Bending And Folding

Flex circuitry is ideal for many of today’s electronics needs. It is light, compact, and, if properly designed,extremely robust. Because it bends, however, a flex circuit has some very specific requirements that aredifferent from those of traditional rigid circuits. Materials, circuit architecture, placement of features, and thenumber of layers in the circuit must all be considered in the design process. So must the degree to which thecircuit will be bent, how tight the bend will be, how the bend will be formed, and how frequently the circuit will be flexed. By carefully defining the application and design priorities and recognizing the unique demandsmade upon flex circuits, the designer can work within these requirements to realize the technology’s fullpotential.

Calculation of flex length

The most frequently asked question we receive regarding flex circuits is “how much can I bend a flex?” The standard IPC answer is 10 times the thickness of the material. There is a section in IPC-2223 that offers reasonable information on bend radius calculations. But there are other factors that need consideration when designing a flex circuit for high reliability.

With Short flexible areas, four bonded flex layers are easier to bend than 2+2 flex layers with airgap.

A variety of factors can impact a circuit’s performance when flexed. These include:

The closer the neutral bend axis falls to the center of the circuit’s material stack, the more evenly forces will be distributed among the other layers of the circuit when it is flexed
Bend angle – the less a circuit is flexed, the smaller the risk of damage
Thickness of the circuit – less thickness reduces the risk of damage when flexed
Bend radius – a larger radius helps reduce the risk of damage
Frequency of flexing – construction that might not be acceptable for a dynamic application, one in which the circuit will be flexed regularly, may be acceptable in a circuit designed to bend only once for installation
Materials – proper selection of materials for their ability to accommodate flex and the way they transmit those forces to other layers in the bend area will improve performance
Construction – designers should avoid placement in or near the bend area of features that are particularly vulnerable to forces generated in the bend area, or that can weaken surrounding circuit structure when flexed

We performed a manufacturing cost simulation with a real rigid-flex circuit designs and a comparative rigid-cable-rigid equivalent. The component BoMs for comparison differed only in the cable and connectors required for the non-flex version. For our simulation, the traditional design is comprised of four-layer boards that use flexible cable and connectors between them, while the rigid-flex circuit design is a four-layer PCB with two inner flex layers. Manufacturing cost for both designs is based on real PCB fabricator quotes, and includes the cost of assembly.

REDUCE OVERALL THICKNESS IN THE FLEX AREA

Reduce the base copper weight (and the corresponding adhesive thicknesses) or reduce the dielectric thickness.
Use adhesiveless base materials. Adhesiveless materials will usually reduce the starting thickness of each substrate by 12-25um (0.0005”- 0.0010”) when compared to adhesive based substrates.
Eliminate copper plating on the conductors in the flexing area (dynamic region) by utilizing selective (pads plating/button plating-only) allowing the circuit to have increased flexibility.

Because the IPC standards are conservatively written to take into account many factors that can affect circuitresilience, it is possible to safely achieve lower-than-standard bend ratios. However, due to the number offactors that can affect performance at smaller-than-recommended bend ratios, it is highly advisable thatdesigners work with an experienced flex circuit manufacturer in developing such designs.