Flex PCB Design: Materials, Bend Radius and Support

A flex PCB is designed against a different set of constraints from a rigid board, and the difference is mechanical as much as electrical. The board has to bend without breaking, the conductors in the bend region have to survive repeated movement, and the areas that must not bend — where connectors and components sit — have to be made stiff on purpose. Getting those decisions wrong produces a design that works on the bench and fails in the field, usually at the fold.

Base Design Rules and Stack-Up

Flexible circuits follow the same design standards as rigid boards for clearance, tolerance and electrical safety, applied to a material that moves. The published design guidance for flexible printed circuitry covers those requirements, and the standard practice is to start from it rather than from the rules used for rigid laminates.

The stack-up is where flexibility is won or lost. A single-sided circuit bends most freely, a double-sided one less so, and a multilayer flex is stiffer again because the layers constrain each other. The base material is usually a polyimide, and the copper thickness is chosen from the current it must carry: thinner foil bends more easily, thicker foil carries more current and constrains the bend.

Within the stack, dissimilar metals in direct contact are avoided, since a couple between them will migrate under the influence of moisture and voltage. Where a finish is applied, its thickness and uniformity matter because a brittle or uneven plating layer will crack long before the copper beneath it does.

Coverlay and Surface Protection

Flexible circuits do not usually receive conventional solder mask over their whole surface. The protective layer is a coverlay, a film of polyimide with openings for the pads, bonded over the conductors. It is ductile, it adheres well, and it moves with the circuit rather than cracking at the first significant deflection.

Flexible solder mask inks are also used, and they are appropriate for areas that see less bending or where a coverlay’s thickness would be a problem. The two are frequently combined on the same board: coverlay in the areas that flex and operate in demanding conditions, ink where the surface is flat and static.

Whichever is used, the openings must be clean and accurately placed, because they define the areas available for soldering, and an opening that is too small or offset leaves the joint with less pad area than the design intended.

Routing on a Material That Moves

Routing rules on flex follow from the fact that the conductor experiences strain when the circuit bends.

Corners are curved rather than square. A right angle in a flexible circuit concentrates stress at the corner, and repeated bending turns that concentration into a crack. Curves distribute the strain along the conductor instead.

Conductor width and spacing are kept as uniform as the circuit allows, because a width change in a bend region concentrates strain exactly where the conductor is weakest. Where a wider conductor is needed for current, it can be divided into several narrower parallel conductors, which bend more easily than one wide one carrying the same current.

Power and ground distribution is arranged as a network rather than as a single long path, so that no individual conductor is required to bend more than necessary, and so that the impedance of the return path stays low.

Where the circuit carries fast signals, impedance control applies as it does on a rigid board, with the added complication that the geometry changes when the circuit is bent. The practical approach is to keep controlled-impedance routing out of the regions that flex, so the requirement only has to be met by the geometry in its flat state.

Bend Radius and Bend Regions

The single most important mechanical rule concerns the radius of the bend. The tighter the bend, the greater the strain in the outermost material. The accepted guidance is that the inside radius of a static bend should be at least several times the total thickness of the flexible circuit, and that a bend that will be flexed repeatedly needs a larger radius still.

The second rule is that the bend region should not contain anything that cannot tolerate the strain. Conductors in a bend should run perpendicular to the bend line rather than parallel to it, because a conductor aligned with the fold is stretched along its length at its weakest point. Pads and plated through holes are kept out of the bend; a plated barrel is the least tolerant structure a flexible circuit can contain, since the plating is thin, brittle and discontinuous at the corners of the hole.

The third rule concerns the copper itself. In regions that bend, a solid copper plane is stiffer and more prone to cracking than an open pattern such as a cross-hatched area, which distributes the copper across a larger area of material and allows the laminate to deform with it. The trade is electrical: an open pattern has different impedance and current capacity, so the decision belongs with the electrical design rather than to mechanical convenience.

Where the bend will be repeated in service, the design should also identify the bend as a region with its own tolerance allowance, since the coverlay and the copper both change the effective radius that the assembly achieves. The practices used for bend areas specifically are described in the note on rigid-flex bend area design.

Drilling and Etching on Flexible Material

Flexible laminates behave differently under the drill. The material is more prone to the bit wandering, and the edges of the hole are more likely to raise a burr. Keeping hole sizes reasonable, avoiding clusters of small holes in a small area and supporting the material during drilling all improve the result.

Etching behaves differently too. The fine features that a flexible circuit can carry are produced with an etch process that removes copper from the sides of the conductor as well as from the surface, and the compensation applied to the artwork must account for the material and the thickness rather than being carried over from a rigid process. Keeping conductor widths and spacings similar across the design helps, because the etch behaves most predictably on uniform geometry.

Support During Assembly

A flexible circuit that arrives at the assembly line without support is difficult to print paste onto and difficult to place components on, because it does not hold its own shape. The design response is to define the areas that must be supported: the regions that carry components and the regions that will be handled by the equipment.

Two methods are used. A stiffener, described below, can be added where the circuit must behave rigidly. Alternatively, the circuit can be delivered attached to a carrier panel that holds it flat through assembly and is removed afterwards, which allows standard assembly equipment to process it without modification.

Where a flexible circuit will be reflowed, its moisture behaviour deserves attention. Polyimide absorbs water, and absorbed moisture flashes into steam at reflow temperature, which produces blistering between layers. Drying before assembly, and keeping the parts in dry storage between drying and use, is part of the process specification rather than an optional precaution.

Stiffeners and Reinforcement

A stiffener is a piece of rigid material bonded to the back of the flexible circuit to make a local area behave rigidly. Common materials are FR-4, polyimide of greater thickness and thin metal.

The places that need one are predictable: under connectors, because a plugging and unplugging cycle applies force that a flexible circuit alone will transmit into the solder joints; under the component areas of a circuit that will be handled during assembly; and at the point where the circuit mates with a rigid board.

The stiffener’s thickness and position determine how the area behaves, and its edges define a step that the coverlay has to cross. Both details belong in the fabrication data, because a stiffener added later interferes with the finish, the coverlay openings and the assembly plan.

Electrical Performance in the Field

Two environmental effects are worth designing for explicitly. Temperature changes cause the copper and the polyimide to expand at different rates, and a conductor that is constrained at both ends experiences that difference as stress. And moisture affects the dielectric properties of the laminate, which changes the impedance of a controlled-impedance line if the design has not accounted for it.

The fabrication process for these constructions, including the treatment of holes that pass through both rigid and flexible sections, is covered in the article on hole processing and rigid-flex fabrication, and the review that catches the mechanical as well as the electrical issues is set out in the layout quality checklist.

FAQ

How tight can a bend be? The inside radius is a multiple of the circuit’s total thickness, and the multiplier is larger for a bend that flexes repeatedly than for one that is formed once and left in place.

Can a plated hole be placed in a bend? It should not be. The plating is thin and brittle, and it is the first thing to crack when the circuit is flexed.

When is a stiffener needed? Wherever the circuit must behave rigidly — under connectors, under areas that will be handled during assembly wherever else force would otherwise reach the solder joints.