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.

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.

Flexible circuit

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 
Flexible circuit

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.‚Äč

Flexible circuit