyimide, and there is no reason for them to match.

The second is the distance from the stiffener edge to the bend. This is the number that causes the most failures, because a stiffener edge is a hard transition from rigid to flexible, and the bend will concentrate exactly at that line.

The rule is to keep the bend a defined distance away from the stiffener edge, comparable to the radius the bend will use. A bend that starts immediately at the edge of a stiffener becomes a crease, and the copper cracks there. Where the mechanical design cannot provide that distance, the stiffener should be tapered or its edge moved, and our guide to flex bend radius sets out how the radius relates to the stack.

A rounded or chamfered stiffener edge is a partial remedy, since it distributes the transition over a short distance rather than concentrating it on a straight line. It is worth specifying where the bend has to be close to the stiffener.

Design Rules

• Define the stiffener on the mechanical drawing as a separate item, with its material, thickness, outline and which layers it is bonded to.
• Keep the stiffener edge at least one bend radius away from any bend, and chamfer the edge where that is not possible.
• Extend the stiffener beyond a connector body by several millimetres on the side where the force is applied.
• Do not place a stiffener over a via, a coverlay opening or any feature that needs to flex.
• Keep the stiffener clear of exposed copper where the material is conductive.
• Match the stiffener thickness to the requirement at that site rather than using one thickness for the whole circuit.
• Allow for the stiffener in the overall thickness budget where the assembly slides into a housing or into a connector.
• Specify how the stiffener is bonded and whether the adhesive is the same as the coverlay adhesive, since the two have different thermal limits.

The thickness budget is the item most often forgotten. A flex with a coverlay, a stiffener and an adhesive layer can be several times thicker at that point than elsewhere, and a design that is dimensioned from the flex alone will not fit the mechanical envelope. The three dimensional model should include the stiffener from the beginning rather than being updated after the flex is fabricated.

Assembly and Handling

A stiffener changes how a flex is assembled, and the changes are worth planning.

Where the stiffener is bonded by the fabricator, the flex arrives as a finished part with a rigid area, and the assembly process handles it as a rigid-flex item rather than as a film. Where the stiffener is added by the assembly house, the adhesive and the alignment become part of the assembly process, and the mechanical drawing has to say which arrangement applies.

Pick and place on a stiffened area is straightforward, because the surface behaves like a rigid board. Placement on an unstiffened film is not, and that is one reason a component site on a flex usually has a stiffener whether or not the mechanical design demands one. Our article on flex assembly covers the handling and fixturing that a flexible circuit requires.

Testing is affected as well. A stiffened area can be probed with normal tooling, while an unsupported film needs a support fixture underneath. Where a test fixture is being designed, the stiffener location belongs in the fixture drawing, and it is cheaper to define it before the fixture is built than to add a support pad afterwards.

FAQ

• Does every flex need a stiffener? No. It is needed where a component, connector, probe point or mechanical interface requires a rigid surface, and unnecessary elsewhere.
• What is the most common stiffener material? FR-4, because it is cheap, available in standard thicknesses and easy to machine. Polyimide and stainless steel are used where thinness or higher stiffness is required.
• Can a stiffener be on both sides? Yes, and it is sometimes used to stiffen a connector area symmetrically so the flex does not bow when the connector is inserted.
• Does a stiffener affect impedance? Not significantly at the frequencies involved in a flex, but it can change the mechanical behaviour of a controlled impedance line if it changes the local geometry. The electrical effect is normally ignored.
• Can a stiffener be removed? It can be cut away in rework, but the adhesive usually damages the coverlay. A stiffener should be treated as permanent.

Summary

A stiffener converts part of a flexible circuit into a rigid one so that a connector can be soldered, a component can be mounted, a tongue can be inserted or a probe can be applied. FR-4, polyimide, stainless steel and aluminium each suit a different combination of thickness, stiffness and thermal requirement.

Two rules decide whether the design works. The stiffener has to extend past the feature that needs support, rather than ending at its edge, and its own edge has to be kept away from any bend by at least a bend radius, or the flex will crease at the transition and crack. Those constraints belong on the mechanical drawing as well as in the layout, and they should be confirmed with the flex fabricator and the assembly partner before the circuit is built, because a stiffener that is added in the wrong place is worse than no stiffener at all.