Flexible circuit Bending

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The reliability of flex circuits depends on careful design. Factors that can affect that reliability include:

  • symmetry of design
  • bend angle
  • circuit thickness
  • static vs. dynamic application
  • choice of materials
  • placement of features
  • forming technique
  • conductor routing
  • discontinuities in the bend area

Careful planning and attention to these factors should produce a circuit that suits its application and delivers all the benefits of flex circuitry at the lowest cost. When in doubt about how to best achieve specific goals in a flex circuit application, an experienced manufacturer can be invaluable in evaluating and balancing requirements, answering questions, and providing solutions.

A variety of factors can impact a circuit’s performance when flexed. These FLEX circuits 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
  • The 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 – a 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.

Flex PCBs have been a key enabler of modern high-density electronics, but achieving this density requires thinner layers and finer lines. Conventional three-layer flex circuits comprised of copper, polyimide, and bonding adhesives are giving way to thinner, smoother two-layer flex circuits that forego the adhesive layer – the copper is instead deposited directly on the polyimide. These two-layer circuits may be as thin as 30 µm, with line spacing as fine as 15 µm (0.6 mils). It’s imperative, therefore, that the processed panels are handled extremely carefully to avoid causing wrinkles, tension, or scratches.

Special design for manufacturing (DFM) software tools for flex circuits help neutralize production problems during the design stage. These advanced tools are used to fully automate manual editing sessions, reducing errors and critical cycle time. Among today’s available flex DFMs are automatic joint curving and surface smoothing, and automated overlay and solder mask optimization that makes design faster, higher quality, and more accurate.

its eliminate these joints, making them much more reliable and able to deliver overall higher product quality and longevity. So while rigid-flex PCB technology is certainly not new, various considerations now make it much more viable – not the least of which is cost.

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