Flex Circuit Boards Handle Bending Cycles

The base material that makes up a flexible circuit’s core is essential in determining the board’s overall physical and electrical properties. PI (polyimide) and PET films are common flex substrates, although thin flexible-epoxy-and-glass fiber cores also exist. These materials, in conjunction with a rigid circuit stackup and appropriate stiffeners and adhesives, provide the structural support that keeps the traces, pads and other components on the flex circuit in contact with the copper, preventing corrosion and damage.

The type of copper foil used for a flex circuit board will impact the ability of that circuit to handle repeated bending cycles. A standard electrodeposited copper foil (EPCC) will not do the job; a higher-grade rolled annealed copper foil is required for use in dynamic, flex applications. The annealing process changes the grain structure of the foil, making it more springy in the Z deflection direction. It also enables the copper to stretch before fatigue cracking occurs.

Conductor pattern and routing is another critical component in a flex circuit’s ability to handle multiple bending cycles. Copper traces should be routed through bend areas as close to perpendicular as possible. Conductor patterns should be minimized to reduce the stress concentrations that can lead to failure. Pad fillets are also important to increase etch yields and improve copper strength in flex-to-rigid transition areas.

How Do Flex Circuit Boards Handle Bending Cycles?

Flex circuits that are designed for a dynamic environment require a greater number of bending cycles than those intended for static, non-dynamic environments. IPC 2223 provides a general guideline, but engineers need to take the specific application into account when designing the flex circuit and determining how many bending cycles it can expect to see over its lifecycle.

A good way to assess the number of bending cycles a flex circuit can sustain is by using a flex-to-rigid stress meter. These meters can measure the forces exerted on a flex circuit and provide a warning when the stresses reach critical levels.

When a flex circuit is expected to see a large number of bending cycles, designers should work with the manufacturer to ensure that they can meet those expectations by designing the circuit with the right material and thickness. If a flex circuit is not thick enough, it may be damaged during the assembly process or by the product it will be used in when it’s in motion.

In addition to ensuring that the flex circuit is thick enough, designers should consider whether panel plating or pad-only plating (button plating) is best for the application. The latter involves depositing copper on the pad and via sites only, improving etch yields and providing control over conductor thickness and spacing in flex-to-rigid zones. This method is also preferred by some manufacturers for reducing the number of manufacturing steps and minimizing cost. A final consideration is the need to include stiffeners on both the top and bottom of the flex circuit. These can be added to the flex material by adding a layer of either epoxy, acrylic or hot-melt. This is a labor-intensive process that adds to the production costs, so careful collaboration between design and assembly engineering teams is necessary to avoid any costly surprises.